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Article Adhesion Performance between Solid Waste and Bitumen Based on Surface Energy Theory 1 1 2 1,2 1, Yu Sun , Haoyu Zhu , Jian Liao , Rong Zhang and Chongwei Huang * Department of Transportation Engineering, University of Shanghai for Science and Technology, Shanghai 200093, China Shanghai Pudong Road and Bridge Group Co., Ltd., Shanghai 202106, China * Correspondence: firstname.lastname@example.org Abstract: Natural aggregates are gradually experiencing resource shortages and price accumula- tions with the rapid development rate of the highway construction industry. Therefore, new mate- rials with social and economic benefits must be identified to replace natural aggregates. Limestone was selected as the representative of natural aggregates, and waste glass and waste ceramics were chosen as the representatives of solid waste in this article. This was undertaken to compare their physical and mechanical properties and the adhesion performance between bitumen and aggregates and to investigate the applicability of solid waste in asphalt pavement. The chemical and mineral compositions of these materials were tested by an X-ray fluorescence spectrometer and an X-ray powder diffractometer. The adhesion performance between bitumen and aggregates was analyzed by establishing adhesion and spalling models based on surface energy theory. Results show that the sizes of glass and ceramics must be small, characteristics which are unsuitable for use in high-grade roads. Glass and ceramics are more adhesive to bitumen than limestone and are suitable for areas with a considerable amount of rainfall and insufficient drainage systems. This article can promote the recycling of glass and ceramics and also provide a research idea for verifying the suitability of other solid waste in asphalt pavement. Keywords: road engineering; solid waste; surface energy theory; adhesion; spalling Citation: Sun, Y.; Zhu, H.; Liao, J.; 1. Introduction Zhang, R.; Huang, C. Adhesion Performance between Solid Waste An increasing number of roads have been constructed in recent years. However, road and Bitumen Based on Surface construction requires an abundant amount of natural aggregate resources, which will Energy Theory. Appl. Sci. 2023, 13, 2. cause a shortage in resources and rising prices . Therefore, finding new materials with https://doi.org/10.3390/app13010002 social and economic benefits to replace natural aggregates is an inevitable trend. Solid waste, such as construction and demolition waste, must be examined first, considering Academic Editor: Luís Picado Santos renewable resources. Construction and demolition waste is solid waste generated during Received: 1 November 2022 the construction or demolition process of buildings. If a long-term neglect attitude toward Revised: 17 December 2022 construction and demolition waste is adopted, then such an attitude will have a poor im- Accepted: 18 December 2022 pact on the urban environment, living conditions, and land quality. Therefore, the effec- Published: 20 December 2022 tive and reasonable utilization of solid waste in road engineering is of considerable im- portance to solve the shortage of natural aggregates. Glass and ceramics can be seen everywhere in daily life. These materials are typically Copyright: © 2022 by the authors. Li- used as utensils in industries and belong to construction and demolition waste, both ac- censee MDPI, Basel, Switzerland. counting for a large proportion of all solid waste. In some developed countries in Europe This article is an open access article and America, waste glass accounts for 4–8% of urban waste generated yearly. In China, distributed under the terms and con- the annual output of waste glass is approximately 10.4 billion kg , and the output of ditions of the Creative Commons At- waste ceramics is approximately 18 billion kg . The development of the construction tribution (CC BY) license (https://cre- industry has made natural resources scarce, and the reasonable disposal of solid waste ativecommons.org/licenses/by/4.0/). Appl. Sci. 2023, 13, 2. https://doi.org/10.3390/app13010002 www.mdpi.com/journal/applsci Appl. Sci. 2023, 13, 2 2 of 20 generated from other industries is a problem faced by countries all over the world. There- fore, scholars have conducted a considerable amount of research from multiple angles and aspects, thus aiming to achieve the goal of solid wastes replacing natural aggregates. Waste glass was used in urban road construction in many places in the United States in the 1960s. The asphalt–glass mixture design has currently been incorporated into the spec- ification by more than 10 states in the United States . In New South Wales, Australia, waste glass has been used in municipal road construction, replacing some natural aggre- gates. The mechanical properties of new aggregates are within the specification require- ments, and their overall permeability is also improved . Six kinds of waste glass were added in asphalt mixture, thus improving the reflective performance of new asphalt pave- ment. However, the dynamic stability, water damage resistance, and low-temperature crack resistance performance were reduced, leading to the conclusion that the recom- mended glass content should not be more than 9% . The adhesion performance between bitumen and glass was found to be poor, which can be improved by adding modified bitumen or stripping agent; the strength and road performance of the mixture met the requirements of the specification . Instead of natural aggregates, many research exper- iments on asphalt mixture using glass have been conducted, and the final research results show that the use of glass in the range of 5% to 15% has the best effect . The effect of glass fiber on the mechanical properties of concrete under different dosages and lengths was examined through a combination of theoretical analysis, laboratory tests, and engi- neering applications. The best length of glass fiber is 12 mm, and the optimal volume rate is 1% . Waste ceramics were first used in building materials by the United States and the Soviet Union in the 1920s. Instead of sand and gravel aggregates, Japan used waste ceramics in cement pavement in the 1960s and 1970s . Ceramics were later applied to asphalt mixtures to study their road performance; results revealed that ceramics can meet the requirements of medium-grade and low-grade roads when their content does not ex- ceed 30% . Ceramics content should also not exceed 40% during coarse aggregate re- placement of SMA-13. The road performance meets the requirements of relevant specifi- cations during such a replacement, and the heat insulation effect can increase from 140% to 360% . Waste ceramics were also used to replace gravel aggregates to design ceramic cement pavement; this replacement demonstrated that the mechanical properties of ce- ramics are better than those of ordinary cement concrete when their content is 50% to 70% . Ceramic particles and powders were used as concrete aggregates and admixtures in different test programs, showing that the compressive strength of recycled mixture grad- ually rose as the number of ceramic particles increased . Surface energy theory is a theory that studies the essential reasons for adhesion be- tween substances. Surface energy theory has been studied for a long time in the field of construction materials. In 1997, surface energy theory was first applied in the field of con- struction materials. The surface energy index was used as a test evaluation index to inves- tigate the water damage and fatigue resistance performances of asphalt pavements . The bitumen–aggregate adhesion model  and spalling model  were established in recent years through theoretical analysis based on surface energy theory. The evaluation results are consistent with the actual situation, proving the effectiveness of the theoretical method based on surface energy theory. The change in adhesion work, between modified bitumen mixed with lime and granite, was analyzed on the basis of surface energy theory to study the effect of lime on the adhesion performance of bitumen fundamentally. The results show that the addition of lime can increase the surface energy of bitumen, raising the adhesion work between bitumen and aggregates and improving the adhesion perfor- mance of bitumen . The spalling phenomenon of the asphalt mixture under different conditions was analyzed on the basis of surface energy theory and pull-off testing, thus providing theoretical and experimental bases for improving the water damage resistance of asphalt mixtures . The steam adsorption method was used to obtain the surface energy parameters of broken gravel samples at 20 °C, and the plug-in method was utilized to obtain the surface energy parameters of bitumen with anti-stripping agents with four Appl. Sci. 2023, 13, 2 3 of 20 different contents (0%, 0.2%, 0.4%, and 0.6%) at 20 °C. Furthermore, the binding energy between broken gravel and bitumen with different contents of anti-stripping agents, and the surface energy evaluation index of water damage resistance, can be calculated . The static drop and plug-in methods were used to measure the surface energy parameters of bitumen and aggregates to obtain the optimal content of anti-stripping agents in an asphalt–granite mixture from a microscopic viewpoint; the index ER was also calculated, which indicates the adhesion performance between bitumen and aggregates. Therefore, the optimal content of the anti-stripping agent is 0.4% . Typically, when scholars study the use of solid waste in road construction, they usu- ally first conduct basic performance tests on solid waste to compare their differences with natural aggregates. Different amounts of solid waste are then added into the asphalt mix- ture, and the performance of the mixture is compared with different solid waste contents to obtain the best content. Finally, surface energy theory is used to establish the bitumen– aggregate adhesion and spalling models, and different comprehensive indicators (ER and CER) are then utilized to judge the bitumen–aggregate adhesion performance and further determine the water damage resistance of the mixture. In addition, an analysis of the ben- efits from solid waste is conducted. Most studies on the basic properties of solid waste only test the relevant indexes of the aggregates according to the relevant requirements. These indexes are mainly macroscopic factors, but the chemical and mineral compositions that cause these changes are disregarded. In addition, the technology of using waste glass and waste ceramics in asphalt mixtures has not been popularized. Simultaneously, studies on using surface energy theory, aiming to analyze the adhesion performance between bi- tumen and glass or asphalt and ceramics, are also remarkably few. This article investigates the use of waste glass and waste ceramics in an asphalt mix- ture based on the above analysis. Tests and analyses of chemical and mineral composi- tions will be conducted for the basic properties of waste glass and waste ceramics. Surface energy theory will also be used to establish the adhesion and spalling models to examine the adhesion performance between bitumen and glass, or bitumen and ceramics, to fur- ther determine their water damage resistance. The final conclusions can provide a theo- retical basis for the future widespread use of glass or ceramic aggregates in asphalt mix- tures. 2. Materials 2.1. Chemical Composition Three different kinds of aggregates of limestone, waste glass, and waste ceramics were used in this study. Their chemical compositions were analyzed by an X-ray fluores- cence spectrometer , which is an analysis method of material chemical composition, performed using primary X-ray photons to shoot atoms in the sample to be tested to gen- erate fluorescence (secondary X-ray). The mass of each aggregate was 5 g, and they were made into powder before the test and each test would be conducted 3 times. In order to avoid contingency during detection, we prepared 15 g for each aggregate and made it into powder, and then divided it into 3 groups, 5 g per group. We tested each aggregate 3 times because each group was sent for detecting. The chemical compositions of the three aggre- gates were detected by a special detector, which is a high-accuracy detector, and its detec- tion data almost have no errors. The final detection data of each aggregate’s chemical com- position were equal to the average value of the sum of the three groups’ detection data, each group of detection data being equal for each aggregate. The test result is expressed by the oxide content corresponding to the contained element in the sample. The analysis results are shown in Table 1. Appl. Sci. 2023, 13, 2 4 of 20 Table 1. Chemical composition of aggregates (%). Limestone Glass Ceramics Na₂O 0.09 12 1.31 6.16 3.31 0.38 MgO 2.18 1.09 16.1 Al₂O₃ SiO₂ 7.88 67.1 63.1 P₂O₅ 0.05 0.02 0.02 SO₃ 0.02 0.54 0.11 Cl - 0.09 - K₂O 0.15 0.35 1.58 60.3 9.23 1.79 CaO TiO₂ 0.14 0.08 0.26 Cr₂O₃ 0.01 - - MnO 0.08 0.01 0.03 Fe₂O₃ 1.23 0.32 0.77 0.01 - - NiO 0.01 - 0.01 CuO ZnO - - 0.26 SrO 0.02 - 0.02 ZrO₂ - 0.03 0.01 Pt 0.06 - 0.06 Total 78.39 94.17 85.81 The table shows the following: limestone mainly contains Ca, Si, Mg, Al, Fe, and Na elements; glass mainly contains Si, Na, Ca, Mg, and Al elements; ceramics mainly contain Si, Al, Ca, K, and Na elements. The main element of limestone is Ca, and the main element of glass and ceramics is Si. 2.2. Mineral Composition The mineral composition of three aggregates was analyzed by X-ray powder diffrac- tometer, which is an analysis method of material mineral composition relying on X-ray scattering from different atoms to interfere with each other and produce strong X-ray dif- fraction in some special directions. In this method, the wavelength set to 1.5406 m, the starting angle set to 10°, the ending angle set to 80°, and the step width set to 0.01°. The mass of each aggregate was 5 g, and they were made into powder before the test and each test would be conducted 3 times. In order to avoid contingency during detection, we pre- pared 15 g for each aggregate and made it into powder, and then divided it into 3 groups, 5 g per group. We analyzed each aggregate 3 times because each group was sent for de- tecting. The chemical composition of the three aggregates was detected by a special detec- tor, which is a high-accuracy detector, and its detection data almost have no error. The final detection data of each aggregate’s chemical composition was equal to the average value of the sum of three groups’ detection data, each group of detection data for each aggregate equal. The test results reflected the double diffraction angle of the sample and the corresponding intensity value. The results data reveal that Jade software was used to perform analysis to obtain the main mineral components contained in aggregates. The X- ray diffraction patterns  are shown in Figures 1–3). Appl. Sci. 2023, 13, 2 5 of 20 Figure 1. XRD results of limestone. Figure 2. XRD results of glass. Figure 3. XRD results of ceramics. Appl. Sci. 2023, 13, 2 6 of 20 Figure 1–3 show that limestone and ceramics have characteristic peaks, proving their crystallinity, while no characteristic peaks are visible for glass, which proves that it is amorphous. The main component of limestone is CaCO3. The shape of the characteristic peak of CaCO3 is most evident, and the degree of crystallinity is highest, under a diffrac- tion angle of 29.35°. Meanwhile, the characteristic peak is nearly invisible, the intensity is poor, and an amorphous state is almost observed under a diffraction angle of 23.09°, which may demonstrate an activity. Similarly, the main component of ceramics is SiO2, which has the highest degree of crystallinity under a diffraction angle of 26.65° and is in an amorphous state at 51.11°. 2.3. Physical Properties The physical properties of the three aggregates were assessed to determine three as- pects: the apparent density, water absorption, and the content of needle and flake-shaped particles. The apparent density of an aggregate has a direct relationship with the porosity of its mixture, and a considerable amount of data prove that the porosity of a mixture containing low-density aggregate will be high. The water absorption of an aggregates lies in its water content under a saturated state. Aggregates with high water absorption have a poor adhesion performance to bitumen under normal circumstances. The research data of NCHRP show that compacting the aggregate on site becomes difficult as the content of needle and flake-shaped particles in the aggregate increases. Aggregates, assessed with the apparent density and water absorption tests, were passed through a 2.36 mm standard sieve during the test process to remove fine aggregates; aggregates in the content of needle and flake-shaped particles test were passed through a 4.75 mm standard sieve. Each phys- ical index test was conducted twice, and the final result was equal to the average value of the sum of data of two tests. Namely, the D-value of the data of two apparent density measurements did not exceed 2%, the D-value of the data of two water absorption meas- urements did not exceed 0.2%, and the D-value of the data of two content of needle and flake-shaped particles measurements did not exceed 20%. The final test results are shown in Figure 4 and the three physical properties’ standards in China are shown in Table 2. Apparent density Content of needle and flake shape particles 3.0 3.0 2.881 Water absorption 72.05 2.656 2.513 2.5 2.5 2.0 2.0 50 40.8 1.5 1.5 1.0 1.0 0.81 0.5 0.5 0.30 0.10 0.0 0.0 Limestone Glass Ceramics Limestone Glass Ceramics Materials Materials Figure 4. Physical properties of aggregates. Table 2. Local standards of physical properties. Physical Properties High-Grade Highway Low-Grade Highway Apparent density ≥2.60 ≥2.45 Water absorption (%) ≤2.0 ≤3.0 Content of needle and flake- ≤12 ≤20 shaped particles (%) -3 Apparent density(g·m ) Water absorption(%) Content of needle and flake shape particles(%) Appl. Sci. 2023, 13, 2 7 of 20 Considering the apparent density, limestone has the highest, followed by ceramics and then glass. Therefore, limestone and ceramics can be used in all grades of roads. Con- sidering water absorption, the order of the three aggregates is the same as above, and all three aggregates can be used in all grades of roads. Considering the content of needle and flake-shaped particles, glass becomes the maximum and limestone becomes the mini- mum; thus, the three aggregates far exceed the specification requirements. Waste glass and waste ceramics are not recommended for application in highways and first-grade highways during actual use based on the three indicators. Simultaneously, waste glass and waste ceramics should be processed by suitable crushing machinery to ensure that sufficiently small particle sizes are achieved. Excessively large particle sizes will markedly increase the content of needle and flake-shaped particles. 2.4. Mechanical Properties The mechanical properties of the three aggregates were tested in terms of two aspects of the crushing and wearing values. The crushing value of the aggregate represents its capability to resist external loads, and the wearing value represents its capability to resist being worn down. A grain size of each aggregate was selected between 9.5 and 13.2 mm in the crushing test, and each group of samples was approximately 3000 g. Meanwhile, the grain size of aggregates in the abrasion test was between 4.75 and 9.5 mm and between 9.5 and 16.0 mm, each group of samples weighing approximately 2500 g. The screening status of each material in the preparation stage of the test is shown in Figure 5, and the machines for the two tests are shown in Figure 6. The crushing value is equal to the ratio of the mass of fine aggregates, passing the 2.36 mm standard sieve from the sample after the test to the initial mass of the sample. The wearing value is equal to the ratio of the mass of fine aggregates, passing the 1.70 mm standard sieve from the sample after test to the initial mass of the sample. The crushing value and the wearing value are expressed by Equation (1) and Equation (2), respectively. The test results are shown in Figure 7, and the two mechanical properties’ standards in China are shown in Table 3. (a) Limestone screening (b) Glass screening (c) Ceramics screening Figure 5. Raw material preparation. Appl. Sci. 2023, 13, 2 8 of 20 (a) Crushing test machine (b) Abrasion test machine Figure 6. Test machines. Q =×100% (1) Q m m where: : Crushing value of aggregate, %; : Initial mass of the sample, g; : Mass a 0 a of fine aggregates passing the 2.36 mm standard sieve from the sample after test, g. Q =×100% (2) where: Q : Wearing value of aggregate, %; m : Initial mass of the sample, g; m : Mass b 0 a of fine aggregates passing the 1.70 mm standard sieve from the sample after test, g. Crushing value 35 33.9 35 Wearing value 31.8 31.1 30 30 25.4 25 25 23.55 21.8 20 20 15 15 10 10 5 5 0 0 Limestone Glass Ceramics Materials Figure 7. Mechanical properties of aggregates. Table 3. Local standards of mechanical properties. Mechanical Properties High-Grade Highway Low-Grade Highway Crushing value ≤26 ≤30 Wearing value ≤28 ≤35 Considering crushing value, glass is the highest, followed by ceramics and then lime- stone; limestone and ceramics can be used in all grades of roads. Considering wearing value, the order of the three aggregates is similar to that of the crushing value, but only limestone can be used in all grades of roads. Combining the two indicators, the particle Crushing value(%) Wearing value(%) Appl. Sci. 2023, 13, 2 9 of 20 size of waste glass and waste ceramics should be treated as little as possible in the actual usage, and using these materials in high-grade highways is not recommended. 3. Methodology 3.1. Surface Energy Theory Substances exist in three states: solid, liquid, and gas. Substances in different states will form the following five interfaces upon contact with each other: solid–solid, solid– liquid, solid–gas, liquid–liquid, and liquid–gas. The internal molecules of an existing sub- stance in a solid or liquid state are subjected to forces of equal magnitude and opposite directions in all directions, finally achieving a state of equilibrium. However, considering molecules on the surface of the substance, the resultant force is not zero because the gas molecules acting on them are far weaker than the attraction of the internal molecules to them. That is, the forces are not equal in all directions, and a tendency to move inside remains. Thus, the molecules on the surface have higher potential energy than that inside. This potential energy is called surface energy, which is represented by the Greek letter γ , and its unit is N/m or J/m . The surface energy of a substance comprises two parts: van der Waals force and Lewis acid–base force , which are expressed by Equation (3). LW AB γγ=+γ (3) mm m LW −2 where: γ : Surface energy of the substance, mJ·m ; γ : Van der Waals force of sub- m m AB −2 −2 stance, mJ·m ; γ : Lewis acid–base force of the substance, mJ·m . The Lewis acid–base force of the substance also comprises two parts, namely Lewis acidic force and Lewis basic force, which are expressed by Equation (4). AB +− (4) γγ =2 γ mmm + − −2 where: γ : Lewis acid force of substance, mJ·m ; γ : Lewis basic force of substance, m m −2 mJ·m . Similarly, the surface energy of the interface formed by any two substances also com- prises van der Waals force and Lewis acid–base force, as expressed by Equation (5). The van der Waals force and Lewis acid–base force are, respectively, expressed by Equations (6) and (7). Equation (8) is the final expression of the surface energy. LW AB γγ=+γ pq pq pq (5) LW LW LW (6) γγ=−γ pq ( p q ) AB ++ − − (7) γγ=− 2 γ γ −γ pq ( p q ) ( p q ) LW LW LW LW +− +− − + +− γ =γ +γ −2γ γ +222 γγγ−−+ γγγ 2γγ (8) pq p q p q pp p q p q qq LW −2 where: γ : Surface energy of the interface between two substances, mJ·m ; γ : Van pq pq AB −2 der Waals force of the interface between two substances, mJ·m ; γ : Lewis acid–base pq −2 force of the interface between two substances, mJ·m . 3.2. Adhesion Performance between Bitumen and Aggregates The original liquid asphalt and solid surfaces of the aggregate will disappear to form a new solid–liquid interface when the liquid bitumen and the aggregate contact each other Appl. Sci. 2023, 13, 2 10 of 20 and the adhesion reaction occurs. This process will result in a reduction in the Gibbs free energy . The Gibbs free energy is the energy consumed during the entire adhesion process, which is the D-value between the final and initial free energy of the system, as expressed by ΔG . A large adhesion work, W strengthens the solid and liquid bond. Gibbs free energy and adhesion work are equal in value and opposite in sign (Equation (9). The Gibbs free energy of the system is expressed by Equations (10)–(12), and the ad- hesion work is expressed by Equation (13). WG =−Δ (9) as as Δ= G γγ− −γ (10) as as a s LW LW LW LW +− + − − + +− ΔG= γ +γ −22 γ γ + γγ−−+ 2 γγ 2 γ γ 2 γ γ as ( a s a s a a a s a s s s (11) LW +− LW +− −+γγ22 γ −γ + γγ ) ) (aaa (s ss LW LW +− − + Δ= G −22 γγ − γ γ −2 γ γ (12) as a s a s a s LW LW +− − + W =2+ γγ 2 γ γ+2 γ γ (13) as a s a s a s −2 where: W : Adhesion work between bitumen and aggregate, mJ·m ; ΔG : Gibbs free as as −2 energy change value during the adhesion process of bitumen and aggregate, mJ·m . 3.3. Spalling between Bitumen and Aggregate The separation of the bitumen and aggregate from the bonded state is the spalling. The analysis of the chemical and mineral compositions of glass and ceramics, based on surface potential theory (Figure 8) in Section 2.1, revealed that the most abundant element is Si. This provides the surface of the aggregate with a weak negative charge, which is connected to H in the water by chemical adsorption in the form of hydrogen bonds, far exceeding the physical adsorption between the bitumen and the aggregate. Therefore, the water can separate the aggregate from the bitumen and finally realize spalling when the aggregate meets the water. Figure 8. Schematic of bitumen–aggregate spalling. The Gibbs free energy of the spalling process of bitumen and aggregate is expressed by Equations (3), (14) and (16). A substantial spalling work facilitates the easy removal of the bitumen from the surface of the aggregate. The spalling work is expressed by Equation (17). Δ= G γγ+ −γ −γ −2γ (14) asw aw sw as s w Appl. Sci. 2023, 13, 2 11 of 20 LW LW LW LW +− + − − + +− Δ= G γ +γ −2 γ γ +222 γγ − γγ − γγ+2 γγ asw ( a w a w a a a w a w w w ) LW LW LW LW +− +− − + +− ++ γ γ −2+ γ γ 2 γγ −2 γγ −2 γγ+2 γγ ( s ws w ss sw sw ww ) (15) LW LW LW LW +− +− − + +− −+ γ γ −2+ γ γ 2 γγ −2 γγ −2 γ γ+2 γγ ( a s a s aa a s as s s ) LW +− −2+γγ2 γ (www ) LW LW + − −+ LW LW + − −+ ΔG =−2 γγ −−− 2 γ γ 22 γ γ γγ −−22 γ γ γ γ asw a w a w a w s w s w s w (16) LW LW +− − + ++22 γγ γ γ+2 γ γ as a s a w WG =−Δ asw asw LW LW + − −+ LW LW + − −+ =22 γ γ + γγ++2 γ γ 2 γ γ ++2 γγ 2 γ γ aw a w a w s w s w s w (17) LW LW +− − + −−22 γγ γ γ−2 γ γ as a s a s −2 where: W : Spalling work of bitumen and aggregate, mJ·m ; ΔG : Gibbs free energy asw asw −2 change value during the spalling process of bitumen and aggregate, mJ·m . 4. Experimental Program 4.1. Scheme The measurement method of the surface energy parameters of solid materials aims to use liquid reagents with known surface energy parameters to wet the solid to be tested and calculate the contact angle between the gas–liquid and solid–liquid interfaces. Equa- tion (19) is obtained in accordance with the Young–Dupre Equation, which is expressed by Equation (18). The contact angle measured in the experiment is substituted into Equa- tion (19), and the obtained equation sets will be solved to identify the surface energy pa- rameters of solid materials. γ⋅= cosθ γγ− (18) ls 1 ls LW LW LW LW +−+− γ +− γ 22 γ γ + γγ −2 γγ ls l s l l l s LW +− γθ⋅= cos γ +2 γγ − ls 1 ( ss ) −+ + − −+22 γγ γ γ ls s s (19) 1c + osθ () 1 LW LW +− − + γγ=+ γ γγ+ γγ ll s l s l s where: θ : Number of degrees of contact angles between the gas–liquid and solid–liquid interfaces, °; l : Liquids with known surface energy parameters; s : Solid to be measured. Appl. Sci. 2023, 13, 2 12 of 20 The 70# bitumen and the SBS-modified bitumen were selected in this article as the bitumen research objects to obtain the surface energy parameters of bitumen and aggre- gates, and the hanging plate method was used to measure the contact angles. Limestone, glass, and ceramics were selected as the aggregate research objects, and the contact angles were measured by the sessile drop method . The two measurement methods should be both tested 5 times when measuring the contact angle of each solid and each liquid, and the final result is equal to the average value of the sum of the data of five tests, and the D-values of the data of five measurements must not exceed 30%. The hanging plate method, which is suitable for measuring flake-shaped solids, is based on the principle that the resultant force of the substance is zero when balanced. The substance force is then analyzed, and the value of the contact angle is obtained through equation sets. The sessile drop method is a measurement method, based on optical image analysis, which can directly measure the contact angle of solids that cannot be made into flake, as is shown in Figure 9. The machine used for this test in this manuscript is the SL200B con- tact angle measuring instrument invented by the American Kono Group. The image anal- ysis method used in the test is the tangent line method, as is shown in Figure 10. The test step is to drop the liquid, meeting the specified volume, onto the solid surface, and the software system will automatically generate a virtual circle. By adjusting the radius, hor- izontal line and position of the virtual circle, the virtual circle and the liquid drop will be overlapped to the maximum extent, and then the software will automatically calculate the contact angle. (a) θ≤° 90 (b) 90°≤ θ ≤180° Figure 9. Schematic diagram of contact angle measurement by sessile drop method. (a) Optical image (b) Geometric schematic diagram Figure 10. Tangent line method. Ethylene glycol, formamide, and distilled water were selected as the liquid reagents, and their surface energy parameters are shown in Table 4. Each test must be performed five times to obtain the average value. Appl. Sci. 2023, 13, 2 13 of 20 Table 4. Surface energy parameters of liquid reagents. −2 Surface Energy Parameters/(mJ·m ) Reagents LW AB + − γ γ γ γ γ l l l l l Ethylene 48.30 29.30 19.00 1.92 47.00 glycol Formamide 58.20 39.50 18.70 2.28 39.60 Distilled 72.80 21.80 51.00 25.50 25.50 water 4.2. Results Each bitumen or aggregate was tested with the same reagent five times, and the con- tact angles and average values obtained are, respectively, shown in Tables 5 and 6. Table 5. Bitumen contact angle results (°). 70# Bitumen SBS-Modified Bitumen 1 83.70 80.50 2 83.50 80.10 3 83.40 80.20 Ethylene glycol 4 83.20 81.20 5 83.70 80.60 Average value 83.50 80.52 1 89.70 87.90 2 89.80 86.90 3 89.40 87.50 Formamide 4 90.30 87.50 5 89.40 87.70 Average value 89.72 87.50 1 94.60 93.50 2 94.50 93.70 3 94.70 93.50 Distilled water 4 94.90 93.60 5 95.00 93.20 Average value 94.74 93.50 Table 6. Aggregate contact angle results (°). Limestone Glass Ceramics 1 30.80 29.10 47.50 2 31.20 31.90 45.40 3 30.50 26.40 45.50 Ethylene glycol 4 30.80 29.50 46.50 5 30.50 29.00 45.80 Average value 30.76 29.18 46.14 1 38.00 25.00 31.90 2 39.20 20.50 29.30 3 37.80 23.00 31.90 Formamide 4 38.50 22.40 29.20 5 37.60 22.80 30.50 Average value 38.22 22.74 30.56 Distilled water 1 40.70 50.10 63.90 Appl. Sci. 2023, 13, 2 14 of 20 2 42.60 56.20 50.50 3 41.50 53.00 36.50 4 40.90 56.20 55.80 5 41.00 53.80 51.80 Average value 41.34 53.86 51.70 5. Discussion 5.1. Surface Energy Substituting the data in Tables 5 and 6 into Equation (19), the surface energy param- eters of the bitumen are obtained as shown in Table 7, and the surface energy parameters of the aggregates are shown in Table 8. Table 7. Bitumen surface energy parameters. −2 Surface Energy Parameters/(mJ·m ) Bitumen LW AB − + γ γ γ γ a a a a 70# bitumen 14.51 8.69 5.82 8.26 1.03 SBS-modified 15.51 8.66 6.85 8.08 1.45 bitumen Table 7 reveals that van der Waals force is larger than Lewis acid–base force in the surface energy of the same substance. The Lewis acid force of a substance is larger than its Lewis basic force. Therefore, the Lewis acid–base force of a substance mainly comprises Lewis acid force. Table 8. Aggregate surface energy parameters. −2 Surface Energy Parameters/(mJ·m ) Aggregates LW AB − + γ γ γ γ s s s s Limestone 52.58 49.03 3.56 41.26 0.08 Glass 123.44 103.25 20.19 19.12 5.33 Ceramics 181.57 136.83 44.74 26.83 18.66 Table 8 shows the surface energy parameters of the substance, and reports the same conclusions as those shown in Table 7. In addition, the van der Waals force of a substance is larger than its Lewis acid–base force in this table. Therefore, the surface energy of a substance mainly comprises van der Waals force. 5.2. Adhesion Work Substituting the result data in Tables 7 and 8 into Equation (13), the adhesion work between various bitumen and aggregates can be calculated. The calculation results are shown in Table 9 and Figure 11. Table 9. Adhesion work between bitumen and aggregates. −2 Aggregates Bitumen Adhesion Work/(mJ·m ) 70# bitumen 55.89 Limestone SBS-modified bitumen 58.26 70# bitumen 82.04 Glass SBS-modified bitumen 83.47 70# bitumen 104.29 Ceramics SBS-modified bitumen 105.89 Appl. Sci. 2023, 13, 2 15 of 20 70# bitumen SBS modified bitumen 105.89 104.29 83.47 82.04 58.26 55.89 Limestone Glass Ceramics Materials Figure 11. Adhesion work between bitumen and aggregates. Table 9 and Figure 11 indicate that the adhesion work of ceramics is the highest, fol- lowed by that of glass, and then finally that of limestone, for the same kind of bitumen. A large adhesion work strengthens the adhesion performance between bitumen and aggre- gate. Therefore, ceramics have the strongest bond with bitumen. The adhesion work val- ues of different bitumen–aggregate combinations are remarkably close, indicating that the type of bitumen used has minimal effects on the bitumen–aggregate adhesion process, which mainly depends on the type of aggregate used. 5.3. Spalling Work Substituting the result data in Tables 6 and 7 into Equation (17), the spalling work between various aggregates and bitumen can be calculated. The calculation results are shown in Table 10 and Figure 12. Table 10. Spalling work between bitumen and aggregates. −2 Aggregates Bitumen Spalling work/(mJ·m ) 70# bitumen 143.95 Limestone SBS-modified bitumen 143.14 70# bitumen 147.10 Glass SBS-modified bitumen 147.25 70# bitumen 167.65 Ceramics SBS-modified bitumen 167.63 70# bitumen SBS modified bitumen 167.65 167.63 147.1 147.25 143.95 143.14 Limestone Glass Ceramics Materials Figure 12. Spalling work between bitumen and aggregates. -2 -2 Spalling work（mJ·m ) Adhesion work（mJ·m ) Appl. Sci. 2023, 13, 2 16 of 20 Table 10 and Figure 12 conclude that the spalling work of ceramics is the highest, followed by that of glass and finally that of limestone for the same kind of bitumen. A large spalling work facilitates the easy separation of the aggregate from the bitumen. Therefore, bitumen easily falls off when the ceramic asphalt mixture encounters water. The spalling work values of different bitumen–aggregate combinations are also remarka- bly close, indicating that the type of bitumen used has minimal effects on the bitumen– aggregate spalling process, which mainly depends on the type of aggregate used. 5.4. Overview The adhesion and spalling works between the same bitumen and varying aggregates are different based on the above calculation results of these works. Ceramics are the easi- est to bond with bitumen, and most easily to cause bitumen fall off upon water contact. Therefore, explaining which aggregate has a good overall adhesion performance is diffi- cult considering only the adhesion and spalling works. The adhesion and spalling works of asphalt mixture must be considered together for further analysis and evaluation, intro- ducing the energy parameter ER, as expressed by Equation (20). A larger ER value gener- ally contributes to the superior water damage resistance of this type of asphalt mixture. as ER = (20) asw Figure 13 would be obtained in accordance with the data in Tables 9 and 10. 0.7 70# bitumen 0.63 SBS modified bitumen 0.62 0.6 0.57 0.56 0.5 0.41 0.39 0.4 0.3 0.2 0.1 0.0 Limestone Glass Ceramics Materials Figure 13. ER value between bitumen and aggregates. Figure 13 shows that the ER values of various bitumen–aggregate combinations are different: their ceramic values are the highest, followed by those of glass and finally lime- stone. A larger ER value contributes to the superior water damage resistance of the mix- ture, indicating that the adhesion performance of asphalt–ceramics mixture is the best, followed by that of asphalt–glass, whereas that of asphalt–limestone mixture is the worst. Therefore, waste glass and ceramics are suitable for areas with a considerable amount of rainfall and insufficient drainage systems. 5.5. Laboratory Test Verification The boiling test  was used to study the adhesion performance between three ag- gregates and bitumen, and the conclusion indicates that the adhesion performance be- tween glass and ceramic aggregates and bitumen is actually better than limestone aggre- gate. The first step of a boiling test is to lift the heated aggregate with thread and immerse it in hot bitumen to form a bitumen membrane on the whole surface of aggregate, then use thread to keep the cooled aggregate suspended in the boiling water, put the aggregate ER Appl. Sci. 2023, 13, 2 17 of 20 into the normal temperature water after 3 min and observe the spalling degree of the bi- tumen membrane on its surface. Test pictures of three aggregates are shown in Figures 14–16, and the adhesion performance grade between bitumen and aggregate is deter- mined according to Table 11. (a) Before water boiling (b) During water boiling (c) After water boiling Figure 14. Test pictures of limestone aggregate. (a) Before water boiling (b) During water boiling (c) After water boiling Figure 15. Test pictures of glass aggregate. (a) Before water boiling (b) During water boiling (c) After water boiling Figure 16. Test pictures of ceramic aggregate. Appl. Sci. 2023, 13, 2 18 of 20 Table 11. Adhesion performance grade between bitumen and aggregate. Spalling Situation of Bitumen Membrane on Aadhesion Performance Grade Aggregate Surface The percentage of spalling area is close to 0% 5 The percentage of spalling area is less than 10% 4 The percentage of spalling area is less than 30% 3 The percentage of spalling area is more than 30% 2 All bitumen floats on the water 1 The evaluation results of adhesion performance grade between various aggregates and bitumen are shown in Table 12. Table 12. Adhesion performance grade between various aggregates and bitumen. Aggregate Limestone Glass Ceramics Adhesion performance grade 3 5 5 However, the evaluation results of the adhesion performance grade between bitu- men and aggregate, according to Table 11, are easily affected by subjective factors. Due to the irregular shape of aggregate, it is easy to generate large errors when estimating the spalling area, and it is difficult to distinguish the adhesion performance differences be- tween various aggregates and bitumen. Therefore, the test will also weigh the mass of the bitumen on the aggregate surface before and after water boiling, and calculate the bitu- men spalling rate during the water boiling process (the mass of spalling bitumen/the ini- tial mass of bitumen × 100%) to evaluate the adhesion performance between various ag- gregates and bitumen accurately. The calculation results of the bitumen spalling rate dur- ing the water boiling process are shown in Table 13. Table 13. Bitumen spalling rate during the water boiling process. Aggregate Limestone Glass Ceramics Bitumen spalling rate 30% 15% 10% Tables 12 and 13 both show that the adhesion performance between glass or ceramics and bitumen is better than that between limestone and bitumen, which verifies the above conclusion based on surface energy theory. Other relevant tests with municipal solid waste incineration (MSWI) and bottom ash aggregate (abbreviation BAA), which contains plenty of glass and ceramics, were also conducted. The conclusion indicates that the water damage resistance of asphalt mixture added with BAA is better than that of ordinary asphalt mixture . Because BAA is a mixture mainly composed of glass and ceramics, the conclusion can also prove that add- ing glass aggregate or ceramic aggregate to an asphalt mixture can improve its water dam- age resistance, verifying the above conclusion made on the basis of surface energy theory. 6. Conclusions Solid waste was studied and compared with natural aggregate limestone in this arti- cle. The study was performed with consideration given to the following five aspects: chemical composition, mineral composition, physical properties, mechanical properties of aggregates, as well as the adhesion performance between bitumen and aggregate. The main research results are as follows. The apparent density and water absorption demonstrate the same order: limestone showed the highest performance, followed by ceramic and then glass, thereby meeting the specification requirements. The content of needle and flake-shaped particles shows Appl. Sci. 2023, 13, 2 19 of 20 that the glass has the highest level, followed by ceramics and then limestone, demonstrat- ing that neither meets the specification requirements. The crushing and abrasion values are in the same order with the content of needle and flake-shaped particles, which meet the specification requirements. The particle size of waste glass and waste ceramics should be treated as small as possible in the actual usage, and their use in highways and first- grade highways is not recommended. The type of bitumen used has minimal effects on the adhesion performance of asphalt mixtures, which is mainly dependent on the type of aggregate. The ER value of various bitumen–aggregate combinations shows that ceramics have the highest performances, fol- lowed by glass and then limestone. Therefore, the adhesion performance of the combina- tion of bitumen–ceramics is the best. Consequently, waste glass and waste ceramics are suitable for areas with a considerable amount of rainfall and insufficient drainage systems. This article also has some limitations. Only two types of bitumen (70# bitumen and SBS-modified bitumen) are used to study the effect of type of bitumen on the adhesion performance of asphalt mixture. The conclusion lacks enough persuasiveness due to the few types of bitumen examined. Only glass and ceramics are selected as solid wastes, and they have limited contribu- tion to the application of solid waste in asphalt mixtures. Author Contributions: Conceptualization, Y.S.; methodology, Y.S.; formal analysis, H.Z.; investiga- tion, J.L. and R.Z.; resources, R.Z.; data curation, J.L.; writing—original draft preparation, H.Z.; writ- ing—review and editing, C.H.; supervision, C.H.; funding acquisition, Y.S. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by [Shanghai Sailing Program] grant number [20YF1431900] And the APC was funded by [Science and Technology Commission of Shanghai Municipality]. Data Availability Statement: Not applicable. Acknowledgments: The author is grateful to the Shanghai Sailing Program (20YF1431900) for sup- porting this study. The author would also like to thank the tutors and students for their assistance in preparing the paper. Furthermore, the author is grateful to fellow authors whose studies have been a good starting point for the current research and are used as references in this paper. 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Applied Sciences – Multidisciplinary Digital Publishing Institute
Published: Dec 20, 2022
Keywords: road engineering; solid waste; surface energy theory; adhesion; spalling
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