Performance of Polymer Cementitious Coatings for High-Voltage Electrical Infrastructure

Bowen Xu; Hui Li; Dan V. Bompa; Ahmed Y. Elghazouli; Jiangbo Chen

doi:10.3390/infrastructures6090125

Performance of Polymer Cementitious Coatings for High-Voltage Electrical Infrastructure

Xu, Bowen;Li, Hui;Bompa, Dan V.;Elghazouli, Ahmed Y.;Chen, Jiangbo 2021-09-02 00:00:00 infrastructures Article Performance of Polymer Cementitious Coatings for High-Voltage Electrical Infrastructure 1 , 2 3 4 2 Bowen Xu * , Hui Li , Dan V. Bompa , Ahmed Y. Elghazouli and Jiangbo Chen Department of Civil Engineering, Xi’an Jiaotong-Liverpool University, Suzhou 215123, China State Key Laboratory of Power Grid Environmental Protection, High Voltage Department, China Electric Power Research Institute, Wuhan 430074, China; lihui2@epri.sgcc.com.cn (H.L.); chenjiangbo@epri.sgcc.com.cn (J.C.) Department of Civil and Environmental Engineering, University of Surrey, Guildford GU2 7HX, UK; d.bompa@surrey.ac.uk Department of Civil and Environmental Engineering, Imperial College, London SW7 2AZ, UK; a.elghazouli@imperial.ac.uk * Correspondence: Bowen.Xu@xjtlu.edu.cn Abstract: This paper investigates the electrical, thermal and mechanical properties as well as the environmental performance of polymer cementitious composites (PCCs) as sustainable coating materials for underground power cables and as high-voltage insulators. Particular focus is placed on the optimised mix design and the effect of the manufacturing method on the performance of PCCs, incorporating liquid styrene and acrylic (SA) monomers, wollastonite and muscovite. Microstructural investigations, together with results from strength tests, indicate that the manufacturing method is a key performance parameter. Experimental results show that PCC mixes containing 25% SA emulsion, 12.5% wollastonite and no muscovite provide the most favourable dielectric properties from the mixes investigated. The PCC material has a dielectric strength up to 16.5 kV/mm and a dielectric Citation: Xu, B.; Li, H.; Bompa, D.V.; loss factor lower than 0.12. Additional experiments also show that PCC has good thermal stability Elghazouli, A.Y.; Chen, J. and thermal conductivity. The mechanical strength tests indicate that PCC specimens possess reliable Performance of Polymer strengths which are applicable in structural design. Environmental assessments also show that Cementitious Coatings for PCCs possess signiﬁcantly lower embodied energy and embodied carbon than conventional plastic High-Voltage Electrical Infrastructure. insulating materials. Infrastructures 2021, 6, 125. https:// doi.org/10.3390/infrastructures6090125 Keywords: manufacturing; electrical properties; thermal properties; mechanical properties; cementi- tious composites; environmental assessment Academic Editor: Kevin Paine Received: 14 July 2021 Accepted: 30 August 2021 Published: 2 September 2021 1. Introduction Combinations of cement or alternative ﬁllers such as wollastonite or muscovite and Publisher’s Note: MDPI stays neutral polymeric compounds have been investigated in the past in the form of polymer powders, with regard to jurisdictional claims in water-soluble polymers or polymer emulsions. Polymer composite materials are frequently published maps and institutional afﬁl- used in situations where an increase in durability, adhesion with substrates or thermal or iations. waterprooﬁng properties is required [1–4]. Amongst the most used compounds in combina- tion with cement are styrene acrylic copolymer (SAE), styrene butadiene rubber emulsion (SBR), poly(styrene-co-hydroxyethyl methacrylate) (P(St-co-HEMA)), ethylene-vinyl ac- etate copolymer (EVA) and vinyl acetate-versatic vinylester (VA/VeoVA) [5]. Wollastonite Copyright: © 2021 by the authors. is the mineral with an acicular shape which can also be regarded as a low-carbon non- Licensee MDPI, Basel, Switzerland. hydraulic binder, as carbonation forms a heterogeneous matrix with mechanical properties This article is an open access article similar to those of the conventional hydrated cement pastes [6,7]. Muscovite is a silicate distributed under the terms and mineral occurring as non-ﬁbrous plates and is a good electrical and thermal insulator used conditions of the Creative Commons in electronic equipment and transmitting capacitors, among other applications [8]. Attribution (CC BY) license (https:// Polymers can interact with the cement components in contact with water due to creativecommons.org/licenses/by/ ionic binding and cross-linking. During cement hydration, the polymer ﬁlm formation 4.0/). Infrastructures 2021, 6, 125. https://doi.org/10.3390/infrastructures6090125 https://www.mdpi.com/journal/infrastructures Infrastructures 2021, 6, 125 2 of 20 property is inhibited, and the crystallisation process is changed [9]. The retardant effect, occurring due to the formation of a polymer membrane that encapsulates the cement grains [10], has two thrusts. The ﬁrst is a delaying effect represented by a postponed hydration peak, and the second is a slowing down effect characterised by a reduced main hydration peak [11]. This not only produces an increase in the setting time but also in the porosity of the hardened material [12]. The structure of polymer-modiﬁed mortars is generally homogeneous, with a small diameter and uniformly distributed voids [2]. The polymer-modiﬁed cement microstructure is characterised by a shell conﬁgura- tion of polymer-containing hydrates, polymer granule beads, and nonhydrated cement grain [10]. A continuous ﬁlm of polymer within the matrix forms a polymer/cement ratio of 10%, providing a structure where bicontinuous phases of the polymer and cement hydrates are intertwined [12]. However, the level of interaction between the cement and polymers seems to depend on the polymer properties, where certain types of polymers do not cause signiﬁcant modiﬁcations in the microstructural proﬁle of modiﬁed cement paste. For example, the possible interaction between VA/VeoVA copolymers and Portland cement is excluded [13]. The mechanical properties of such materials are typically enhanced by the presence of polymers [14]. For example, blends of SAE and SBR produce a synergistic effect, particularly with regard to the ﬂexural strength, which is 20–40% higher for SAE/SBR blends compared with that of mono-emulsion-modiﬁed mortars [5]. SAE emulsion only enhances the ﬂexural strength and tensile bond strength of a ternary binder system [15]. Additionally, SAE emulsions have good water reduction and water retention effects in mortar as well as reduced shrinkage rates. The water reduction rate reaches about 40% when the SAE emulsion-to-cement ratio is 20%, while the water retention rate is maintained at around 99% as the emulsion-to-cement ratio reaches 10% or more [16]. SAE emulsions have been successfully used in combination with cement and sil- ica fume to enhance the dielectric constant for applications to electromagnetic shielding materials [17,18]. The presence of SB emulsion with an emulsion/cement mass ratio 0.30 increases the relative permittivity (dielectric constant) (2 kHz) from 27 to 43, which is attributed to the compatibility between the cement and the emulsion solid (Wang and Chung 2018). In terms of the increase in relative permittivity, the cement is the main contributor, followed by the emulsion–cement interface. Additionally, due to the same interactions, the relative permittivity of the cement paste from 10 kHz to 1 MHz is de- creased by 28% when silica fume is used, yet it is increased through the use of SB emulsion by 21% [19]. It was also shown that between PVA emulsion (polyvinyl acetate) and the styrene-acrylic SA copolymer, the latter performs better, as its matrix allows a reduction in the percolation threshold to a larger extent [17]. Wollastonite ﬁbers are used as ﬁllers in ceramics or plastics, as these enhance the water vapour transmission in paint ﬁlms [20]. More recently, combinations with cement or other ﬁllers have been investigated [6,21]. Accelerated carbonation tests showed that 25 wt.% of natural wollastonite in wollastonite-Portland cement pastes exhibits a high compressive strength over 80 Mpa [22]. A combination of wollastonite particles replacing cement at a rate of 50% and recycled waste ceramic aggregates reduced the workability of the concrete but increased the compressive, tensile and ﬂexural strengths by about 24%, 4% and 10%, respectively, in comparison with the reference concrete [21]. Moreover, cement mortars with 12% and 15% synthetic wollastonite had higher compressive and ﬂexural strengths than the control mortar [7]. Mechanically activated muscovite can react with hydration products in cement pastes such as calcium hydroxide to form calcium silicate hydrate [23,24]. Although wollastonite can be regarded as a non-hydraulic binder, and muscovite has been used as a reactive powder to produce hardened cementitious composites, these ﬁllers received limited research attention, even though they can provide enhanced thermal and electrical insulation properties. Moreover, studies regarding their electric and electromag- netic performance are lacking. The inﬂuence of polymer emulsions such as styrene-acrylic Infrastructures 2021, 6, 125 3 of 20 copolymers on the microstructural properties, electrical and thermal insulation as well as the mechanical properties of the cementitious materials incorporating wollastonite or muscovite has not been examined before [25]. To this end, a series of experimental studies are proposed in this investigation to examine the performance of polymer-cement-wollastonite (PCW) and polymer-cement- wollstonite/muscovite (PCW/M) composites. The general ﬂow chart of the experimental investigation embedded in this study is shown in Figure 1, with the details demonstrated in the following sections. An ‘optimised’ mix design and a suitable manufacturing method for styrene-acrylic polymer cement composites are proposed. The study presents the experimental results and assessment of the factors inﬂuencing the electric, thermal, mi- crostructural and mechanical properties of PCCs, as well as a comparative environmental assessment. This work is part of a China Electric Power Research Institute-funded project that aims to develop sustainable electric insulating materials that can be used as structural elements [25]. The results provide key information on the fundamental properties of PCC materials and contribute to the development of cementitious construction materials with high-value electrical and thermal performance. Figure 1. Flow chart of the experimental investigation. 2. Materials and Mixes 2.1. Material Details The polymer cementitious composites (PCCs) described in this paper were provided with liquid styrene and acrylic (SA) monomers, wollastonite and muscovite. Portland cement PO 42.5R with a speciﬁc gravity of 3.10 was used as the main binder of the mixes [26]. The cement composition included 22.77% SiO , 5.15% Fe O , 8.26% Al O , 53.25% CaO, 2 2 3 2 3 1.85% MgO, 0.36% Na O, 0.85% K O and 2.34% SO . 2 2 3 Aside from cement, the mixes included two types of ﬁllers: admixtures and tap water. One of the ﬁllers took the form of calcium metasilicate or wollastonite ﬁbres (CaSiO : about 48.28% CaO and 51.72% SiO ), which are white to yellowish-brown naturally occurring minerals. These ﬁbres were used in other studies to improve the strengths of the com- posite material, with the aim to enhance its thermal properties and reduce its electrical conductivity [21,22]. The second ﬁller, hydrated aluminium potassium silicate or muscovite (Al2K2O6Si), is a silicate mineral occurring as non-ﬁbrous plates. This is a good electrical and thermal Infrastructures 2021, 6, 125 4 of 20 insulator used in electronic equipment and transmitting capacitors [8]. Muscovite provided by a local supplier was also considered ﬁller in PCC to improve the workability and electric insulation of the material. Micro-size (5–20 m) wollastonite and muscovite particles were chosen to enhance the compactness of the mixture and provide enhanced thermal and electrical properties. The polymer emulsion used in the PCC mix was a commercial copolymer of liquid styrene and acrylic (SA) monomers [27]. These were chosen to contribute to the water and heat resistance as well as to aid insulation and material ﬂexibility. The SA emulsion (XQS-200) used in these mixes had a solid particle content of 48–52%, monomer content of 0.1–0.5%, a pH of 8–9 and a viscosity between 1000–2000 (mPass). Additionally, a polycarboxylate superplasticiser and an aqueous defoamer were employed to improve the workability of the mixture [28,29]. 2.2. Manufacturing Methods A total of 51 mixes of polymer cementitious composites (PCCs) consisting of more than 400 disc samples, with a diameter of 50 mm and a thickness of 2 mm, were prepared to determine the dielectric strengths of the material and the optimum mix ratio. In order to perform a detailed study regarding the inﬂuence of the manufacturing methods on the insulation properties of PCC materials, mixes with and without muscovite particles were prepared for each method. In total, 8 types of mixes with a minimum of 4 samples for each mix were prepared. For all mixing procedures, disc samples (Ø50 2 mm) were prepared for material investigations, while cylindrical (Ø5 12.5 mm) and prismatic specimens (Ø5 5 12.5 mm) were prepared for the compressive and ﬂexural experiments. Details of each preparation process and the sample details are discussed below. After manufacturing, all samples were kept in a curing chamber (YH-40-B) for 14 days and oven dried at a temperature of 60 C prior to testing. A rotary mixing machine with a capacity of 5 L was used for the mould casting (MC). Figure 2 shows the samples obtained through this mixing. Through a smoother appearance, from left to right, images of samples 1–6 showed gradual improvement in mix homogeneity and less porosity. The mixing rate was 200 rot/min. Initially, the styrene and acrylic (SA) emulsion and water were mixed together in the container for 1–2 min, and the water-to-cement ratio (w/c) was 0.25. The cement and ﬁller particles were then mixed for 3–5 min. The ready mixed SA emulsion and additives were then added, and all the materials were mixed for another 20 min. Figure 2. Optimisation of samples from mould casting with gradual improvement in compaction: (a) very poor, (b) poor, (c) acceptable, (d) good, (e) very good and (f) excellent. (Note: the lowest sample number ‘1’ indicates a poor homogeneity, and the highest sample number ‘6’ indicates excellent homogeneity). Infrastructures 2021, 6, 125 5 of 20 The vacuum mixing (VM) method aims to reduce the air content in the paste to the minimum during the mixing procedure. The mixing system used in this study contained a three-neck round container, an electric rotary rod and a vacuum pump (SHZ-D(III)). As before, the SA emulsion and designed amount of water (w/c = 0.25) were mixed in the container ﬁrst and were then followed by the cement and ﬁller particles. All the materials were mixed for 20 min, while the air in the container was vacuumed during the last 5 min. Through this procedure, the air content in the mixture created by mixing was reduced to the minimum, while the moisture ratio was also maintained. The centrifugal moulding (CM) of concrete and mortar was ﬁrst used for the casting of thin wall pipes. Compared with conventional casting methods, CM allows further compaction of the cement and ﬁller particles. In this study, use was made of a centrifuge (TG16-WS) with a maximum speed of 16,000 rot/min, a speed accuracy of 30 rot/min, a maximum capacity of 6 50 mL and a relative centrifugal acceleration of 23,669 g m/s (g represents gravity’s acceleration). The cement, SA emulsion and ﬁller granules were all mixed together. The same w/c ratio (0.25) was maintained in this casting method, while most of the water content was segregated from the mix under the centrifugal force. The mixture was then placed in plastic test tubes and centrifuged at a speed of 200 rot/min for 10 min. The powder press moulding (PPM) method has been widely used in the manufactur- ing of insulation elements (e.g., ceramic chips). In this study, a tablet press machine (DY-30) was used for the casting of PCC samples. The cement and ﬁller granules were dry mixed separately ﬁrst in the mixing container for 2–3 min. The SA emulsion was then added to the dry powder and mixed for another 5 min. Additional water was sprayed on the surface of the dry mixes with further mixing for 2–3 min. The mixes were then idled for 2 h to ensure the evaporation of added water. The mixtures were subsequently placed in the moulding system with non-spill spoons. A compressive pressure of 10 MPa was applied on the sample and maintained for 10 s. 2.3. Mix Optimisation In the ﬁrst step, a total of 21 mixes with cement and SA emulsion ratios between 0% and 50% were prepared as listed in Table 1, using the mould casting method (MC) described above. Mix references adopt the format TTxx, in which xx indicates the amount of SA emulsion in grams added to the mix while maintaining constant cement and admixtures. After selecting the mixes with the highest dielectric strength [30,31], the dielectric loss, thermogravimetric and infrared spectroscopy analyses, microstructural investigations as well as the thermal conductivity and main mechanical properties of the specimens manufactured through different methods were assessed, and they are described in detail in Sections 3–5. Table 1. Initial mixes with cement and SA emulsion (mass in each sample). Sample Number of Superplasticiser + Cement (g) SA(g) Wollastonite (g) Muscovite (g) Water (g) Number Mixes (-) Defoamer (g) TT00-TT20 21 40 0–20 0 0 0.32 10 TW01-TW16 15 40 10 1–15 0 0.32 10 TM01-TM16 15 40 10 5 1–15 0.32 10 As shown from the test set-up and the results in Figure 3, from a total of 21 mixes consisting of cement, SA, admixtures and water only, the highest dielectric strength of the cement paste was reached by adding 25% SA emulsion. A further increase to the added emulsion caused a reduction in strength. The SA emulsion formed a grid structure between the cement particles which enhanced the insulation properties of the material. Further increasing the emulsion, on the other hand, caused poor compatibility between the SA particles and the cement paste. Consequently, 25% SA emulsion (i.e., 10 g in each sample) was used in the optimum mix design. As shown in Table 1, a water-to-cement ratio of 0.25 Infrastructures 2021, 6, 125 6 of 20 Infrastructures 2021, 6, x FOR PEER REVIEW 6 of 21 was maintained during the casting to achieve a relatively compact mix and materials with adequate strength [32]. (a) (b) Figure 3. (a) Test machine and sketch of electrodes and specimen. (b) Change in dielectric strength Figure 3. (a) Test machine and sketch of electrodes and specimen. (b) Change in dielectric strength with components added. with components added. The following 15 mixes were prepared to investigate the inﬂuence of wollastonite on The following 15 mixes were prepared to investigate the influence of wollastonite on the insulation and workability of the PCC material. The mix reference from Tables 1 and 2 the insulation an adopts d worthe kabformat ility of TWxx, the PCin C ma which terial. xx Th indicates e mix re the fere amount nce from of Ta wollastonite ble 1 and 2 in grams added adopts the format TWxx, in which xx indicates the amount of wollastonite in grams added to the mix, whilst the cement, SA emulsion and admixtures were kept constant. As shown to the mix, whilst in Fig theur ceme e 3b,nt, the SA dielectric emulsionstr and ength adm ofixt the ures mixes wereincr kept eased constant. signiﬁcantly As shown as the amount of in Figure 3b, the wollastonite dielectric strength reached 12.5% of the(i.e., mixes 5 g incr in each eased sample), signific which antly was as the chosen amount for the of optimum mix. The last 15 mixes listed in Table 1 were prepared to investigate the inﬂuence of muscovite wollastonite reached 12.5% (i.e., 5 g in each sample), which was chosen for the optimum particles on the dielectric strength of the PCC. As for the previous mixes, the mix format mix. The last 15 mixes listed in Table 1 were prepared to investigate the influence of mus‐ TMxx includes the muscovite content in grams, whilst the other components were kept covite particles on the dielectric strength of the PCC. As for the previous mixes, the mix constant. Figure 3b indicates that although muscovite provided high insulation properties, format TMxx includes the muscovite content in grams, whilst the other components were the mix of muscovite with polymer-cement pastes reduced the dielectric strength of the mix. kept constant. Figure 3b indicates that although muscovite provided high insulation prop‐ This was caused by the poor bonding between the muscovite powder and SA particles, erties, the mix of muscovite with polymer‐cement pastes reduced the dielectric strength and segregation of the ﬁller and cement paste was also observed. of the mix. This was caused by the poor bonding between the muscovite powder and SA particles, and segregation of the filler and cement paste was also observed. Table 2. Properties of composite samples from different manufacturing methods. Table 1. Initial mixes with cement and SA emulsion (mass in each sample). Dielectric Wollastonite Muscovite Superplasticiser Method Group Type Cement (g) SA (g) Water (g) Strength (g) (g) Super +p Defoamer lasti‐ (g) (kV/mm) Sample Number of Cement SA(g) Wollastonite (g) Muscovite (g) ciser + Water (g) MC S1 PCW C 40 10 5 0 0.32 10 6.1 Number Mixes (‐) (g) Defoamer (g) S5 PCW/MC 40 10 5 3.2 0.32 10 7.8 VM S2 PCWC 40 10 5 0 0.32 10 9.4 TT00‐TT20 21 40 0‐20 0 0 0.32 10 S6 PCW/MC 40 10 5 3.2 0.32 10 9.7 TW01‐TW16 15 40 10 1–15 0 0.32 10 CM S3 PCWC 40 10 5 0 0.32 10 13.2 S7 PCW/MC 40 10 5 3.2 0.32 10 11.4 TM01‐TM16 15 40 10 5 1–15 0.32 10 PM S4 PCWC 40 10 5 0 0.32 + 16.5 S8 PCW/MC 40 10 5 3.2 0.32 + 14.1 Table 2. Properties of composite samples from different manufacturing methods. Superplas‐ 3. Electric Insulation Properties Wollaston‐ Dielectric Cement SA Muscovite ticiser + Water The samples prepared were divided into eight groups as shown in Table 2 and Figure 4 Method Group Type ite Strength (g) (g) (g) Defoamer (g)* in order to investigate the electrical and mechanical behaviour of the polymer-cement- (g) (kV/mm) (g) wollastonite composite (PCWC) and polymer-cement-wollastonite/muscovite composite MC S1 PCW C 40 (PCW/MC) 10 materials. 5 The amounts 0 of cement, 0.32 SA emulsion 10 and wollastonite 6.1 were chosen according to the mix optimisation described above. Particular focus was placed on the S5 PCW/MC 40 10 5 3.2 0.32 10 7.8 inﬂuence of the manufacturing methods on the properties of the PCC materials. The inves- VM S2 PCWC 40 10 5 0 0.32 10 9.4 tigated properties included the dielectric strength, dielectric loss, thermal stability, infrared S6 PCW/MC 40 10 5 3.2 0.32 10 9.7 CM S3 PCWC 40 10 5 0 0.32 10 13.2 S7 PCW/MC 40 10 5 3.2 0.32 10 11.4 PM S4 PCWC 40 10 5 0 0.32 + 16.5 Infrastructures 2021, 6, x FOR PEER REVIEW 7 of 21 S8 PCW/MC 40 10 5 3.2 0.32 + 14.1 3. Electric Insulation Properties The samples prepared were divided into eight groups as shown in Table 2 and Figure 4 in order to investigate the electrical and mechanical behaviour of the polymer‐cement‐ wollastonite composite (PCWC) and polymer‐cement‐wollastonite/muscovite composite (PCW/MC) materials. The amounts of cement, SA emulsion and wollastonite were chosen Infrastructures 2021, 6, 125 7 of 20 according to the mix optimisation described above. Particular focus was placed on the influence of the manufacturing methods on the properties of the PCC materials. The in‐ vestigated properties included the dielectric strength, dielectric loss, thermal stability, in‐ (IR) spectroscopy analysis, microstructural analysis, thermal conductivity and mechanical frared (IR) spectroscopy analysis, microstructural analysis, thermal conductivity and me‐ properties. The experimental procedures and corresponding results are discussed in detail chanical properties. The experimental procedures and corresponding results are dis‐ in this section. cussed in detail in this section. (a) (b) Figure 4. Sample characteristics for composite materials with (a) wollastonite and (b) wollstonite and muscovite. Figure 4. Sample characteristics for composite materials with (a) wollastonite and (b) wollstonite and muscovite. 3.1. Dielectric Strength 3.1. Dielectric Strength As mentioned before, the dielectric strength (EB) is defined as the maximum voltage As mentioned before, the dielectric strength (EB) is deﬁned as the maximum voltage that the material with a unit thickness can withstand (kV/mm) before electrical break‐ that the material with a unit thickness can withstand (kV/mm) before electrical breakdown down occurs (i.e., when current flows through the material). A total of 24 disc samples occurs (i.e., when current ﬂows through the material). A total of 24 disc samples with a size with a size of Ø50 × 2 mm (three specimens in each group) were manufactured using the of Ø50 2 mm (three specimens in each group) were manufactured using the four methods four methods with and without muscovite granules. Tests using a JY6611 machine were with and without muscovite granules. Tests using a JY6611 machine were performed under performed under the ambient temperature (25 °C). According to procedures recom‐ the ambient temperature (25 C). According to procedures recommended by the codiﬁed mended by the codified provisions mentioned before, an AC voltage with a frequency of provisions mentioned before, an AC voltage with a frequency of 50 Hz was applied to the 50 Hz was applied to the test specimen through an electrode, which was increased at a test specimen through an electrode, which was increased at a rate of 2 kV/s from 0. The rate of 2 kV/s from 0. The test was manually stopped when the first spark occurred. test was manually stopped when the ﬁrst spark occurred. The dielectric strengths depicted in Figure 5 for the specimens from each group The dielectric strengths depicted in Figure 5 for the specimens from each group are are listed in Table 2, determined as the average value of all three specimens. It can listed in Table 2, determined as the average value of all three specimens. It can be noted be noted that the manufacturing method had a signiﬁcant inﬂuence on the dielectric that the manufacturing method had a significant influence on the dielectric strengths of strengths of the PCCs. Samples from the S1-PCW/MC and S5-PCWC groups, which were the PCCs. Samples from the S1‐PCW/MC and S5‐PCWC groups, which were manufac‐ manufactured from mould casting, provided the lowest dielectric strengths. Compared tured from mould casting, provided the lowest dielectric strengths. Compared with with specimens from the other groups, the S1-PCW/MC and S5-PCWC specimens had the specimens from the other groups, the S1‐PCW/MC and S5‐PCWC specimens had the highest porosities, as the mixing procedure also introduced a large amount of air content. highest porosities, as the mixing procedure also introduced a large amount of air con‐ Compared with specimens from the S1-PCW/MC and S5-PCWC groups, the specimens tent. Compared with specimens from the S1‐PCW/MC and S5‐PCWC groups, the speci‐ from groups S2-PCW/MC and S6-PCWC, which were manufactured through vacuum mixing, pr mens ovided from ar ound groups a 30% S2‐PCW increase /MCin and dielectric S6‐PCW str Cength. , which The were vacuuming manufactured procedur through e vac‐ reduced the air content in the mixtures, leading to enhanced insulation. uum mixing, provided around a 30% increase in dielectric strength. The vacuuming pro‐ Specimens prepared through centrifugal moulding (S3-PCWC and S7-PCW/MC) cedure reduced the air content in the mixtures, leading to enhanced insulation. provided a twofold dielectric strength compared with their counterparts manufactured via conventional mixing. The centrifugal force removed most of the water and air content, which led to a much more compact mixture. The powder press moulding method provided specimens with the highest dielectric strengths, since it required almost zero added water in mixtures and no air content was introduced during the manufacturing. It is also noted that the muscovite particles did not enhance the insulation properties of the composite material as mentioned before. Hence, further experiments were limited to the PCWC samples only (i.e., groups S1–S4). Infrastructures 2021, 6, 125 8 of 20 Figure 5. Dielectric strength results for different mixing procedures. The dielectric strengths of the PCCs from this study showed values between 6.1 and 16.5 kV/mm, which are within the ranges obtained for common insulating materials such as glass (5–10 kV/mm) and electro-ceramics (8–15 kV/mm) [33]. Other studies also indicated that PCCs incorporating acrylic and vinyl ester have dielectric strengths between 6 and 9 kV/mm [34,35]. Consequently, the PCC mixes developed in this study provided adequate dielectric strengths to be applied in most high-voltage insulating systems. 3.2. Dielectric Loss Dielectric loss of a solid material under the inﬂuence of an alternating electric ﬁeld, represented by the dielectric loss tangent angle tan, deﬁnes the electrical energy dissipated by the motion of dipoles and free charges within the dielectric material as polarisation switches directions. A high value of tan indicates a large amount of electric energy loss, which is converted to thermal energy. The admittance of a dielectric material under an alternating voltage is equivalent to a parallel combination of an energy lossless capacitor C’ and resistance R, which causes the loss of electrical energy as shown in the top left of Figure 6. In this study, the testing arrangements included a bridge circuit, a vacuum container with solid electrodes, a vacuum pump and a high voltage power supply with temperature adjustment [36]. Figure 6. Dielectric loss tangent of the PCC material under different temperatures and voltages and an idealisation of the dielectric medium as the capacitor and resistor (top right panel). The dielectric loss tests in this study focused on the PCC specimens manufactured through a press powder method which provided the highest dielectric strength, as men- tioned in the previous section. The test specimen with the size of Ø50 2 mm was placed between two solid electrodes in the vacuum container. Various levels of alternating volt- ages (U = 500 V, 1000 V and 1500 V) of 50 Hz were applied to the circuit to investigate the inﬂuence of the external electric ﬁeld on the dielectric behaviour of the PCC material. Infrastructures 2021, 6, 125 9 of 20 In addition, the temperature in the container was controlled between 35 C and 90 C to investigate the inﬂuence of the temperature on the dielectric loss of the PCC material. It can be noted from Figure 6 that the applied voltage had no inﬂuence on the dielectric loss tangent of the PCC material, while the higher temperature led to a higher tan. In general, all the PCC specimens manufactured through press powder moulding had a tan lower than 0.12. The introduction of polymer components such as the SA employed in this study greatly reduces the dielectric loss of PCC materials compared with the conventional concrete mixtures, which possess a tan between 0 and 101 [37]. Studies of PCC mixes incorporating various types of insulating ﬁllers, such as recycled rubber and cycloaliphatic epoxy resin, indicated a reduction of tan in the range from 10-2 to 10-1 [38], which is in accordance with the results in this study. The tan factor of the PCC in this study showed similar values compared to other types of insulating coating materials, such as silicon rubber (0.05–0.20) [39]. Conversly, the dielectric loss factor of the PCC showed much higher values than other types of widely used insulating materials such as cross-linked polyethylene (from 10-3 to 10-4) and SF6 gas (less than 10-7) under a frequency of 50 Hz [40–42]. Based on these results, the low cost and high durability of the PCC materials also provide the potential for application of such materials in cable protection covers and high-voltage insulators [34]. 4. Thermal Properties 4.1. Thermal Conductivity Thermal conductivity () (W/mK or W/m C) deﬁnes the amount of heat (W) trans- ferred in 1 s through a material with an area of 1 m , which has a thickness of 1 m and a temperature difference between the two surfaces of 1 K (or C). In this study, the Transient Plane Source (TPS) method [43] was used for measuring the thermal conductivity of the PCC materials. Compared with the other widely used methods such as Transient Hot Strip (THS) [44], which requires samples with speciﬁc geometries [45], the TPS method only needs samples with two parallel smooth surfaces (e.g., disc samples of Ø50 2 mm). As indicated schematically in the top right side panel of Figure 7, a 10–20-m thick pure nickel disk resistor covered on both sides with Kapton foil for protection and electric insulation, whose resistance is in a linear relationship with the temperature, was utilised in the TPS method. This resistor performed simultaneously as the heat source and temperature sensor for the thermal conductivity measurement. During the test, the sensor was sandwiched between two tested insulating specimens. Figure 7. Thermal conductivity test. (a) DRE thermal conductivity parameter test machine. (b) Test results. The automated DRE-IV machines (DRE-IV) employed in this study to measure the thermal conductivity of the PCCs, were in accordance with the requirement of ISO 22007- 2 [46], as shown in Figure 7a. The thermal conductivity of the PCC samples from groups S1–S4 are listed in Table 3 and Figure 7. It can be noted that the of the PCCs was between Infrastructures 2021, 6, 125 10 of 20 1.5 and 2.54 W/mK, depending on the manufacturing method. Studies have shown that the thermal conductivity of concrete depends on its mix components and increases with the increase in density and compactness of the mix [47,48]. The polymer emulsion and antifoaming agent applied in the PCC mixes, in combination with the manufacturing methods, reduced the porosity of the material, reaching = 2.54 W/mK. Table 3. Average electrical, thermal and mechanical properties of the PWC composites tested. Dielectric Thermal Compressive Flexural Method Group Strength SD * Conductivity SD Strength SD Strength SD (kV/mm) (W/mK) (MPa) (MPa) MC S1 6.1 0.5 1.54 0.17 38.2 3.7 7.2 0.6 VM S2 9.4 0.5 1.60 0.07 40.5 4.5 12.0 1.0 CM S3 13.2 1.5 1.65 0.08 65.8 4.9 15.7 1.8 PM S4 16.5 0.6 2.56 0.20 80.7 6.8 20.5 1.8 * The standard deviations (SDs) of the corresponding data. The PCC outperformed the typically used coatings and insulating materials in terms of their thermal conductivity factors. For example, cross-linked polyethylene had = 0.5 W/mK [49], silicone rubber had = 0.2 W/mK [50], SF6 gas had = 0.014 W/mK and insulating oils were around = 0.13 W/mK. As heat dissipation is essential for electrical devices and cables, materials with a high value of are desirable for insulating coatings and mediums [51]. The high heat conductivity factor, combined with the thermal stability test results presented in the following section, indicate that the PCCs provided reliable thermal properties compared with existing insulating materials. 4.2. Thermalgravimetric Analysis The use of thermal analysis, including differential thermal analysis and derivative thermogravimetry, enables a description of the reactions that occur in cementitious compos- ites with an increase in temperature [52]. Typically, hydrated cement pastes include four main components: tricalcium silicate (C3S), dicalcium silicate (C2S), tricalcium aluminate (C3A) and tetracalcium aluminoferrite (C4AF) [53]. In cement-based materials, the main hydration products are calcium silicate hydrate (C-S-H) and portlandite (calcium hydroxide - CH). A number of research studies showed that under an increase in temperature, cement pastes and concretes develop a set of reactions. At temperatures in the range of 30–105 C, the free water and part of the bound water evaporates, whilst at around 120 C, it becomes nonexistent [53]. Furthermore, within 110–180 C, ettringite and gypsum decompose, and water loss from the carboaluminate hydrates occurs [54,55]. In addition to the above, C-S-H and carboaluminate hydrates decompose at tempera- tures above 180 C but below 300 C. The last two stages include dihydroxylation of the portlandite (calcium hydroxide Ca(OH) ) at 450–550 C [56] and decarbonation of calcium carbonate CaCO at 700–900 C [57]. At this stage, depending on the mix proportions and properties of the constituents, 5–30% wt% weight loss can occur. In the particular cases of polymer concretes, the range where weight loss occurs due to polymer pyrolysis is around 350–430 C [1,10]. In terms of the ﬁllers used in this paper, wollastonite is a stable material with low weight loss up to 1000 C, which can only occur due to the presence of small traces of impurities (e.g., calcium carbonate) [58]. To investigate the thermal stability of the PCC mixes, thermogravimetric analyses (TGA) of the selected PCC samples were further carried out in a thermal analyser (Netzsch STA 409PC). The working temperature was established between 30 and 800 C with a heating rate of 20 C/min in a nitrogen environment. As shown in Table 3, TGA was performed with samples from groups S1–S4 (without muscovite granules), which were in accordance with the groups for the thermal conductivity tests. As indicated by the black curves in Figure 8, samples from all the groups lost 5% of their mass at temperatures (T5%) of 170–300 C. The highest thermal stability up to 300 C Infrastructures 2021, 6, x FOR PEER REVIEW 11 of 21 °C, it becomes nonexistent [53]. Furthermore, within 110–180 °C, ettringite and gypsum decompose, and water loss from the carboaluminate hydrates occurs [54,55]. In addition to the above, C‐S‐H and carboaluminate hydrates decompose at temper‐ atures above 180 °C but below 300 °C. The last two stages include dihydroxylation of the portlandite (calcium hydroxide Ca(OH)2) at 450–550 °C [56] and decarbonation of calcium carbonate CaCO3 at 700–900 °C [57]. At this stage, depending on the mix proportions and properties of the constituents, 5–30% wt% weight loss can occur. In the particular cases of polymer concretes, the range where weight loss occurs due to polymer pyrolysis is around 350–430 °C [1,10]. In terms of the fillers used in this paper, wollastonite is a stable material with low weight loss up to 1000 °C, which can only occur due to the presence of small traces of impurities (e.g., calcium carbonate) [58]. To investigate the thermal stability of the PCC mixes, thermogravimetric analyses (TGA) of the selected PCC samples were further carried out in a thermal analyser (Netzsch STA 409PC). The working temperature was established between 30 and 800 °C with a heating rate of 20 °C/min in a nitrogen environment. As shown in Table 3, TGA was per‐ formed with samples from groups S1–S4 (without muscovite granules), which were in Infrastructures 2021, 6, 125 11 of 20 accordance with the groups for the thermal conductivity tests. As indicated by the black curves in Figure 8, samples from all the groups lost 5% of their mass at temperatures (T5%) of 170–300 °C. The highest thermal stability up to 300 °C was shown for S4. This was followed by decomposition of the hydration products. It can was shown for S4. This was followed by decomposition of the hydration products. It be noted that samples from S1, S2 and S3 showed a mass loss of more than 2% at a tem‐ can be noted that samples from S1, S2 and S3 showed a mass loss of more than 2% at a temperatur perature between e between 100 100 andand 200 200 °C. No C. No pyro pyr lysolysis is of the of the polymer polymer or inorg or inor anic gan po icwders powders oc‐ occurr curreded at atthis th istempera temperatur ture; e;hence, hence, the the ma mass ss los loss s was was cause caused d by by th the e vaporisation vaporisation of of the the water water content contentand andother other volatiles. volatilesThis . This was was also also captur capted ured thr through ough the the ﬁrst first derivative derivati ofve the of weight, the weight, repr esented represented by the bygr the ey curves. grey curves. The ﬁrst Thepeak first was peak observed was obsar eround ved aro 100und C, 10 which 0 °C, corresponded to water evaporation, followed by decomposition of the hydration products. which corresponded to water evaporation, followed by decomposition of the hydration A 10 percent (T10%), mass loss was observed at 329–372 C. This was associated with a products. A 10 percent (T10%), mass loss was observed at 329–372 °C. This was associated relatively sudden drop in the weight loss–temperature curves (TGA) and the highest peaks with a relatively sudden drop in the weight loss–temperature curves (TGA) and the high‐ in the derivative curves (DTG). As shown in previous studies, cementitious materials are est peaks in the derivative curves (DTG). As shown in previous studies, cementitious ma‐ relatively stable in the range of 300–400 C without any major component being eliminated terials are relatively stable in the range of 300–400 °C without any major component being from the composite. Implicitly, the mass loss and corresponding peaks on the derivative eliminated from the composite. Implicitly, the mass loss and corresponding peaks on the curves were associated with polymer pyrolysis, which was shown to occur between 350 derivative curves were associated with polymer pyrolysis, which was shown to occur be‐ and 430 C. tween 350 and 430 °C. Infrastructures 2021, 6, x FOR PEER REVIEW 12 of 21 Figure 8. Relationship between remaining mass as a percentage and temperature of samples from (a) group S1, (b) group Figure 8. Relationship between remaining mass as a percentage and temperature of samples from (a) group S1, (b) group S2, (c) group S3 and (d) group S4. S2, (c) group S3 and (d) group S4. In terms of the best thermal performance, samples from groups S1–S3 showed an In terms of the best thermal performance, samples from groups S1–S3 showed an increase in compactness, leading to an increase in thermal stability, which was in accord‐ increase in compactness, leading to an increase in thermal stability, which was in accor- ance with the influence on the dielectric strengths as presented previously. Although the dance with the inﬂuence on the dielectric strengths as presented previously. Although constituent quantities were the same, samples from S4 on the other hand showed signifi‐ the constituent quantities were the same, samples from S4 on the other hand showed cant degradation between 300 and 400 °C. This was attributed to the higher porosity of signiﬁcant degradation between 300 and 400 C. This was attributed to the higher porosity the samples, which allowed a faster thermal transfer and hence implicitly accelerated the of the samples, which allowed a faster thermal transfer and hence implicitly accelerated polymer pyrolysis. Out of the first three mixing procedures, the best performance was the polymer pyrolysis. Out of the ﬁrst three mixing procedures, the best performance was obtained obtained for for sa samples mples from from the the S3 S3 group group,, which which had had aa rrelatively elatively high high T5% T5% and and the the highest highest T10% T10%,, whilst whilst the the remain remaining ing mass mass at at 800 800 °C C was was ar aroun ound d 76 76% % of of the the iin nit itial, ial, wh which ich was was 12 12%, %, 8% and 3% higher than that of S1, S2 and S3, respectively. It is worth noting that weight loss 8% and 3% higher than that of S1, S2 and S3, respectively. It is worth noting that weight loss above ab5% ovewould 5% would occur occ aru ound r aroun 900–1000 d 900–10C00when °C wh the en wollastonite the wollastonite would would suffer sustr ffer uctural struc‐ changes [52,58]. tural changes [52,58]. 5. Microstructure and Mechanical Properties 5.1. Infrared Spectroscopy Analysis Fourier‐transform infrared spectroscopy (FTIR) can be represented by a curve depict‐ ing the relationship between infrared light absorbance or transmittance and wave‐ numbers, which can also by represented in terms of a wavelength or frequency. Through this procedure, a beam of infrared light of a different wavelength is transmitted onto the molecules of the substance, and certain wavelengths of infrared rays are absorbed to form an infrared absorption spectrum of the molecule. The absorbance–wavenumber curve provides an indication of the chemical reactions in the material. The FTIR curves, resulting from an analysis of samples S1–S4 with FTIR equipment (PerkinElmer Spectrum 100 FT‐ IR), are shown in Figure 9. Figure 9. FTIR curves of the samples analysed. Infrastructures 2021, 6, 125 12 of 20 5. Microstructure and Mechanical Properties 5.1. Infrared Spectroscopy Analysis Fourier-transform infrared spectroscopy (FTIR) can be represented by a curve depict- ing the relationship between infrared light absorbance or transmittance and wavenumbers, which can also by represented in terms of a wavelength or frequency. Through this proce- dure, a beam of infrared light of a different wavelength is transmitted onto the molecules of the substance, and certain wavelengths of infrared rays are absorbed to form an infrared absorption spectrum of the molecule. The absorbance–wavenumber curve provides an indication of the chemical reactions in the material. The FTIR curves, resulting from an analysis of samples S1–S4 with FTIR equipment (PerkinElmer Spectrum 100 FT-IR), are shown in Figure 9. Figure 9. FTIR curves of the samples analysed. According to the FTIR curves in Figure 9, the calcium hydroxide peak indicating consumption of portlandite in the mortars was around 3643 cm , which is clearly observed in Figure 9 [59,60]. In the 3300–3500 cm range, a broader peak corresponding to O-H vibrations appeared as a result of hydration due to the presence of water in the air [61]. Three other peaks were visible in the range of 2100–3300 cm , with a maximum near 3000 cm corresponding to C-H, which is characteristic for SAE [62]. An absorption peak was further observed around 1734 cm , corresponding to C=O in the SAE powder- modiﬁed cement pastes [60]. Moreover, the peak around 1620 cm could be attributed to the C-C stretching vibration, which is characteristic for wollastonite [63]. The peak observed at 1431 cm was attributed to the calcium carbonate typical for cementitious materials, which ranges between 1410 and 1510 cm [64]. Additionally, the same peak can also be attributed to wollastonite, which can contains CaO [65]. Typical sulphate absorption bands were generally present in the range of 1100–1200 cm due to S–O stretching vibration [66]. The peak at 1100 cm of the composite could be assigned to Si-O, which is considered ﬁngerprint evidence for the degree of polymerization with the formation of the calcium silica hydrate phase as a result of hydration of the cement [59,66], whilst around 900 cm , this was due to Si-O-Ca stretching present in wollastonite [65]. Through the FTIR analysis, it was shown that quantitative variations in terms of absorbance values occurred as a function of the manufacturing methods of the PCW composites. A clear difference was observed between S4 and the curves of the other three groups (S1–S3). Implicitly, the inherent different porosity and free water in the sample had an impact on the absorption of infrared light. On the other hand, the FTIR results for samples from the S1–S3 groups showed a similar hydration process and reactions within the material with some inﬂuence from the manufacturing method. 5.2. Microstructural Analysis In cementitious materials, the microscopic morphology, pore size, grain boundaries and degree of agglomeration of the original materials and their products determine the Infrastructures 2021, 6, 125 13 of 20 ﬁnal properties of the materials. The particle type and their size distribution and, implicitly, the speciﬁc surface of the mineral admixtures strongly inﬂuence the hydration speed of the process and the demand for water [67]. Cement powders are characterised by very heterogeneous and irregular particle structures with shape factors between one and ﬁve, which are affected by fragmentation and tend to form agglomerates [68]. As mentioned before, the main hydration products in cement pastes are either inner products when C-S-H forms within the boundary of the original particles or outer products when C-S-H and calcium hydroxide form in what was originally water-ﬁlled space [69]. On the other hand, wollastonite has an acicular morphology with crystal lengths at least two times greater than their width, while the shape coefﬁcient of wollastonite (crystal diameter divided by thickness) can be higher than 15 [58]. Wollastonite is regarded as a non-hydraulic binder, since on-carbonation forms a heterogeneous matrix in a similar manner to those of the conventional hydrated cement pastes, resulting in the formation of two main products: calcium carbonate (CaCO ) and amorphous silica gel (SiO ) [6]. 3 2 Materials such as styrene-acrylic emulsions used in the mixes described in this paper have several effects at the microstructural level, such as obstructing the ﬁne pores network inside the hydrates, ﬁlling larger pores and forming membranes that encapsulate cement grains [70]. These characteristics are observed in Figure 10, which depicts scanning electron micro- scope (SEM) images of the selected samples using the manufacturing methods described in Section 2. The microstructural investigations were undertaken using a microscope (JEOL JSM-6510 LV) at two locations of randomly selected samples. Prior to studying the samples, these were spread over a conductive tape and covered in gold ﬁlm. The microstructural examination showed the typical hydration products found in cement pastes for the S1 and S3 manufacturing methods, similar to the characteristics observed in other investi- gations (e.g., [71]). The S2 SEM images seemed to show a less porous structure in which membrane-like substances (hydrates and polymers) bonded the aggregate together, cross- ing the pores like a bridge [10,70]. Ultimately, the results of S4 showed an increased porous media in comparison with the other manufacturing methods (S1–S3) and a non-hydrated cement particle, strengthening the results obtained from the thermogravimetric analysis in Section 4.2. Figure 10. Microstructure images. Infrastructures 2021, 6, 125 14 of 20 5.3. Mechanical Properties In addition to the above material properties, the mechanical strengths of the PCC material were also investigated in this study, including the uniaxial compression and ﬂexural strengths. As introduced previously, cylindrical samples (Ø5 12.5 mm) and prismatic samples (5 5 12.5 mm) were prepared for the two types of tests. These were performed under a displacement ratio of 0.2 mm/min, typical for structures under static loading [72]. The compressive and ﬂexural strengths of the PCC samples from groups S1–S4 are shown in Table 3. It is noted that the manufacturing methods have a signiﬁcant inﬂuence on the strengths of PCC materials. The PCC samples from mould casting showed the lowest compressive and ﬂexural strengths (38.2 MPa and 7.2 MPa, respectively), while specimens from group S2 (VM) had corresponding strengths of 40.5 MPa and 12 MPa, respectively. The enhancement in strength was mainly due to the reduction in the air content in the specimens from the vacuuming procedure. The specimens from group S3 showed twofold the compressive and ﬂexural strengths compared with their counterparts from group S1. Similar to the enhancement in dielectric strength, the specimens prepared from CM were highly compacted, with considerably less water and air content due to the centrifugal force. It can be noted that the highest values of compressive (up to 81 MPa) and ﬂexural strengths (up to 21 MPa) were acquired with PCC materials provided through the powder press moulding method, since it introduced almost zero added water and air content during manufacturing. The SA emulsion provided much higher enhancement to the ﬂexural strength of the PCC materials due to the potential higher tensile strength of the SA emulsion and the overall improvement in the cement hydrate–aggregate bond [2]. Similar to the inﬂuence on the dielectric and thermal properties of the PCC materials, the samples from groups S1–S4 showed an increase in strength due to the increase in the mixture’s compactness and reduction in the water content. It is important to note that the PCC materials provided reliable compressive (up to 81 MPa) and ﬂexural strengths (up to 21 MPa) which were suitable for application as structural materials [73,74]. The high strengths of the PCC materials provide potential applications of such materials in load resistance systems for the protection of telecommunication and electrical power cables. Compared with the direct burial method used in the laying of power cables, the highly durable, low-cost tunnel of PCC material would offer additional protection for the cables from corrosion and other hazards such as ground motions and rodents chewing on them. 6. Environmental Assessments Power transmission network infrastructures, such as electric insulating cables, con- tain substantial amounts of materials that possess large amounts of energy [75]. The most commonly used electric insulating materials are plastics such as polyvinyl chloride (PVC), polyethylene (PE), high-density polyethylene (HDPE) and cross-linked polyethy- lene (XLPE). Synthetic plastic polymers have great potential for providing thermal and electric insulation, high mechanical and ductile properties and water and chemical resis- tance [76,77]. Polyethylene and PVC materials account for over 95% of the waste plastics in the world, and their essential polymerization of ethane (CH2=CH2) indicates that carbon and hydrogen are the backbone of their produced waste [78]. A comparative environmental assessment of the innovative polymer cementitious con- crete system and of the plastic materials used as cable insulating materials was undertaken, as is shown in Table 4. The assessment considered all energy inputs (megajoules) and carbon emissions (kg of carbon dioxide equivalent) from the extraction of raw materials for manufacturing 1 tonne of material to its complete disposal at the end of life. It was shown that the plastic materials possessed signiﬁcantly high embodied energy (around 77,000 MJ/t) which was almost twice the energy density of diesel fuel (i.e., 43,000 MJ/t). The average greenhouse gas (GHG) emissions of the listed plastic materials during their life cycle was around 3300 kg CO eq./t, which is 1.3 times higher than structural steel (i.e., 2500 kg CO eq./t) and 20 times higher than typical concrete materials (i.e., 130 kg CO 2 2 Infrastructures 2021, 6, 125 15 of 20 eq./t) [79]. From this comparison, it is evident that the production of polymer materials led to a much more severe environmental impact compared with conventional construction materials. Table 4. GHG emissions and embodied energy of plastic insulating materials [75,78–83]. Embodied Energy Embodied Carbon Embodied Embodied Carbon/m Material Speciﬁc Gravity (-) 3 3 3 (MJ/t) (kg CO eq./t) Energy/m (MJ/m ) (kg CO eq./m ) 2 2 PVC 77,000 3100 1.40 107,800 4340 PE 76,700 3100 0.96 73,600 2970 XLPE 76,700 4400 0.92 70,560 4050 HDPE 77,000 2700 1.96 150,920 5300 An environmental assessment of the PCC materials investigated in this paper is shown in Table 5, considering the embodied energy and embodied carbon of each component of the mixture. Cement is associated with about 1000 kg CO eq./t embodied carbon and 5500 MJ/t embodied energy, which is the main cause of the environmental load of con- crete [84,85]. Table 5 shows that the styrene-acrylic copolymer had high values of embodied energy and embodied carbon (i.e., 37,400 MJ/t and 1500 kg CO eq./t, respectively) which were comparable to the plastic materials, as shown in Table 4, since SA is also formed through the polymerization of ethane [80,86]. Table 5. GHG emissions and embodied energy of each PCC and its constituents [84–89]. Embodied Embodied Embodied Carbon Components Embodied Amount per Speciﬁc 3 3 Carbon (kg Energy/m of /m of PCC (kg (Group S3) Energy (MJ/t) Sample (g) Gravity (-) 3 3 CO eq./t) PCC (MJ/m ) CO eq./m ) 2 2 Cement 5500 1000 40 3.11 6430 1200 SA 37,400 1500 10 1.08 10,920 438 Wollastonite - - 5 2.75 - - Muscovite - - 0 2.77 - - Superplasticiser 9000 250 0.32 1.08 84 2.3 Sum 17,434 1640 A prototype of a 6-m long PCC tube network (shown in Figure 11) was manufactured and used as a reference for the environmental assessment in this paper. The tube, designed to carry 35 kV, had an outer diameter of 190 mm and an inner diameter of 100 mm. Instead of using traditional plastic materials, a PCC layer was manufactured using centrifugal moulding (CM) to provide insulation and mechanical protection. Electrical characteristic tests indicated that the insulation resistance of the cable tunnel was 4500 MW with a dielectric loss of 4.5%, which met the requirements. Figure 11. Prototype of a 35-kV PCC cable tunnel. Tables 4 and 5 compare the embodied energies and embodied carbon of conventional plastic insulation materials and those of the PCC tube prototype using the respective spe- Infrastructures 2021, 6, 125 16 of 20 ciﬁc gravity of each component. The conventional plastic materials possessed an average 3 3 embodied energy of 100,000 MJ/m and GHG emissions of 4000 kg CO eq./m . On the other hand, the PCC had a total embodied energy and embodied carbon of 17,434 MJ/m and 1640 kg CO eq./m , respectively. Hence, the amount of energy required to manu- facture a cable tunnel with appropriate properties for practical application while being comparable with conventional systems, was reduced by more than 80%, while the embod- ied carbon was halved. 7. Concluding Remarks This paper described an experimental investigation and an environmental assessment of polymer cementitious composites (PCCs) as sustainable coating materials for under- ground power cables and as high-voltage insulators. A detailed account of more than 400 tests on PCC disc samples incorporating liquid styrene and acrylic (SA) monomers (wollastonite and muscovite) was given. Prior to describing the sample manufacturing methods, the selected polymer emulsion and ﬁllers, as well as the mix optimisation proce- dures, were presented in detail. Focus was given to the inﬂuence of the four manufacturing methods on the dielectric and mechanical properties of the PCC. Additional thermogravi- metric, infrared spectroscopy and scanning electron microscope analyses were carried out to offer detailed insight into the microstructure of the PCC. Mix optimisation studies have shown that PCCs with 25% SA emulsion provide the highest dielectric strength from the mixtures investigated, while a further increase in SA causes poor compatibility between constituents. Although muscovite possesses high insulating properties compared with the other materials considered in this paper, the poor bonding between the muscovite powder and SA particle as well as the cement paste reduced the dielectric strength of the PCC. A mix containing 25% SA emulsion, 12.5% wollastonite and no muscovite was shown to offer the best balance in terms of strength and microstructure. The optimised PCC material could reach dielectric strengths up to 16.5 kV/mm, depending on the manufacturing methods, which is above the value of common insulating materials such as glass and electro-ceramics. The dielectric loss factor tan of the PCC materials was lower than 0.12, which is comparable to silicone rubber. Thermal conductivity tests and thermogravimetric analyses also showed that the PCCs had relatively high conductivity ( > 1.3 W/mK) and good thermal stability, respectively. This indicates a superior heat dissipation property in comparison with existing alternatives. As for the above material properties, the mechanical strengths and the microstructure are also greatly inﬂuenced by the manufacturing methods. The compressive and tensile strength of the PCC materials increased with the reduction in water content and porosity and reached maximum values of about 80 MPa and 20 MPa, respectively. Environmental assessments showed that the PCC possessed signiﬁcantly lower embodied energy and embodied carbon than conventional plastic insulating materials. The study presented in the paper indicates that PCC materials have great potential for high-voltage electrical insulating infrastructure. Compared with conventional plastic- based insulating materials such as PE and XLPE, the PCC possesses higher strength, better thermal properties, lower environmental impacts and reasonable electrical properties. Future studies can also focus on optimising the mixing design of the PCC composites to further improve the dielectric strength and reduce the dielectric loss of the material, as well as undertake full-scale tests on members and assessing the performance in service of infrastructure components incorporating the PCC. Author Contributions: Conceptualization, B.X., H.L. and J.C.; methodology, H.L. and D.V.B.; soft- ware, H.L.; validation, B.X., H.L. and D.V.B.; formal analysis, B.X. and D.V.B.; investigation, B.X., D.V.B. and A.Y.E.; resources, H.L. and J.C.; data curation, B.X. and D.V.B.; writing—original draft preparation, B.X. and D.V.B.; writing—review and editing, B.X., D.V.B. and A.Y.E.; supervision, A.Y.E. and J.C.; funding acquisition, H.L. and J.C. All authors have read and agreed to the published version of the manuscript. Infrastructures 2021, 6, 125 17 of 20 Funding: This research was funded by China Electric Power Research Institute, grant number GY71-15-054, GYW17201500088. Data Availability Statement: The data presented in this study are available on request from the corresponding author. Acknowledgments: The ﬁnancial support of China Electric Power Research Institute (GY71-15-054, GYW17201500088) is gratefully acknowledged. 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DOI: 10.3390/infrastructures6090125
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Abstract

infrastructures Article Performance of Polymer Cementitious Coatings for High-Voltage Electrical Infrastructure 1 , 2 3 4 2 Bowen Xu * , Hui Li , Dan V. Bompa , Ahmed Y. Elghazouli and Jiangbo Chen Department of Civil Engineering, Xi’an Jiaotong-Liverpool University, Suzhou 215123, China State Key Laboratory of Power Grid Environmental Protection, High Voltage Department, China Electric Power Research Institute, Wuhan 430074, China; lihui2@epri.sgcc.com.cn (H.L.); chenjiangbo@epri.sgcc.com.cn (J.C.) Department of Civil and Environmental Engineering, University of Surrey, Guildford GU2 7HX, UK; d.bompa@surrey.ac.uk Department of Civil and Environmental Engineering, Imperial College, London SW7 2AZ, UK; a.elghazouli@imperial.ac.uk * Correspondence: Bowen.Xu@xjtlu.edu.cn Abstract: This paper investigates the electrical, thermal and mechanical properties as well as the environmental performance of polymer cementitious composites (PCCs) as sustainable coating materials for underground power cables and as high-voltage insulators. Particular focus is placed on the optimised mix design and the effect of the manufacturing method on the performance of PCCs, incorporating liquid styrene and acrylic (SA) monomers, wollastonite and muscovite. Microstructural investigations, together with results from strength tests, indicate that the manufacturing method is a key performance parameter. Experimental results show that PCC mixes containing 25% SA emulsion, 12.5% wollastonite and no muscovite provide the most favourable dielectric properties from the mixes investigated. The PCC material has a dielectric strength up to 16.5 kV/mm and a dielectric Citation: Xu, B.; Li, H.; Bompa, D.V.; loss factor lower than 0.12. Additional experiments also show that PCC has good thermal stability Elghazouli, A.Y.; Chen, J. and thermal conductivity. The mechanical strength tests indicate that PCC specimens possess reliable Performance of Polymer strengths which are applicable in structural design. Environmental assessments also show that Cementitious Coatings for PCCs possess signiﬁcantly lower embodied energy and embodied carbon than conventional plastic High-Voltage Electrical Infrastructure. insulating materials. Infrastructures 2021, 6, 125. https:// doi.org/10.3390/infrastructures6090125 Keywords: manufacturing; electrical properties; thermal properties; mechanical properties; cementi- tious composites; environmental assessment Academic Editor: Kevin Paine Received: 14 July 2021 Accepted: 30 August 2021 Published: 2 September 2021 1. Introduction Combinations of cement or alternative ﬁllers such as wollastonite or muscovite and Publisher’s Note: MDPI stays neutral polymeric compounds have been investigated in the past in the form of polymer powders, with regard to jurisdictional claims in water-soluble polymers or polymer emulsions. Polymer composite materials are frequently published maps and institutional afﬁl- used in situations where an increase in durability, adhesion with substrates or thermal or iations. waterprooﬁng properties is required [1–4]. Amongst the most used compounds in combina- tion with cement are styrene acrylic copolymer (SAE), styrene butadiene rubber emulsion (SBR), poly(styrene-co-hydroxyethyl methacrylate) (P(St-co-HEMA)), ethylene-vinyl ac- etate copolymer (EVA) and vinyl acetate-versatic vinylester (VA/VeoVA) [5]. Wollastonite Copyright: © 2021 by the authors. is the mineral with an acicular shape which can also be regarded as a low-carbon non- Licensee MDPI, Basel, Switzerland. hydraulic binder, as carbonation forms a heterogeneous matrix with mechanical properties This article is an open access article similar to those of the conventional hydrated cement pastes [6,7]. Muscovite is a silicate distributed under the terms and mineral occurring as non-ﬁbrous plates and is a good electrical and thermal insulator used conditions of the Creative Commons in electronic equipment and transmitting capacitors, among other applications [8]. Attribution (CC BY) license (https:// Polymers can interact with the cement components in contact with water due to creativecommons.org/licenses/by/ ionic binding and cross-linking. During cement hydration, the polymer ﬁlm formation 4.0/). Infrastructures 2021, 6, 125. https://doi.org/10.3390/infrastructures6090125 https://www.mdpi.com/journal/infrastructures Infrastructures 2021, 6, 125 2 of 20 property is inhibited, and the crystallisation process is changed [9]. The retardant effect, occurring due to the formation of a polymer membrane that encapsulates the cement grains [10], has two thrusts. The ﬁrst is a delaying effect represented by a postponed hydration peak, and the second is a slowing down effect characterised by a reduced main hydration peak [11]. This not only produces an increase in the setting time but also in the porosity of the hardened material [12]. The structure of polymer-modiﬁed mortars is generally homogeneous, with a small diameter and uniformly distributed voids [2]. The polymer-modiﬁed cement microstructure is characterised by a shell conﬁgura- tion of polymer-containing hydrates, polymer granule beads, and nonhydrated cement grain [10]. A continuous ﬁlm of polymer within the matrix forms a polymer/cement ratio of 10%, providing a structure where bicontinuous phases of the polymer and cement hydrates are intertwined [12]. However, the level of interaction between the cement and polymers seems to depend on the polymer properties, where certain types of polymers do not cause signiﬁcant modiﬁcations in the microstructural proﬁle of modiﬁed cement paste. For example, the possible interaction between VA/VeoVA copolymers and Portland cement is excluded [13]. The mechanical properties of such materials are typically enhanced by the presence of polymers [14]. For example, blends of SAE and SBR produce a synergistic effect, particularly with regard to the ﬂexural strength, which is 20–40% higher for SAE/SBR blends compared with that of mono-emulsion-modiﬁed mortars [5]. SAE emulsion only enhances the ﬂexural strength and tensile bond strength of a ternary binder system [15]. Additionally, SAE emulsions have good water reduction and water retention effects in mortar as well as reduced shrinkage rates. The water reduction rate reaches about 40% when the SAE emulsion-to-cement ratio is 20%, while the water retention rate is maintained at around 99% as the emulsion-to-cement ratio reaches 10% or more [16]. SAE emulsions have been successfully used in combination with cement and sil- ica fume to enhance the dielectric constant for applications to electromagnetic shielding materials [17,18]. The presence of SB emulsion with an emulsion/cement mass ratio 0.30 increases the relative permittivity (dielectric constant) (2 kHz) from 27 to 43, which is attributed to the compatibility between the cement and the emulsion solid (Wang and Chung 2018). In terms of the increase in relative permittivity, the cement is the main contributor, followed by the emulsion–cement interface. Additionally, due to the same interactions, the relative permittivity of the cement paste from 10 kHz to 1 MHz is de- creased by 28% when silica fume is used, yet it is increased through the use of SB emulsion by 21% [19]. It was also shown that between PVA emulsion (polyvinyl acetate) and the styrene-acrylic SA copolymer, the latter performs better, as its matrix allows a reduction in the percolation threshold to a larger extent [17]. Wollastonite ﬁbers are used as ﬁllers in ceramics or plastics, as these enhance the water vapour transmission in paint ﬁlms [20]. More recently, combinations with cement or other ﬁllers have been investigated [6,21]. Accelerated carbonation tests showed that 25 wt.% of natural wollastonite in wollastonite-Portland cement pastes exhibits a high compressive strength over 80 Mpa [22]. A combination of wollastonite particles replacing cement at a rate of 50% and recycled waste ceramic aggregates reduced the workability of the concrete but increased the compressive, tensile and ﬂexural strengths by about 24%, 4% and 10%, respectively, in comparison with the reference concrete [21]. Moreover, cement mortars with 12% and 15% synthetic wollastonite had higher compressive and ﬂexural strengths than the control mortar [7]. Mechanically activated muscovite can react with hydration products in cement pastes such as calcium hydroxide to form calcium silicate hydrate [23,24]. Although wollastonite can be regarded as a non-hydraulic binder, and muscovite has been used as a reactive powder to produce hardened cementitious composites, these ﬁllers received limited research attention, even though they can provide enhanced thermal and electrical insulation properties. Moreover, studies regarding their electric and electromag- netic performance are lacking. The inﬂuence of polymer emulsions such as styrene-acrylic Infrastructures 2021, 6, 125 3 of 20 copolymers on the microstructural properties, electrical and thermal insulation as well as the mechanical properties of the cementitious materials incorporating wollastonite or muscovite has not been examined before [25]. To this end, a series of experimental studies are proposed in this investigation to examine the performance of polymer-cement-wollastonite (PCW) and polymer-cement- wollstonite/muscovite (PCW/M) composites. The general ﬂow chart of the experimental investigation embedded in this study is shown in Figure 1, with the details demonstrated in the following sections. An ‘optimised’ mix design and a suitable manufacturing method for styrene-acrylic polymer cement composites are proposed. The study presents the experimental results and assessment of the factors inﬂuencing the electric, thermal, mi- crostructural and mechanical properties of PCCs, as well as a comparative environmental assessment. This work is part of a China Electric Power Research Institute-funded project that aims to develop sustainable electric insulating materials that can be used as structural elements [25]. The results provide key information on the fundamental properties of PCC materials and contribute to the development of cementitious construction materials with high-value electrical and thermal performance. Figure 1. Flow chart of the experimental investigation. 2. Materials and Mixes 2.1. Material Details The polymer cementitious composites (PCCs) described in this paper were provided with liquid styrene and acrylic (SA) monomers, wollastonite and muscovite. Portland cement PO 42.5R with a speciﬁc gravity of 3.10 was used as the main binder of the mixes [26]. The cement composition included 22.77% SiO , 5.15% Fe O , 8.26% Al O , 53.25% CaO, 2 2 3 2 3 1.85% MgO, 0.36% Na O, 0.85% K O and 2.34% SO . 2 2 3 Aside from cement, the mixes included two types of ﬁllers: admixtures and tap water. One of the ﬁllers took the form of calcium metasilicate or wollastonite ﬁbres (CaSiO : about 48.28% CaO and 51.72% SiO ), which are white to yellowish-brown naturally occurring minerals. These ﬁbres were used in other studies to improve the strengths of the com- posite material, with the aim to enhance its thermal properties and reduce its electrical conductivity [21,22]. The second ﬁller, hydrated aluminium potassium silicate or muscovite (Al2K2O6Si), is a silicate mineral occurring as non-ﬁbrous plates. This is a good electrical and thermal Infrastructures 2021, 6, 125 4 of 20 insulator used in electronic equipment and transmitting capacitors [8]. Muscovite provided by a local supplier was also considered ﬁller in PCC to improve the workability and electric insulation of the material. Micro-size (5–20 m) wollastonite and muscovite particles were chosen to enhance the compactness of the mixture and provide enhanced thermal and electrical properties. The polymer emulsion used in the PCC mix was a commercial copolymer of liquid styrene and acrylic (SA) monomers [27]. These were chosen to contribute to the water and heat resistance as well as to aid insulation and material ﬂexibility. The SA emulsion (XQS-200) used in these mixes had a solid particle content of 48–52%, monomer content of 0.1–0.5%, a pH of 8–9 and a viscosity between 1000–2000 (mPass). Additionally, a polycarboxylate superplasticiser and an aqueous defoamer were employed to improve the workability of the mixture [28,29]. 2.2. Manufacturing Methods A total of 51 mixes of polymer cementitious composites (PCCs) consisting of more than 400 disc samples, with a diameter of 50 mm and a thickness of 2 mm, were prepared to determine the dielectric strengths of the material and the optimum mix ratio. In order to perform a detailed study regarding the inﬂuence of the manufacturing methods on the insulation properties of PCC materials, mixes with and without muscovite particles were prepared for each method. In total, 8 types of mixes with a minimum of 4 samples for each mix were prepared. For all mixing procedures, disc samples (Ø50 2 mm) were prepared for material investigations, while cylindrical (Ø5 12.5 mm) and prismatic specimens (Ø5 5 12.5 mm) were prepared for the compressive and ﬂexural experiments. Details of each preparation process and the sample details are discussed below. After manufacturing, all samples were kept in a curing chamber (YH-40-B) for 14 days and oven dried at a temperature of 60 C prior to testing. A rotary mixing machine with a capacity of 5 L was used for the mould casting (MC). Figure 2 shows the samples obtained through this mixing. Through a smoother appearance, from left to right, images of samples 1–6 showed gradual improvement in mix homogeneity and less porosity. The mixing rate was 200 rot/min. Initially, the styrene and acrylic (SA) emulsion and water were mixed together in the container for 1–2 min, and the water-to-cement ratio (w/c) was 0.25. The cement and ﬁller particles were then mixed for 3–5 min. The ready mixed SA emulsion and additives were then added, and all the materials were mixed for another 20 min. Figure 2. Optimisation of samples from mould casting with gradual improvement in compaction: (a) very poor, (b) poor, (c) acceptable, (d) good, (e) very good and (f) excellent. (Note: the lowest sample number ‘1’ indicates a poor homogeneity, and the highest sample number ‘6’ indicates excellent homogeneity). Infrastructures 2021, 6, 125 5 of 20 The vacuum mixing (VM) method aims to reduce the air content in the paste to the minimum during the mixing procedure. The mixing system used in this study contained a three-neck round container, an electric rotary rod and a vacuum pump (SHZ-D(III)). As before, the SA emulsion and designed amount of water (w/c = 0.25) were mixed in the container ﬁrst and were then followed by the cement and ﬁller particles. All the materials were mixed for 20 min, while the air in the container was vacuumed during the last 5 min. Through this procedure, the air content in the mixture created by mixing was reduced to the minimum, while the moisture ratio was also maintained. The centrifugal moulding (CM) of concrete and mortar was ﬁrst used for the casting of thin wall pipes. Compared with conventional casting methods, CM allows further compaction of the cement and ﬁller particles. In this study, use was made of a centrifuge (TG16-WS) with a maximum speed of 16,000 rot/min, a speed accuracy of 30 rot/min, a maximum capacity of 6 50 mL and a relative centrifugal acceleration of 23,669 g m/s (g represents gravity’s acceleration). The cement, SA emulsion and ﬁller granules were all mixed together. The same w/c ratio (0.25) was maintained in this casting method, while most of the water content was segregated from the mix under the centrifugal force. The mixture was then placed in plastic test tubes and centrifuged at a speed of 200 rot/min for 10 min. The powder press moulding (PPM) method has been widely used in the manufactur- ing of insulation elements (e.g., ceramic chips). In this study, a tablet press machine (DY-30) was used for the casting of PCC samples. The cement and ﬁller granules were dry mixed separately ﬁrst in the mixing container for 2–3 min. The SA emulsion was then added to the dry powder and mixed for another 5 min. Additional water was sprayed on the surface of the dry mixes with further mixing for 2–3 min. The mixes were then idled for 2 h to ensure the evaporation of added water. The mixtures were subsequently placed in the moulding system with non-spill spoons. A compressive pressure of 10 MPa was applied on the sample and maintained for 10 s. 2.3. Mix Optimisation In the ﬁrst step, a total of 21 mixes with cement and SA emulsion ratios between 0% and 50% were prepared as listed in Table 1, using the mould casting method (MC) described above. Mix references adopt the format TTxx, in which xx indicates the amount of SA emulsion in grams added to the mix while maintaining constant cement and admixtures. After selecting the mixes with the highest dielectric strength [30,31], the dielectric loss, thermogravimetric and infrared spectroscopy analyses, microstructural investigations as well as the thermal conductivity and main mechanical properties of the specimens manufactured through different methods were assessed, and they are described in detail in Sections 3–5. Table 1. Initial mixes with cement and SA emulsion (mass in each sample). Sample Number of Superplasticiser + Cement (g) SA(g) Wollastonite (g) Muscovite (g) Water (g) Number Mixes (-) Defoamer (g) TT00-TT20 21 40 0–20 0 0 0.32 10 TW01-TW16 15 40 10 1–15 0 0.32 10 TM01-TM16 15 40 10 5 1–15 0.32 10 As shown from the test set-up and the results in Figure 3, from a total of 21 mixes consisting of cement, SA, admixtures and water only, the highest dielectric strength of the cement paste was reached by adding 25% SA emulsion. A further increase to the added emulsion caused a reduction in strength. The SA emulsion formed a grid structure between the cement particles which enhanced the insulation properties of the material. Further increasing the emulsion, on the other hand, caused poor compatibility between the SA particles and the cement paste. Consequently, 25% SA emulsion (i.e., 10 g in each sample) was used in the optimum mix design. As shown in Table 1, a water-to-cement ratio of 0.25 Infrastructures 2021, 6, 125 6 of 20 Infrastructures 2021, 6, x FOR PEER REVIEW 6 of 21 was maintained during the casting to achieve a relatively compact mix and materials with adequate strength [32]. (a) (b) Figure 3. (a) Test machine and sketch of electrodes and specimen. (b) Change in dielectric strength Figure 3. (a) Test machine and sketch of electrodes and specimen. (b) Change in dielectric strength with components added. with components added. The following 15 mixes were prepared to investigate the inﬂuence of wollastonite on The following 15 mixes were prepared to investigate the influence of wollastonite on the insulation and workability of the PCC material. The mix reference from Tables 1 and 2 the insulation an adopts d worthe kabformat ility of TWxx, the PCin C ma which terial. xx Th indicates e mix re the fere amount nce from of Ta wollastonite ble 1 and 2 in grams added adopts the format TWxx, in which xx indicates the amount of wollastonite in grams added to the mix, whilst the cement, SA emulsion and admixtures were kept constant. As shown to the mix, whilst in Fig theur ceme e 3b,nt, the SA dielectric emulsionstr and ength adm ofixt the ures mixes wereincr kept eased constant. signiﬁcantly As shown as the amount of in Figure 3b, the wollastonite dielectric strength reached 12.5% of the(i.e., mixes 5 g incr in each eased sample), signific which antly was as the chosen amount for the of optimum mix. The last 15 mixes listed in Table 1 were prepared to investigate the inﬂuence of muscovite wollastonite reached 12.5% (i.e., 5 g in each sample), which was chosen for the optimum particles on the dielectric strength of the PCC. As for the previous mixes, the mix format mix. The last 15 mixes listed in Table 1 were prepared to investigate the influence of mus‐ TMxx includes the muscovite content in grams, whilst the other components were kept covite particles on the dielectric strength of the PCC. As for the previous mixes, the mix constant. Figure 3b indicates that although muscovite provided high insulation properties, format TMxx includes the muscovite content in grams, whilst the other components were the mix of muscovite with polymer-cement pastes reduced the dielectric strength of the mix. kept constant. Figure 3b indicates that although muscovite provided high insulation prop‐ This was caused by the poor bonding between the muscovite powder and SA particles, erties, the mix of muscovite with polymer‐cement pastes reduced the dielectric strength and segregation of the ﬁller and cement paste was also observed. of the mix. This was caused by the poor bonding between the muscovite powder and SA particles, and segregation of the filler and cement paste was also observed. Table 2. Properties of composite samples from different manufacturing methods. Table 1. Initial mixes with cement and SA emulsion (mass in each sample). Dielectric Wollastonite Muscovite Superplasticiser Method Group Type Cement (g) SA (g) Water (g) Strength (g) (g) Super +p Defoamer lasti‐ (g) (kV/mm) Sample Number of Cement SA(g) Wollastonite (g) Muscovite (g) ciser + Water (g) MC S1 PCW C 40 10 5 0 0.32 10 6.1 Number Mixes (‐) (g) Defoamer (g) S5 PCW/MC 40 10 5 3.2 0.32 10 7.8 VM S2 PCWC 40 10 5 0 0.32 10 9.4 TT00‐TT20 21 40 0‐20 0 0 0.32 10 S6 PCW/MC 40 10 5 3.2 0.32 10 9.7 TW01‐TW16 15 40 10 1–15 0 0.32 10 CM S3 PCWC 40 10 5 0 0.32 10 13.2 S7 PCW/MC 40 10 5 3.2 0.32 10 11.4 TM01‐TM16 15 40 10 5 1–15 0.32 10 PM S4 PCWC 40 10 5 0 0.32 + 16.5 S8 PCW/MC 40 10 5 3.2 0.32 + 14.1 Table 2. Properties of composite samples from different manufacturing methods. Superplas‐ 3. Electric Insulation Properties Wollaston‐ Dielectric Cement SA Muscovite ticiser + Water The samples prepared were divided into eight groups as shown in Table 2 and Figure 4 Method Group Type ite Strength (g) (g) (g) Defoamer (g)* in order to investigate the electrical and mechanical behaviour of the polymer-cement- (g) (kV/mm) (g) wollastonite composite (PCWC) and polymer-cement-wollastonite/muscovite composite MC S1 PCW C 40 (PCW/MC) 10 materials. 5 The amounts 0 of cement, 0.32 SA emulsion 10 and wollastonite 6.1 were chosen according to the mix optimisation described above. Particular focus was placed on the S5 PCW/MC 40 10 5 3.2 0.32 10 7.8 inﬂuence of the manufacturing methods on the properties of the PCC materials. The inves- VM S2 PCWC 40 10 5 0 0.32 10 9.4 tigated properties included the dielectric strength, dielectric loss, thermal stability, infrared S6 PCW/MC 40 10 5 3.2 0.32 10 9.7 CM S3 PCWC 40 10 5 0 0.32 10 13.2 S7 PCW/MC 40 10 5 3.2 0.32 10 11.4 PM S4 PCWC 40 10 5 0 0.32 + 16.5 Infrastructures 2021, 6, x FOR PEER REVIEW 7 of 21 S8 PCW/MC 40 10 5 3.2 0.32 + 14.1 3. Electric Insulation Properties The samples prepared were divided into eight groups as shown in Table 2 and Figure 4 in order to investigate the electrical and mechanical behaviour of the polymer‐cement‐ wollastonite composite (PCWC) and polymer‐cement‐wollastonite/muscovite composite (PCW/MC) materials. The amounts of cement, SA emulsion and wollastonite were chosen Infrastructures 2021, 6, 125 7 of 20 according to the mix optimisation described above. Particular focus was placed on the influence of the manufacturing methods on the properties of the PCC materials. The in‐ vestigated properties included the dielectric strength, dielectric loss, thermal stability, in‐ (IR) spectroscopy analysis, microstructural analysis, thermal conductivity and mechanical frared (IR) spectroscopy analysis, microstructural analysis, thermal conductivity and me‐ properties. The experimental procedures and corresponding results are discussed in detail chanical properties. The experimental procedures and corresponding results are dis‐ in this section. cussed in detail in this section. (a) (b) Figure 4. Sample characteristics for composite materials with (a) wollastonite and (b) wollstonite and muscovite. Figure 4. Sample characteristics for composite materials with (a) wollastonite and (b) wollstonite and muscovite. 3.1. Dielectric Strength 3.1. Dielectric Strength As mentioned before, the dielectric strength (EB) is defined as the maximum voltage As mentioned before, the dielectric strength (EB) is deﬁned as the maximum voltage that the material with a unit thickness can withstand (kV/mm) before electrical break‐ that the material with a unit thickness can withstand (kV/mm) before electrical breakdown down occurs (i.e., when current flows through the material). A total of 24 disc samples occurs (i.e., when current ﬂows through the material). A total of 24 disc samples with a size with a size of Ø50 × 2 mm (three specimens in each group) were manufactured using the of Ø50 2 mm (three specimens in each group) were manufactured using the four methods four methods with and without muscovite granules. Tests using a JY6611 machine were with and without muscovite granules. Tests using a JY6611 machine were performed under performed under the ambient temperature (25 °C). According to procedures recom‐ the ambient temperature (25 C). According to procedures recommended by the codiﬁed mended by the codified provisions mentioned before, an AC voltage with a frequency of provisions mentioned before, an AC voltage with a frequency of 50 Hz was applied to the 50 Hz was applied to the test specimen through an electrode, which was increased at a test specimen through an electrode, which was increased at a rate of 2 kV/s from 0. The rate of 2 kV/s from 0. The test was manually stopped when the first spark occurred. test was manually stopped when the ﬁrst spark occurred. The dielectric strengths depicted in Figure 5 for the specimens from each group The dielectric strengths depicted in Figure 5 for the specimens from each group are are listed in Table 2, determined as the average value of all three specimens. It can listed in Table 2, determined as the average value of all three specimens. It can be noted be noted that the manufacturing method had a signiﬁcant inﬂuence on the dielectric that the manufacturing method had a significant influence on the dielectric strengths of strengths of the PCCs. Samples from the S1-PCW/MC and S5-PCWC groups, which were the PCCs. Samples from the S1‐PCW/MC and S5‐PCWC groups, which were manufac‐ manufactured from mould casting, provided the lowest dielectric strengths. Compared tured from mould casting, provided the lowest dielectric strengths. Compared with with specimens from the other groups, the S1-PCW/MC and S5-PCWC specimens had the specimens from the other groups, the S1‐PCW/MC and S5‐PCWC specimens had the highest porosities, as the mixing procedure also introduced a large amount of air content. highest porosities, as the mixing procedure also introduced a large amount of air con‐ Compared with specimens from the S1-PCW/MC and S5-PCWC groups, the specimens tent. Compared with specimens from the S1‐PCW/MC and S5‐PCWC groups, the speci‐ from groups S2-PCW/MC and S6-PCWC, which were manufactured through vacuum mixing, pr mens ovided from ar ound groups a 30% S2‐PCW increase /MCin and dielectric S6‐PCW str Cength. , which The were vacuuming manufactured procedur through e vac‐ reduced the air content in the mixtures, leading to enhanced insulation. uum mixing, provided around a 30% increase in dielectric strength. The vacuuming pro‐ Specimens prepared through centrifugal moulding (S3-PCWC and S7-PCW/MC) cedure reduced the air content in the mixtures, leading to enhanced insulation. provided a twofold dielectric strength compared with their counterparts manufactured via conventional mixing. The centrifugal force removed most of the water and air content, which led to a much more compact mixture. The powder press moulding method provided specimens with the highest dielectric strengths, since it required almost zero added water in mixtures and no air content was introduced during the manufacturing. It is also noted that the muscovite particles did not enhance the insulation properties of the composite material as mentioned before. Hence, further experiments were limited to the PCWC samples only (i.e., groups S1–S4). Infrastructures 2021, 6, 125 8 of 20 Figure 5. Dielectric strength results for different mixing procedures. The dielectric strengths of the PCCs from this study showed values between 6.1 and 16.5 kV/mm, which are within the ranges obtained for common insulating materials such as glass (5–10 kV/mm) and electro-ceramics (8–15 kV/mm) [33]. Other studies also indicated that PCCs incorporating acrylic and vinyl ester have dielectric strengths between 6 and 9 kV/mm [34,35]. Consequently, the PCC mixes developed in this study provided adequate dielectric strengths to be applied in most high-voltage insulating systems. 3.2. Dielectric Loss Dielectric loss of a solid material under the inﬂuence of an alternating electric ﬁeld, represented by the dielectric loss tangent angle tan, deﬁnes the electrical energy dissipated by the motion of dipoles and free charges within the dielectric material as polarisation switches directions. A high value of tan indicates a large amount of electric energy loss, which is converted to thermal energy. The admittance of a dielectric material under an alternating voltage is equivalent to a parallel combination of an energy lossless capacitor C’ and resistance R, which causes the loss of electrical energy as shown in the top left of Figure 6. In this study, the testing arrangements included a bridge circuit, a vacuum container with solid electrodes, a vacuum pump and a high voltage power supply with temperature adjustment [36]. Figure 6. Dielectric loss tangent of the PCC material under different temperatures and voltages and an idealisation of the dielectric medium as the capacitor and resistor (top right panel). The dielectric loss tests in this study focused on the PCC specimens manufactured through a press powder method which provided the highest dielectric strength, as men- tioned in the previous section. The test specimen with the size of Ø50 2 mm was placed between two solid electrodes in the vacuum container. Various levels of alternating volt- ages (U = 500 V, 1000 V and 1500 V) of 50 Hz were applied to the circuit to investigate the inﬂuence of the external electric ﬁeld on the dielectric behaviour of the PCC material. Infrastructures 2021, 6, 125 9 of 20 In addition, the temperature in the container was controlled between 35 C and 90 C to investigate the inﬂuence of the temperature on the dielectric loss of the PCC material. It can be noted from Figure 6 that the applied voltage had no inﬂuence on the dielectric loss tangent of the PCC material, while the higher temperature led to a higher tan. In general, all the PCC specimens manufactured through press powder moulding had a tan lower than 0.12. The introduction of polymer components such as the SA employed in this study greatly reduces the dielectric loss of PCC materials compared with the conventional concrete mixtures, which possess a tan between 0 and 101 [37]. Studies of PCC mixes incorporating various types of insulating ﬁllers, such as recycled rubber and cycloaliphatic epoxy resin, indicated a reduction of tan in the range from 10-2 to 10-1 [38], which is in accordance with the results in this study. The tan factor of the PCC in this study showed similar values compared to other types of insulating coating materials, such as silicon rubber (0.05–0.20) [39]. Conversly, the dielectric loss factor of the PCC showed much higher values than other types of widely used insulating materials such as cross-linked polyethylene (from 10-3 to 10-4) and SF6 gas (less than 10-7) under a frequency of 50 Hz [40–42]. Based on these results, the low cost and high durability of the PCC materials also provide the potential for application of such materials in cable protection covers and high-voltage insulators [34]. 4. Thermal Properties 4.1. Thermal Conductivity Thermal conductivity () (W/mK or W/m C) deﬁnes the amount of heat (W) trans- ferred in 1 s through a material with an area of 1 m , which has a thickness of 1 m and a temperature difference between the two surfaces of 1 K (or C). In this study, the Transient Plane Source (TPS) method [43] was used for measuring the thermal conductivity of the PCC materials. Compared with the other widely used methods such as Transient Hot Strip (THS) [44], which requires samples with speciﬁc geometries [45], the TPS method only needs samples with two parallel smooth surfaces (e.g., disc samples of Ø50 2 mm). As indicated schematically in the top right side panel of Figure 7, a 10–20-m thick pure nickel disk resistor covered on both sides with Kapton foil for protection and electric insulation, whose resistance is in a linear relationship with the temperature, was utilised in the TPS method. This resistor performed simultaneously as the heat source and temperature sensor for the thermal conductivity measurement. During the test, the sensor was sandwiched between two tested insulating specimens. Figure 7. Thermal conductivity test. (a) DRE thermal conductivity parameter test machine. (b) Test results. The automated DRE-IV machines (DRE-IV) employed in this study to measure the thermal conductivity of the PCCs, were in accordance with the requirement of ISO 22007- 2 [46], as shown in Figure 7a. The thermal conductivity of the PCC samples from groups S1–S4 are listed in Table 3 and Figure 7. It can be noted that the of the PCCs was between Infrastructures 2021, 6, 125 10 of 20 1.5 and 2.54 W/mK, depending on the manufacturing method. Studies have shown that the thermal conductivity of concrete depends on its mix components and increases with the increase in density and compactness of the mix [47,48]. The polymer emulsion and antifoaming agent applied in the PCC mixes, in combination with the manufacturing methods, reduced the porosity of the material, reaching = 2.54 W/mK. Table 3. Average electrical, thermal and mechanical properties of the PWC composites tested. Dielectric Thermal Compressive Flexural Method Group Strength SD * Conductivity SD Strength SD Strength SD (kV/mm) (W/mK) (MPa) (MPa) MC S1 6.1 0.5 1.54 0.17 38.2 3.7 7.2 0.6 VM S2 9.4 0.5 1.60 0.07 40.5 4.5 12.0 1.0 CM S3 13.2 1.5 1.65 0.08 65.8 4.9 15.7 1.8 PM S4 16.5 0.6 2.56 0.20 80.7 6.8 20.5 1.8 * The standard deviations (SDs) of the corresponding data. The PCC outperformed the typically used coatings and insulating materials in terms of their thermal conductivity factors. For example, cross-linked polyethylene had = 0.5 W/mK [49], silicone rubber had = 0.2 W/mK [50], SF6 gas had = 0.014 W/mK and insulating oils were around = 0.13 W/mK. As heat dissipation is essential for electrical devices and cables, materials with a high value of are desirable for insulating coatings and mediums [51]. The high heat conductivity factor, combined with the thermal stability test results presented in the following section, indicate that the PCCs provided reliable thermal properties compared with existing insulating materials. 4.2. Thermalgravimetric Analysis The use of thermal analysis, including differential thermal analysis and derivative thermogravimetry, enables a description of the reactions that occur in cementitious compos- ites with an increase in temperature [52]. Typically, hydrated cement pastes include four main components: tricalcium silicate (C3S), dicalcium silicate (C2S), tricalcium aluminate (C3A) and tetracalcium aluminoferrite (C4AF) [53]. In cement-based materials, the main hydration products are calcium silicate hydrate (C-S-H) and portlandite (calcium hydroxide - CH). A number of research studies showed that under an increase in temperature, cement pastes and concretes develop a set of reactions. At temperatures in the range of 30–105 C, the free water and part of the bound water evaporates, whilst at around 120 C, it becomes nonexistent [53]. Furthermore, within 110–180 C, ettringite and gypsum decompose, and water loss from the carboaluminate hydrates occurs [54,55]. In addition to the above, C-S-H and carboaluminate hydrates decompose at tempera- tures above 180 C but below 300 C. The last two stages include dihydroxylation of the portlandite (calcium hydroxide Ca(OH) ) at 450–550 C [56] and decarbonation of calcium carbonate CaCO at 700–900 C [57]. At this stage, depending on the mix proportions and properties of the constituents, 5–30% wt% weight loss can occur. In the particular cases of polymer concretes, the range where weight loss occurs due to polymer pyrolysis is around 350–430 C [1,10]. In terms of the ﬁllers used in this paper, wollastonite is a stable material with low weight loss up to 1000 C, which can only occur due to the presence of small traces of impurities (e.g., calcium carbonate) [58]. To investigate the thermal stability of the PCC mixes, thermogravimetric analyses (TGA) of the selected PCC samples were further carried out in a thermal analyser (Netzsch STA 409PC). The working temperature was established between 30 and 800 C with a heating rate of 20 C/min in a nitrogen environment. As shown in Table 3, TGA was performed with samples from groups S1–S4 (without muscovite granules), which were in accordance with the groups for the thermal conductivity tests. As indicated by the black curves in Figure 8, samples from all the groups lost 5% of their mass at temperatures (T5%) of 170–300 C. The highest thermal stability up to 300 C Infrastructures 2021, 6, x FOR PEER REVIEW 11 of 21 °C, it becomes nonexistent [53]. Furthermore, within 110–180 °C, ettringite and gypsum decompose, and water loss from the carboaluminate hydrates occurs [54,55]. In addition to the above, C‐S‐H and carboaluminate hydrates decompose at temper‐ atures above 180 °C but below 300 °C. The last two stages include dihydroxylation of the portlandite (calcium hydroxide Ca(OH)2) at 450–550 °C [56] and decarbonation of calcium carbonate CaCO3 at 700–900 °C [57]. At this stage, depending on the mix proportions and properties of the constituents, 5–30% wt% weight loss can occur. In the particular cases of polymer concretes, the range where weight loss occurs due to polymer pyrolysis is around 350–430 °C [1,10]. In terms of the fillers used in this paper, wollastonite is a stable material with low weight loss up to 1000 °C, which can only occur due to the presence of small traces of impurities (e.g., calcium carbonate) [58]. To investigate the thermal stability of the PCC mixes, thermogravimetric analyses (TGA) of the selected PCC samples were further carried out in a thermal analyser (Netzsch STA 409PC). The working temperature was established between 30 and 800 °C with a heating rate of 20 °C/min in a nitrogen environment. As shown in Table 3, TGA was per‐ formed with samples from groups S1–S4 (without muscovite granules), which were in Infrastructures 2021, 6, 125 11 of 20 accordance with the groups for the thermal conductivity tests. As indicated by the black curves in Figure 8, samples from all the groups lost 5% of their mass at temperatures (T5%) of 170–300 °C. The highest thermal stability up to 300 °C was shown for S4. This was followed by decomposition of the hydration products. It can was shown for S4. This was followed by decomposition of the hydration products. It be noted that samples from S1, S2 and S3 showed a mass loss of more than 2% at a tem‐ can be noted that samples from S1, S2 and S3 showed a mass loss of more than 2% at a temperatur perature between e between 100 100 andand 200 200 °C. No C. No pyro pyr lysolysis is of the of the polymer polymer or inorg or inor anic gan po icwders powders oc‐ occurr curreded at atthis th istempera temperatur ture; e;hence, hence, the the ma mass ss los loss s was was cause caused d by by th the e vaporisation vaporisation of of the the water water content contentand andother other volatiles. volatilesThis . This was was also also captur capted ured thr through ough the the ﬁrst first derivative derivati ofve the of weight, the weight, repr esented represented by the bygr the ey curves. grey curves. The ﬁrst Thepeak first was peak observed was obsar eround ved aro 100und C, 10 which 0 °C, corresponded to water evaporation, followed by decomposition of the hydration products. which corresponded to water evaporation, followed by decomposition of the hydration A 10 percent (T10%), mass loss was observed at 329–372 C. This was associated with a products. A 10 percent (T10%), mass loss was observed at 329–372 °C. This was associated relatively sudden drop in the weight loss–temperature curves (TGA) and the highest peaks with a relatively sudden drop in the weight loss–temperature curves (TGA) and the high‐ in the derivative curves (DTG). As shown in previous studies, cementitious materials are est peaks in the derivative curves (DTG). As shown in previous studies, cementitious ma‐ relatively stable in the range of 300–400 C without any major component being eliminated terials are relatively stable in the range of 300–400 °C without any major component being from the composite. Implicitly, the mass loss and corresponding peaks on the derivative eliminated from the composite. Implicitly, the mass loss and corresponding peaks on the curves were associated with polymer pyrolysis, which was shown to occur between 350 derivative curves were associated with polymer pyrolysis, which was shown to occur be‐ and 430 C. tween 350 and 430 °C. Infrastructures 2021, 6, x FOR PEER REVIEW 12 of 21 Figure 8. Relationship between remaining mass as a percentage and temperature of samples from (a) group S1, (b) group Figure 8. Relationship between remaining mass as a percentage and temperature of samples from (a) group S1, (b) group S2, (c) group S3 and (d) group S4. S2, (c) group S3 and (d) group S4. In terms of the best thermal performance, samples from groups S1–S3 showed an In terms of the best thermal performance, samples from groups S1–S3 showed an increase in compactness, leading to an increase in thermal stability, which was in accord‐ increase in compactness, leading to an increase in thermal stability, which was in accor- ance with the influence on the dielectric strengths as presented previously. Although the dance with the inﬂuence on the dielectric strengths as presented previously. Although constituent quantities were the same, samples from S4 on the other hand showed signifi‐ the constituent quantities were the same, samples from S4 on the other hand showed cant degradation between 300 and 400 °C. This was attributed to the higher porosity of signiﬁcant degradation between 300 and 400 C. This was attributed to the higher porosity the samples, which allowed a faster thermal transfer and hence implicitly accelerated the of the samples, which allowed a faster thermal transfer and hence implicitly accelerated polymer pyrolysis. Out of the first three mixing procedures, the best performance was the polymer pyrolysis. Out of the ﬁrst three mixing procedures, the best performance was obtained obtained for for sa samples mples from from the the S3 S3 group group,, which which had had aa rrelatively elatively high high T5% T5% and and the the highest highest T10% T10%,, whilst whilst the the remain remaining ing mass mass at at 800 800 °C C was was ar aroun ound d 76 76% % of of the the iin nit itial, ial, wh which ich was was 12 12%, %, 8% and 3% higher than that of S1, S2 and S3, respectively. It is worth noting that weight loss 8% and 3% higher than that of S1, S2 and S3, respectively. It is worth noting that weight loss above ab5% ovewould 5% would occur occ aru ound r aroun 900–1000 d 900–10C00when °C wh the en wollastonite the wollastonite would would suffer sustr ffer uctural struc‐ changes [52,58]. tural changes [52,58]. 5. Microstructure and Mechanical Properties 5.1. Infrared Spectroscopy Analysis Fourier‐transform infrared spectroscopy (FTIR) can be represented by a curve depict‐ ing the relationship between infrared light absorbance or transmittance and wave‐ numbers, which can also by represented in terms of a wavelength or frequency. Through this procedure, a beam of infrared light of a different wavelength is transmitted onto the molecules of the substance, and certain wavelengths of infrared rays are absorbed to form an infrared absorption spectrum of the molecule. The absorbance–wavenumber curve provides an indication of the chemical reactions in the material. The FTIR curves, resulting from an analysis of samples S1–S4 with FTIR equipment (PerkinElmer Spectrum 100 FT‐ IR), are shown in Figure 9. Figure 9. FTIR curves of the samples analysed. Infrastructures 2021, 6, 125 12 of 20 5. Microstructure and Mechanical Properties 5.1. Infrared Spectroscopy Analysis Fourier-transform infrared spectroscopy (FTIR) can be represented by a curve depict- ing the relationship between infrared light absorbance or transmittance and wavenumbers, which can also by represented in terms of a wavelength or frequency. Through this proce- dure, a beam of infrared light of a different wavelength is transmitted onto the molecules of the substance, and certain wavelengths of infrared rays are absorbed to form an infrared absorption spectrum of the molecule. The absorbance–wavenumber curve provides an indication of the chemical reactions in the material. The FTIR curves, resulting from an analysis of samples S1–S4 with FTIR equipment (PerkinElmer Spectrum 100 FT-IR), are shown in Figure 9. Figure 9. FTIR curves of the samples analysed. According to the FTIR curves in Figure 9, the calcium hydroxide peak indicating consumption of portlandite in the mortars was around 3643 cm , which is clearly observed in Figure 9 [59,60]. In the 3300–3500 cm range, a broader peak corresponding to O-H vibrations appeared as a result of hydration due to the presence of water in the air [61]. Three other peaks were visible in the range of 2100–3300 cm , with a maximum near 3000 cm corresponding to C-H, which is characteristic for SAE [62]. An absorption peak was further observed around 1734 cm , corresponding to C=O in the SAE powder- modiﬁed cement pastes [60]. Moreover, the peak around 1620 cm could be attributed to the C-C stretching vibration, which is characteristic for wollastonite [63]. The peak observed at 1431 cm was attributed to the calcium carbonate typical for cementitious materials, which ranges between 1410 and 1510 cm [64]. Additionally, the same peak can also be attributed to wollastonite, which can contains CaO [65]. Typical sulphate absorption bands were generally present in the range of 1100–1200 cm due to S–O stretching vibration [66]. The peak at 1100 cm of the composite could be assigned to Si-O, which is considered ﬁngerprint evidence for the degree of polymerization with the formation of the calcium silica hydrate phase as a result of hydration of the cement [59,66], whilst around 900 cm , this was due to Si-O-Ca stretching present in wollastonite [65]. Through the FTIR analysis, it was shown that quantitative variations in terms of absorbance values occurred as a function of the manufacturing methods of the PCW composites. A clear difference was observed between S4 and the curves of the other three groups (S1–S3). Implicitly, the inherent different porosity and free water in the sample had an impact on the absorption of infrared light. On the other hand, the FTIR results for samples from the S1–S3 groups showed a similar hydration process and reactions within the material with some inﬂuence from the manufacturing method. 5.2. Microstructural Analysis In cementitious materials, the microscopic morphology, pore size, grain boundaries and degree of agglomeration of the original materials and their products determine the Infrastructures 2021, 6, 125 13 of 20 ﬁnal properties of the materials. The particle type and their size distribution and, implicitly, the speciﬁc surface of the mineral admixtures strongly inﬂuence the hydration speed of the process and the demand for water [67]. Cement powders are characterised by very heterogeneous and irregular particle structures with shape factors between one and ﬁve, which are affected by fragmentation and tend to form agglomerates [68]. As mentioned before, the main hydration products in cement pastes are either inner products when C-S-H forms within the boundary of the original particles or outer products when C-S-H and calcium hydroxide form in what was originally water-ﬁlled space [69]. On the other hand, wollastonite has an acicular morphology with crystal lengths at least two times greater than their width, while the shape coefﬁcient of wollastonite (crystal diameter divided by thickness) can be higher than 15 [58]. Wollastonite is regarded as a non-hydraulic binder, since on-carbonation forms a heterogeneous matrix in a similar manner to those of the conventional hydrated cement pastes, resulting in the formation of two main products: calcium carbonate (CaCO ) and amorphous silica gel (SiO ) [6]. 3 2 Materials such as styrene-acrylic emulsions used in the mixes described in this paper have several effects at the microstructural level, such as obstructing the ﬁne pores network inside the hydrates, ﬁlling larger pores and forming membranes that encapsulate cement grains [70]. These characteristics are observed in Figure 10, which depicts scanning electron micro- scope (SEM) images of the selected samples using the manufacturing methods described in Section 2. The microstructural investigations were undertaken using a microscope (JEOL JSM-6510 LV) at two locations of randomly selected samples. Prior to studying the samples, these were spread over a conductive tape and covered in gold ﬁlm. The microstructural examination showed the typical hydration products found in cement pastes for the S1 and S3 manufacturing methods, similar to the characteristics observed in other investi- gations (e.g., [71]). The S2 SEM images seemed to show a less porous structure in which membrane-like substances (hydrates and polymers) bonded the aggregate together, cross- ing the pores like a bridge [10,70]. Ultimately, the results of S4 showed an increased porous media in comparison with the other manufacturing methods (S1–S3) and a non-hydrated cement particle, strengthening the results obtained from the thermogravimetric analysis in Section 4.2. Figure 10. Microstructure images. Infrastructures 2021, 6, 125 14 of 20 5.3. Mechanical Properties In addition to the above material properties, the mechanical strengths of the PCC material were also investigated in this study, including the uniaxial compression and ﬂexural strengths. As introduced previously, cylindrical samples (Ø5 12.5 mm) and prismatic samples (5 5 12.5 mm) were prepared for the two types of tests. These were performed under a displacement ratio of 0.2 mm/min, typical for structures under static loading [72]. The compressive and ﬂexural strengths of the PCC samples from groups S1–S4 are shown in Table 3. It is noted that the manufacturing methods have a signiﬁcant inﬂuence on the strengths of PCC materials. The PCC samples from mould casting showed the lowest compressive and ﬂexural strengths (38.2 MPa and 7.2 MPa, respectively), while specimens from group S2 (VM) had corresponding strengths of 40.5 MPa and 12 MPa, respectively. The enhancement in strength was mainly due to the reduction in the air content in the specimens from the vacuuming procedure. The specimens from group S3 showed twofold the compressive and ﬂexural strengths compared with their counterparts from group S1. Similar to the enhancement in dielectric strength, the specimens prepared from CM were highly compacted, with considerably less water and air content due to the centrifugal force. It can be noted that the highest values of compressive (up to 81 MPa) and ﬂexural strengths (up to 21 MPa) were acquired with PCC materials provided through the powder press moulding method, since it introduced almost zero added water and air content during manufacturing. The SA emulsion provided much higher enhancement to the ﬂexural strength of the PCC materials due to the potential higher tensile strength of the SA emulsion and the overall improvement in the cement hydrate–aggregate bond [2]. Similar to the inﬂuence on the dielectric and thermal properties of the PCC materials, the samples from groups S1–S4 showed an increase in strength due to the increase in the mixture’s compactness and reduction in the water content. It is important to note that the PCC materials provided reliable compressive (up to 81 MPa) and ﬂexural strengths (up to 21 MPa) which were suitable for application as structural materials [73,74]. The high strengths of the PCC materials provide potential applications of such materials in load resistance systems for the protection of telecommunication and electrical power cables. Compared with the direct burial method used in the laying of power cables, the highly durable, low-cost tunnel of PCC material would offer additional protection for the cables from corrosion and other hazards such as ground motions and rodents chewing on them. 6. Environmental Assessments Power transmission network infrastructures, such as electric insulating cables, con- tain substantial amounts of materials that possess large amounts of energy [75]. The most commonly used electric insulating materials are plastics such as polyvinyl chloride (PVC), polyethylene (PE), high-density polyethylene (HDPE) and cross-linked polyethy- lene (XLPE). Synthetic plastic polymers have great potential for providing thermal and electric insulation, high mechanical and ductile properties and water and chemical resis- tance [76,77]. Polyethylene and PVC materials account for over 95% of the waste plastics in the world, and their essential polymerization of ethane (CH2=CH2) indicates that carbon and hydrogen are the backbone of their produced waste [78]. A comparative environmental assessment of the innovative polymer cementitious con- crete system and of the plastic materials used as cable insulating materials was undertaken, as is shown in Table 4. The assessment considered all energy inputs (megajoules) and carbon emissions (kg of carbon dioxide equivalent) from the extraction of raw materials for manufacturing 1 tonne of material to its complete disposal at the end of life. It was shown that the plastic materials possessed signiﬁcantly high embodied energy (around 77,000 MJ/t) which was almost twice the energy density of diesel fuel (i.e., 43,000 MJ/t). The average greenhouse gas (GHG) emissions of the listed plastic materials during their life cycle was around 3300 kg CO eq./t, which is 1.3 times higher than structural steel (i.e., 2500 kg CO eq./t) and 20 times higher than typical concrete materials (i.e., 130 kg CO 2 2 Infrastructures 2021, 6, 125 15 of 20 eq./t) [79]. From this comparison, it is evident that the production of polymer materials led to a much more severe environmental impact compared with conventional construction materials. Table 4. GHG emissions and embodied energy of plastic insulating materials [75,78–83]. Embodied Energy Embodied Carbon Embodied Embodied Carbon/m Material Speciﬁc Gravity (-) 3 3 3 (MJ/t) (kg CO eq./t) Energy/m (MJ/m ) (kg CO eq./m ) 2 2 PVC 77,000 3100 1.40 107,800 4340 PE 76,700 3100 0.96 73,600 2970 XLPE 76,700 4400 0.92 70,560 4050 HDPE 77,000 2700 1.96 150,920 5300 An environmental assessment of the PCC materials investigated in this paper is shown in Table 5, considering the embodied energy and embodied carbon of each component of the mixture. Cement is associated with about 1000 kg CO eq./t embodied carbon and 5500 MJ/t embodied energy, which is the main cause of the environmental load of con- crete [84,85]. Table 5 shows that the styrene-acrylic copolymer had high values of embodied energy and embodied carbon (i.e., 37,400 MJ/t and 1500 kg CO eq./t, respectively) which were comparable to the plastic materials, as shown in Table 4, since SA is also formed through the polymerization of ethane [80,86]. Table 5. GHG emissions and embodied energy of each PCC and its constituents [84–89]. Embodied Embodied Embodied Carbon Components Embodied Amount per Speciﬁc 3 3 Carbon (kg Energy/m of /m of PCC (kg (Group S3) Energy (MJ/t) Sample (g) Gravity (-) 3 3 CO eq./t) PCC (MJ/m ) CO eq./m ) 2 2 Cement 5500 1000 40 3.11 6430 1200 SA 37,400 1500 10 1.08 10,920 438 Wollastonite - - 5 2.75 - - Muscovite - - 0 2.77 - - Superplasticiser 9000 250 0.32 1.08 84 2.3 Sum 17,434 1640 A prototype of a 6-m long PCC tube network (shown in Figure 11) was manufactured and used as a reference for the environmental assessment in this paper. The tube, designed to carry 35 kV, had an outer diameter of 190 mm and an inner diameter of 100 mm. Instead of using traditional plastic materials, a PCC layer was manufactured using centrifugal moulding (CM) to provide insulation and mechanical protection. Electrical characteristic tests indicated that the insulation resistance of the cable tunnel was 4500 MW with a dielectric loss of 4.5%, which met the requirements. Figure 11. Prototype of a 35-kV PCC cable tunnel. Tables 4 and 5 compare the embodied energies and embodied carbon of conventional plastic insulation materials and those of the PCC tube prototype using the respective spe- Infrastructures 2021, 6, 125 16 of 20 ciﬁc gravity of each component. The conventional plastic materials possessed an average 3 3 embodied energy of 100,000 MJ/m and GHG emissions of 4000 kg CO eq./m . On the other hand, the PCC had a total embodied energy and embodied carbon of 17,434 MJ/m and 1640 kg CO eq./m , respectively. Hence, the amount of energy required to manu- facture a cable tunnel with appropriate properties for practical application while being comparable with conventional systems, was reduced by more than 80%, while the embod- ied carbon was halved. 7. Concluding Remarks This paper described an experimental investigation and an environmental assessment of polymer cementitious composites (PCCs) as sustainable coating materials for under- ground power cables and as high-voltage insulators. A detailed account of more than 400 tests on PCC disc samples incorporating liquid styrene and acrylic (SA) monomers (wollastonite and muscovite) was given. Prior to describing the sample manufacturing methods, the selected polymer emulsion and ﬁllers, as well as the mix optimisation proce- dures, were presented in detail. Focus was given to the inﬂuence of the four manufacturing methods on the dielectric and mechanical properties of the PCC. Additional thermogravi- metric, infrared spectroscopy and scanning electron microscope analyses were carried out to offer detailed insight into the microstructure of the PCC. Mix optimisation studies have shown that PCCs with 25% SA emulsion provide the highest dielectric strength from the mixtures investigated, while a further increase in SA causes poor compatibility between constituents. Although muscovite possesses high insulating properties compared with the other materials considered in this paper, the poor bonding between the muscovite powder and SA particle as well as the cement paste reduced the dielectric strength of the PCC. A mix containing 25% SA emulsion, 12.5% wollastonite and no muscovite was shown to offer the best balance in terms of strength and microstructure. The optimised PCC material could reach dielectric strengths up to 16.5 kV/mm, depending on the manufacturing methods, which is above the value of common insulating materials such as glass and electro-ceramics. The dielectric loss factor tan of the PCC materials was lower than 0.12, which is comparable to silicone rubber. Thermal conductivity tests and thermogravimetric analyses also showed that the PCCs had relatively high conductivity ( > 1.3 W/mK) and good thermal stability, respectively. This indicates a superior heat dissipation property in comparison with existing alternatives. As for the above material properties, the mechanical strengths and the microstructure are also greatly inﬂuenced by the manufacturing methods. The compressive and tensile strength of the PCC materials increased with the reduction in water content and porosity and reached maximum values of about 80 MPa and 20 MPa, respectively. Environmental assessments showed that the PCC possessed signiﬁcantly lower embodied energy and embodied carbon than conventional plastic insulating materials. The study presented in the paper indicates that PCC materials have great potential for high-voltage electrical insulating infrastructure. Compared with conventional plastic- based insulating materials such as PE and XLPE, the PCC possesses higher strength, better thermal properties, lower environmental impacts and reasonable electrical properties. Future studies can also focus on optimising the mixing design of the PCC composites to further improve the dielectric strength and reduce the dielectric loss of the material, as well as undertake full-scale tests on members and assessing the performance in service of infrastructure components incorporating the PCC. Author Contributions: Conceptualization, B.X., H.L. and J.C.; methodology, H.L. and D.V.B.; soft- ware, H.L.; validation, B.X., H.L. and D.V.B.; formal analysis, B.X. and D.V.B.; investigation, B.X., D.V.B. and A.Y.E.; resources, H.L. and J.C.; data curation, B.X. and D.V.B.; writing—original draft preparation, B.X. and D.V.B.; writing—review and editing, B.X., D.V.B. and A.Y.E.; supervision, A.Y.E. and J.C.; funding acquisition, H.L. and J.C. All authors have read and agreed to the published version of the manuscript. Infrastructures 2021, 6, 125 17 of 20 Funding: This research was funded by China Electric Power Research Institute, grant number GY71-15-054, GYW17201500088. Data Availability Statement: The data presented in this study are available on request from the corresponding author. Acknowledgments: The ﬁnancial support of China Electric Power Research Institute (GY71-15-054, GYW17201500088) is gratefully acknowledged. 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Infrastructures – Multidisciplinary Digital Publishing Institute

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Keywords: manufacturing; electrical properties; thermal properties; mechanical properties; cementitious composites; environmental assessment

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Performance of Polymer Cementitious Coatings for High-Voltage Electrical Infrastructure

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Performance of Polymer Cementitious Coatings for High-Voltage Electrical Infrastructure

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