Performance Analysis of Industrial-Waste-Based Artificial Aggregates: CO₂ Uptake and Applications in Bituminous Pavement

Abstract

In order to raise the utilization rate of industrial waste and mitigate issues involving land resource occupation and environmental damage, in this study, industrial-waste-based artificial aggregates (IWAAs) were fabricated using steel slag powders, fly ash, and cement. They were processed under accelerated carbonation and were utilized in a bitumen mixture. During the experiment, the micromorphology, internal structure, and phase composition of IWAAs before and after accelerated carbonation were characterized using X-ray phase analysis, thermal analysis, and scanning electron microscopy (SEM); concurrently, the possibility of IWAAs being used as a partial substitute for natural aggregate to prepare bituminous mixture was qualitatively and quantitatively analyzed based on Marshall’s design procedure in combination with road performance tests and microcosmic analyses. The results indicated that the presence of carbonate crystals brought about by accelerated carbonation was the main phase composition inside the IWAAs, enhancing the microstructure densification and diminishing the immersion expansion and crushing values; this is due to the depletion of the interior Ca-based (e.g., f-CaO and Portlandite) and Mg-based (e.g., periclase and brucite) compounds together with the formation of cement hydration products. Additionally, the 12 h carbonation time obtained the optimum CO₂-sequestration efficiency on the premise of satisfying the performance standard. The expansion rate and crushing value of the IWAAs decreased by 82.21% and 41.58%, respectively, whilst the anti-rutting properties, the moisture damage resistance, and the skid resistance rose by 31.92%, 5.59%, and 10.00%, respectively, in the IWAAs–bituminous mixture. This study lays a foundation for research on the CO₂ sequestration and resource utilization of industrial wastes in bitumen mixtures.

1. Introduction

As the byproduct of manufacturing processes, a large amount of industrial waste (e.g., demolition waste, slags, fly ash, etc.) is produced during the progressive expansion of industrial production activities globally. The long-term disposal of industrial waste occupies significant land resources and further impacts surrounding ecologies and the health of humankind due to the existence of detrimental substances (e.g., solvents, heavy metals, and chemicals) penetrating into the land and groundwater. Meanwhile, with the rapid expansion of the transportation industries, the construction of highway transportation infrastructure has consumed large amounts of high-quality resources [1]. Hence, the comprehensive management and utilization of industrial waste would be an effective approach to resolving problems of resource shortages and ecological degradation [2]. Notably, the production of artificial aggregates (e.g., cementitious, sintered, and alkali-activated types) derived from industrial solid waste utilizing disc granulation have been reported as alternatives for natural aggregates and have gained widespread research attention and applications [3,4]; this involves manufacturing processes such as cold-bonding or sintering techniques [5]. Liu et al. [6] explored the feasibility and performance of artificial aggregates from waste cement slurry and brick masonry powder; they found that extending the sedimentation time would reduce the mineral reactivity and dispersion level of internal waste slurry, exerting an adverse effect on the performance of the artificial aggregates. Fan et al. [7] developed artificial aggregates with bottom ash as the main constituent with cement and calcined clay as auxiliary cementing materials, utilizing the cold-bonding granulation process. They ascertained that the aggregates possessed abundant interior hydration products and dense microstructures; the compression strength of the aggregates approached 2.5 MPa, which could improve the mechanical strength and internal aperture distribution of concrete while lowering resource consumption and CO₂ emissions. Jiang et al. [8] manufactured IWAAs with basic oxygen furnace slag (BOFS) and steel slag and processed the IWAAs pellets in a curing chamber with a high CO₂ concentration. They found that the BOFS–IWAAs mixture achieved a 2.2 times higher anti-crushing ability than that of the atmosphere, owing to the generation of carbonate precipitates.

Steel slag, as a major industrial waste, is regarded as an optimal choice for use and has thus been extensively utilized in the field of construction projects, especially for road engineering, because it has great potential for application in pavement materials; moreover, it ensures mitigation of greenhouse gases under CO₂ sequestration due to its good adhesiveness to bitumen, its abrasiveness, and its rich internal mineral phase (e.g., CaO and MgO) [9,10]. These do not merely diminish the consumption of mineral resources and improve the utilization rate of solid waste, but they also contribute to achieving the “Emission Peak” and “Carbon Neutrality” goals. Many scholars have expressed great enthusiasm for applied technologies utilizing industrial waste in pavement engineering. Wan J. et al. [11] proposed that steel slag should be incorporated as an auxiliary material in functional ultrathin friction course SMA-5 for induced-heat deicing. Shen et al. [12] simulated a skid resistance degradation process for tunnel pavements as a function of dosage and applied loads; they proposed that the surface texture, pore gradation, and micromorphology are the crucial factors influencing the stabilization rate of the skid resistance. However, the high water absorption and complex porous morphologies of steel slag were found to directly impact the water resistance of bituminous mixtures due to the presence of f-CaO content with high hydration expansion [13,14]. Lyu and Guo et al. [15,16] analyzed the evolution of the deterioration of water resistance, utilizing varying dosages of steel slag under dry–wet or freezing–thawing regimes. The results indicated that the substitution of 50% aggregates could be used to obtain an optimal enhancement effect, due to the physical anchoring and chemical adhesion effects. Overall, the incorporation of steel slag with complex porous structures could substantially improve the road performance, utilizing bituminous pavement, with the exception of the skid resistance and water resistance characteristics.

Currently, related research has involved analyses of the properties of artificial aggregates using steel slag as the raw material, motivated by ensuring that we take full advantage of its good performance. The chemical and mineral components of steel slags mainly contain active phases, such as the C₃S–C₂S–RO phase and the f-CaO and f-MgO phases [17,18]; the calcium-containing phases (e.g., free-CaO, β -C₂S, γ-C₂S, C₄AF, and Portlandite) have been found to possess high carbonation reactivity under CO₂-enriched conditions to produce calcium carbonate (CaCO₃) with polymorphism of calcite, aragonite, and vaterite [19]. Hence, steel-slag–artificial aggregates possess high potential for CO₂ sequestration, which not only favors its enhanced properties as engineering materials through carbonation but also mitigates the emissions of greenhouse gas. Zhang et al. [20] prepared artificial aggregates with regular morphology via compression-molding slag powder and proposed that a polycarboxylic (PCE) admixture could improve the degree of carbonation and compaction. Xu et al. [21] analyzed the practicability and effect of biochar on the carbonation efficiency and performance of steel-slag–artificial aggregates, since biochar was supplied for the temporary deposition of CO₂. Liu et al. [22] developed artificial aggregates using steel slag, furnace slag, and biochar and pointed out that the CO₂ adsorptive ability of biochar under low concentrations was conducive to the gaseous storage, diffusion, and carbonation inside the matrix, further stimulating the long-term hydration and strength formation. Mo et al. [23] investigated the carbonated artificial aggregates fabricated by steel slag-fly ash mixture via granulation process and CO₂-sequestration and the strength variation of concrete using carbonated artificial aggregates, confirming that the 28 d compression strength was improved within 36.2~45.5 MPa because its interior relative humidity regulated the autogenous shrinkage of concrete within limits. Furthermore, for further improvement in the carbonated efficiency of slag-based IWAAs, chitosan was used as a catalyst in the carbonation process considering that its chelation with calcium leached from slag content might expedite the calcium leaching [24]. Similarly, amino acids [25,26] and protein [27] have also been incorporated into slag-based IWAAs to increase the carbonation degree and strengthen the mechanical properties through stimulating the Ca²⁺ leaching and dissolution from calcium silicate lattice.

Based on the aforementioned literature, the current investigations involving IWAAs have mainly focused on the property variation before and after carbonation, as well as the mechanical behaviors and enhancement mechanism of its application in concrete from a multiscale analysis; yet, few studies have involved road performance with the utilization of IWAAs in bituminous mixtures. Hence, in this study, new IWAAs were fabricated with steel slag powders as the main component and fly ash and cement as auxiliary cementitious materials for its agglomeration; the performance variation rule and enhancement mechanism were analyzed, as well as the feasibility of its application in IWAAs–bituminous mixture. The experimental design program for this study is illustrated in Figure 1. Herein, the physical property, mineral phase composition, and microstructural morphology of IWAAs with carbonation times (0 h, 6 h, 12 h, 24 h, and 48 h) were investigated through dimensional stability, the crushing value test, XRD, DSC, and TG, and SEM analysis; the road performance (e.g., high-temperature performances, moisture damage resistance and skid resistance) of the IWAAs–bituminous mixture were evaluated by implementing the Marshall, rutting, and BPN value tests, respectively.

2. Materials and Experiments

2.1. Materials

In this study, steel slag with specific surface area of 260 m²/kg produced from Shagang Group in Zhang Jiagang, China, was selected, which was ground by planetary mill and screened by 0.08 mm sieve. In addition, P.O 42.5 cement from Conch Cement Co., Ltd. in Zhang Jiagang, China, was utilized as a supplementary cementitious phase for strength enhancement prior to carbonation, and Class-I fly ash from Shenhua Huashou Power Co., Ltd. in Shanghai, China was used as the filling phase for CO₂ diffusion inside the IWAAs. The related main composition and the XRD pattern of raw steel slag are presented in

Table 1. Main component of materials.

Types	Mass Fraction (%)
	CaO	MgO	SiO2	Al2O3	Fe2O3	K2O	Na2O	SO3	Loss
P.O 42.5 cement	64.02	2.38	19.39	4.81	2.89	0.70	0.13	2.62	2.71
Steel slag	41.83	10.67	9.80	2.59	29.31	0.05	0.10	0.09	1.40
Class-I fly ash	4.02	1.08	48.79	27.86	6.08	1.12	0.39	0.88	6.17

and Figure 2. IWAAs were prepared with steel slag powders as the main component, with fly ash and cement as auxiliary cementitious materials for its agglomeration, whose calcium-containing phases (e.g., free-CaO, C₂S, and Portlandite) possessed a high carbonation reactivity. Notably, as displayed in Figure 2, steel slag contains calcium silicate, C₃S, C₂S, and periclase, as well as a small quantity of lime; herein, the X-ray diffraction peak of Portlandite can be ascribed to interior lime hydration, and the appearance of the CaCO₃ diffraction peak is caused by partial carbonation of portlandite/lime [23].

The graded macadam, manufactured sand, and ground powders from limestone were selected as the coarse, fine aggregate, and mineral powders, respectively, and Shell 70# matrix bitumen with the penetration grade of 60/80 was selected as binder, which were provided by Yonglian Jingzhu Construction Group Co., Ltd. in Zhang Jiagang, China. Its corresponding properties are listed in

Table 2. Properties of 70# bitumen.

Properties	Penetration (25 °C, 100 g, 5 s)/0.1 mm	Ductility (5 cm/min, 15 °C)/cm	Softening Point/°C	Relative Density (25 °C)	Dynamic Viscosity (60 °C)/Pa·s
Results	68.2	>100	48.5	1.012	185.1
Standard requirement	60–80	≥15	≥46	-	≥160

,

Table 3. Properties of coarse aggregates.

Properties	Crushing Value (%)	Polished Value	Los Angeles Abrasion Loss (%)	Soft Stone Content (%)	Needle-like Content (%)
Results	23.9	40.7	18.6	0.51	8.5
Standard requirement	≤26	≥40	≤28	≤3	≤15

,

Table 4. Properties of fine aggregates.

Properties	Apparent Relative Density (g/cm3)	Angularity	Sand Equivalent (%)	Sturdiness (<0.3 mm)	Mud Content (<0.075 mm) (%)
Results	2.781	44.1	75	7.2	0.9
Standard requirement	≥2.50	≥30	≥60	-	≤3

and

Table 5. Properties of mineral powder.

Properties	Apparent Relative Density (g/cm3)	Particle Size Range (%)			Hydrophilic Coefficient	Plasticity Index (%)
		<0.6 mm	<0.15 mm	<0.075 mm
Results	2.674	100	94.9	89.5	0.57	2.5
Standard requirement	≥2.50	-			<1	<4

. In this study, based on preliminary indoor experiments, the IWAAs content for a substitute was 30% by the total mass of coarse aggregate, and the aggregates grading of the AC-13 bituminous mixture is displayed in Figure 3. Subsequently, Marshall proportional designing procedure was implemented to determine the optimal bitumen content in accordance with the standards (JTG F40-2004) [28]; the relevant design parameter is listed in

Table 6. Marshall design parameter.

Bitumen Content (%)	Bulk Gravity (g/cm3)	Voidage (%)	VMA (%)	VFA (%)	Marshall Stability (kN)	Flow Value (mm)
6.4	2.725	3.7	18.49	78.56	16.19	2.94

.

2.2. Preparation and Carbonation Process of IWAAs Specimens

During the preparation process, the manufacture of IWAAs was implemented with steel slag powder as the major phase, cement as the supplementary cementitious phase for the basic microstructure generation prior to carbonation, and fly ash as the filling phase for the CO₂ diffusion at a ratio of 5:3:1 [29]. Prior to its preparation, the optimum moisture rate and maximum dry density were calculated via compaction test. The specimens were compacted to reach 98% of maximum dry density and maintained under the condition of 20 ± 2 °C, with above 90% relative humidity for 7d. Thereafter, the IWAAs specimens were broken and sieved into aggregates with particle sizes of 2.36~13.2 mm utilizing a jaw crusher. In the carbonation process of the IWAAs specimens, the prepared IWAAs were put into a customized carbonation tank, vacuumed to remove the interior air, and processed with 0.2 MPa pressure and 55 °C ambient temperature under 99.0% CO₂ concentration for variable-time carbonation (0 h, 6 h, 12 h, 24 h, and 48 h) [30]. The carbonation and carbonation process are illustrated in Figure 4, and the set carbonation regime is listed in

Table 7. Group assignment of IWAAs.

Grouping	S0	S1	S2	S3	S4
Carbonation time (h)	-	6	12	24	48

.

2.3. Characterization of IWAAs Properties

2.3.1. Mineral Composition Detection

The IWAAs were broken into small cubes with approximately 4 mm, placed into anhydrous ethanol for 24 h, and dried in an 80 °C oven for 6 h. The specimens were ground into 80 μm powders. The mineral composition was analyzed and characterized using X-ray diffraction (Rigaku SmartLab in Tokyo, Japan) at a voltage of 30 kV and a 10 mA current, with an XRD scanning range of 5~80° and a scanning speed of 5°/min. The CO₂ absorption and CaCO₃ production of the prepared IWAAs with variable carbonation times were quantified by a TG/DSC analyzer (Netzsch STA 449 produced in Waldkraiburg, Germany), with a temperature range of 25~900 °C, at a ramp rate of 10 °C/min, and in a nitrogen atmosphere [31]. Notably, the appearance of obvious endothermic peaks at 105 °C, 400~450 °C, and 600~750 °C could be ascribed to the dehydration of C-S-H gel, the decomposition of Portlandite, and the decarbonation of CaCO₃ crystal, respectively. Based on the mass loss variation, the results of the quantity in CO₂ absorption and CaCO₃ production were determined based on Equations (1) and (2).

$$W_{C{O}_2-absorption}=\frac{\Delta m_{600~750°\mathrm{C}}}{m_{900°\mathrm{C}}},$$

(1)

$$W_{CaC{O}_3}=\left(\Delta m_1-\Delta m_2\right)\times \frac{M_{CaC{O}_3}}{M_{C{O}_2}},$$

(2)

where $\Delta m_{600~750°\mathrm{C}}$ = the weight loss within 600~750 °C; $m_{900°\mathrm{C}}$ = the weight at 900 °C; $\Delta m_1, \Delta m_2$ = the weight loss of aggregate before and after carbonation, respectively.

2.3.2. Dimensional Stability and Crushing Value of IWAAs

In accordance with specification (JTG E42-2005) [32], for the dimensional stability test, the IWAAs were loaded into the test mold and compacted, and the test mold was then put into the water tank at 80 °C and heated for 6 h per day. The percentage meter readings were taken 11 times before heating. The expansion rate C was calculated according to Equation (3). In terms of the crushing value test, 3000 g IWAAs with the grain size of 9.5~13.2 mm were dried and loaded into a mold. Subsequently, the specimens were loaded until the total loading approached 400 kN within 10 min and sieved through a sieve of 2.36 mm. The ratio of the screen residue to the total weight was determined as the crushing value. The relevant test process is shown in Figure 5.

$$C=\frac{d_{10}-d_0}{125}\times 100\%,$$

(3)

where d₀ = the initial readings of the dial indicator; d₁₀ = the percentage readings taken on the day following the end of the test heating at 10 d.

2.4. Performance of the IWAAs–Bituminous Mixture

Conforming to the specification (JTG E20-2011) [33], for the rutting test, a rutting plate with dimensions of 300 mm × 300 mm × 50 mm was placed in a 60 °C thermostat for 5 h preheating, and when the condition of 60 °C and 0.7 MPa wheel pressure was detected, the rolling times per 1 mm of permanent deformation were calculated until the deformation stabilized. For the immersed Marshall test, standard Marshall specimens with the dimensions of φ101.6 mm × 63.5 mm were fabricated and placed into a 60 °C water tank for 30 min and 48 h, and a loading rate of 50.8 mm/min until failure was used to calculate the immersion residual stability. In terms of the skid-resistance test, the wheel milling machine was used to manufacture rutting plates, and the friction coefficient of the plate specimen was detected by the BM-Ⅲ type pendulum friction coefficient tester. The aforementioned test procedure is illustrated in Figure 6.

3. Results and Discussion

3.1. Properties and Characteristics of the IWAAs

3.1.1. Microscopic Morphology

Figure 7 displays the microscopic morphology of IWAAs with variable carbonation time and the microstructures of the IWAAs–bitumen interface for a thorough investigation. As illustrated in Figure 7a, the uncarbonated IWAAs possess the loose structure of slag plates with formations of porous and interstices defects, coupled with the attachment of visible fly ash particles on the surface. Overall, the reticulated C-S-H gels (calcium silicate hydrates) and CH crystals (Ca(OH)₂) with hexagonal prisms can be observed, as well as a large quantity of interior micropores [22,34]. In contrast, after 6 h of carbonation (Figure 7b), a small number of carbonate crystals with various irregular morphology (e.g., vaterite and aragonite) were generated among the fly ash particles, together with the intermingling and binding effect of the carbonation and hydration components [35], manifesting as a much denser microstructure. In terms of Figure 7c, the formation and growth of the cubic blocky calcium carbonates in the process of 12 h of carbonation filled parts of the micropores near the surface, yielding a denser structure of IWAAs. As can be seen in Figure 7d,e, after 24 h and 48 h of carbonation, with a further increase in the carbonation degree, the precipitation and agglomeration of abundant calcium carbonate on the IWWAs’ surface could bind the fly ash grain together, replenishing most of the secondary pores inside the IWAAs, blocking the infiltration channel, and further enhancing the densification and homogenization of the IWAAs matrix [17,18]. Notably, the coexistence of CaCO₃ and Mg might induce the generation of aragonite instead of calcite, manifesting as a tiny quantity of CaCO₃ with characteristics of rhombic morphology, which are consistent with the SEM results in this study [34,36]. Obviously, the porous structure on the IWAAs’ surface provided sufficient voids for the nucleation and growth of carbonation crystals; thus, there was no risk-free level of rupture due to excessive structural density [37,38].

As displayed in Figure 7f, the dark areas showed bitumen mortar with a smooth and glossy surface, while the light areas represent IWAAs, as well as the greyish continuous interfacial phase. Based on the micromorphology observation, the pores and structural phase such as carbonate crystals inside the IWAAs were homogeneously and tightly wrapped by bitumen with a certain penetration depth; in addition, the range of the continuous interfacial phase was in a condensed and stable state without obvious interior microdefects. This phenomenon could be explained by the fact that the alkaline phase and porous microstructure of IWAAs could contribute to the formation of surface adhesion due to the solid-anchoring effect and chemical bonding on the interface phase [15]. Furthermore, the result was revealed that the phase composition and microstructure of IWAAs affected the carbonation efficiency, except for the carbonation time.

3.1.2. TG/DSC Thermal Analysis

Figure 8 illustrates the TG/DSC analysis curves of IWAAs with different carbonation times. Obvious endothermic peaks appear at 105 °C, 400~450 °C, and 600~750 °C, which could be induced by the dehydration of C-S-H gel, the decomposition of Portlandite, and the decarbonation of CaCO₃ crystal, respectively [38,39]. Namely, the weight losses within the corresponding temperature ranges were determined to quantify the content of the hydration products, respectively, as listed in

Table 8. CO₂ absorption and CaCO₃ production of the IWAAs.

Grouping	Weight Loss within 600~750 °C/wt.%	Estimated Absorption of CO2/wt.%	Estimated Production of CaCO3/wt.%
S0	0.46	2.53	1.05
S1	6.76	9.93	15.36
S2	7.21	10.06	16.38
S3	7.49	10.13	17.01
S4	8.36	10.75	19.01

. It can be found that the endothermic peak decreased and trended to gentle, while that of CaCO₃ presents a rising trend with carbonation time, indicating that the prolonged carbonation time could facilitate the formation and agglomeration of pure calcite inside IWAAs, which were consistent with the observation in Figure 7. Concurrently, the carbonation efficiency showed a downward trend with carbonation time. For example, the IWAAs after carbonation absorbed an additional 7.40 wt.% (6 h), 7.53 wt.% (12 h), 7.60 wt.% (24 h), and 8.22 wt.% (48 h) compared to the uncarbonated, i.e., the effectiveness within 6 h of the initial carbonation was markedly higher than other later stages (from 6 h to 48 h). Combined with the microscopic morphology, those phenomena could be attributed to the fact that the carbonation could remarkedly exert a replenishing and refinement effect and abate the microporous interconnectivity on its surface, effectively hindering the further ingress and diffusion of CO₂ inside the matrix, resulting in the reduction in the CO₂ uptake and reaction efficiency [39].

3.1.3. Mineral Composition Analysis

Figure 9 presents the XRD pattern of the IWAAs with variable carbonation times. It can be clearly observed that the C₂S and C₃S diffraction peaks’ intensity of CO₂ sequestration IWAAs were reduced compared with the raw IWAAs, where the diffraction peaks intensity of f-CaO inside the raw IWAAs gradually disappeared after carbonation; i.e., the depletion in the f-CaO occurred inside the IWAAs; concurrently, the diffraction peaks’ intensity of Ca(OH)₂, C₂S, and C₃S tended to be weak, while the CaCO₃ peak increased with the extension of the carbonation time, indicating that with the progress in CO₂ uptake, the Ca-based (e.g., f-CaO and Portlandite) and Mg-based (e.g., periclase and brucite) compounds inside the IWAAs might be in chemical reaction with CO₂ to generate stabilized carbonation crystals, effectively boosting the enhancement of the microstructural densification and homogenization and hence improve the mechanical properties [40]. Furthermore, the interior hydration reaction of C₂S and C₃S could occur to generate C-S-H gel and CaCO₃. The corresponding reaction process of CO₂ sequestration is as described in Equations (4)–(9) [41].

$$2\left(3\mathrm{C}\mathrm{a}\mathrm{O}\cdot \mathrm{S}\mathrm{i}{\mathrm{O}}_2\right)+3\mathrm{C}{\mathrm{O}}_2+3{\mathrm{H}}_2\mathrm{O}\to 3\mathrm{C}\mathrm{a}\mathrm{O}\cdot 2\mathrm{S}\mathrm{i}{\mathrm{O}}_2\cdot 3{\mathrm{H}}_2\mathrm{O}+3{\mathrm{CaCO}}_3,$$

(4)

$$2\left(2\mathrm{C}\mathrm{a}\mathrm{O}\cdot \mathrm{S}\mathrm{i}{\mathrm{O}}_2\right)+\mathrm{C}{\mathrm{O}}_2+3{\mathrm{H}}_2\mathrm{O}\to 3\mathrm{C}\mathrm{a}\mathrm{O}\cdot 2\mathrm{S}\mathrm{i}{\mathrm{O}}_2\cdot 3{\mathrm{H}}_2\mathrm{O}+{\mathrm{CaCO}}_3,$$

(5)

$$\mathrm{C}\mathrm{a}\mathrm{O}+{\mathrm{H}}_2\mathrm{O}\to \mathrm{C}\mathrm{a}{\left(\mathrm{O}\mathrm{H}\right)}_2,$$

(6)

$$\mathrm{C}\mathrm{a}{\left(\mathrm{O}\mathrm{H}\right)}_2+\mathrm{C}{\mathrm{O}}_2\to {\mathrm{H}}_2\mathrm{O}+\mathrm{C}{\mathrm{aCO}}_3,$$

(7)

$$\mathrm{M}\mathrm{g}\mathrm{O}+{\mathrm{H}}_2\mathrm{O}\to \mathrm{M}\mathrm{g}{\left(\mathrm{O}\mathrm{H}\right)}_2,$$

(8)

$$\mathrm{M}\mathrm{g}{\left(\mathrm{O}\mathrm{H}\right)}_2+\mathrm{C}{\mathrm{O}}_2\to {\mathrm{H}}_2\mathrm{O}+{\mathrm{MgCO}}_3,$$

(9)

3.2. Performance Analysis of IWAAs

3.2.1. Dimensional Stability

Figure 10 illustrates the immersion expansion rate of IWAAs as function of the carbonation curing time. As can be seen from Figure 10, the expansion rate of uncarbonated IWAAs maintained in 60 °C water for 10d was 2.83%, exceeding the standard requirement (YB/T 4184-2009) [42], which was mainly caused by the hydration and expansion of the interior crystalline phases [18]. With the extension of the carbonation time, the expansion rate of the IWAAs progressively decreased and met the requirement of less than 2%. For instance, the decreases in the expansion rate were 66.08% (S1), 82.21% (S2), 87.63% (S3), and 90.10% (S4) as compared with the uncarbonated, respectively, implying that the extension of the carbonation time might be beneficial for increasing the possibility of a more complete reaction between CO₂ uptake and the internal mineral phase of IWAAs. Meanwhile, the corresponding variation rates exhibited a downtrend with carbonation time, which could be elucidated by the fact that the carbonated phase generated from the mineralization reaction would exert a replenishing and densification effect on the surface, hinder the CO₂ uptake and diffusion into IWAAs during the further carbonation, thereby retarding the progression of obtaining a higher carbonation degree of the IWAAs [41].

3.2.2. Crushing Value

Figure 11 displays the variation in the crushing value of the IWAAs at varying carbonation times. It can be found that the crushing value of the raw IWAAs exceeded 26%, not meeting the upper limit for the surface course of an expressway and first-class highway. Furthermore, in terms of the carbonated IWAAs, the crushing value presented a downward trend with carbonation time; herein, the corresponding reductions were 32.59% (S1), 41.58% (S2), 48.25% (S3), and 51.76% (S4) compared to the uncarbonated, demonstrating that carbonation could efficiently upgrade the anti-crushing performance of the IWAAs. Combined with the microscopic results presented in Section 3.1, this phenomenon could be explained as follows: the extension of carbonation time could promote the formation of a tiny carbonate crystalloid phase with excellent mechanical properties inside the IWAAs particles, along with the depletion in f-CaO, f-MgO, and Ca(OH)₂ as a result of carbonation, which could provide supplemental nucleation sites for the C₂S and C₃S hydration. As a result, the carbonation and hydrated phases could replenish larger voids and be in combination with slag and cement, thereby enhancing the density and mechanical strength of the IWAAs [41]. Notably, IWAAs-S2, S3, and S4 satisfy the requirement for the crushing value of level-Ⅱ aggregates based on the standard (GB/T 14685-2011) [43].

3.3. Performance Characterization of IWAAs–Bituminous Mixture

3.3.1. High-Temperature Rutting Resistance

Figure 12 displays the rutting test results of the bituminous mixture using different IWAAs. It can be observed that the dynamic stability (DS) presents a general uptrend with carbonation time. For example, the increases in the DS values are 16.41% (S1), 31.92% (S2), 46.87% (S3), and 48.41% (S4) compared to the uncarbonated, respectively. This suggests that the longer carbonation time could exert marked effectiveness on the high-temperature behavior of the bituminous mixture, manifesting as the improvement in the resistance to permanent deformation under repeated traffic loading; furthermore, the relevant variation law exhibited a superior consistency with the property characteristics of IWAAs. Combined with the aforementioned microscopic analysis, this could be explained as follows: the interior cement could play the role of an initial skeletal structure inside IWAAs since hydration products bound slag and fly ash together prior to carbonation; afterwards, the mineral crystal from the carbonation of Ca-based and Mg-based compounds inside the slag and cement hydrated products might play a critical role in enhancing the compression properties of the IWAAs, as the crystalline phase formed within the matrix could catalyze the densification and homogenization of the amorphous C-S-H structure, together with its good adhesiveness to bitumen due to its high alkalinity and the solid-anchoring effect from the porous microstructure, thus strengthening the road performance of the IWAAs–bituminous mixture [44,45].

Nonetheless, it is noteworthy that the DS growth rate (i.e., the increase in the DS value divided by the increase in the carbonation time) within the carbonation time range of 0 h to 12 h reached the top, which was 2.74%/h within 0~6 h and 2.59%/h within 6~12 h; in contrast, the DS growth rates corresponding to 12~24 h and 24~48 h were 1.26%/h and 0.06%/h, respectively. This clearly indicates that the growth rate of the DS value and the CO2-sequestration efficiency of IWAAs gradually slowed down with the extension of the carbonation time, which exhibits superior consistency with the results of the mineral composition content in Section 3.1.2. These phenomena might be closely related to the microstructural densification of the IWAAs subjected to variable-time carbonation, as well as the reduction in the interior Ca-based and Mg-based crystal (e.g., f-CaO and periclase) content that could be carbonated.

3.3.2. Moisture Damage Resistance

Figure 13 displays the results of the moisture damage resistance of the bitumen mixture utilizing IWAAs. The moisture damage resistance met the specification (i.e., residual stability ≥ 80%). It can be found that in terms of the Marshall stability (MS), the MS values after immersion were significantly lower than before immersion. This could be because the immersion pumping effect would aggravate the adhesion attenuation of the IWAAs–bitumen, together with the expansion and penetration of microcracks and pore interconnectivity, causing the interfacial adhesion energy to exceed the cohesive energy of the bitumen film [15]. In addition, the MS value showed an upward trend with the extension of the carbonation time irrespective of the immersion condition and the residual stability as well. If we take the residual stability as an example, the increase corresponding to the carbonation time of 6 h, 12 h, 24 h, and 48 h was 2.15% (S1), 5.59% (S2), 7.44% (S3), and 8.85% (S4), as compared to the uncarbonated, respectively. The reasons for these phenomena could be as follows: firstly, CO₂ sequestration could enhance the density and mechanical strength of IWAAs; secondly, the porous microstructure of the slag surface could provide a solid anchoring endpoint for bitumen, guaranteeing the effective interfacial adhesion against the pumping effect of hydrodynamic forces; thirdly, the alkaline-active components inside IWAAs would chemically react with the surface acidity groups of bitumen to generate an adhesive force together with energy exchange [15,16].

3.3.3. Skid-Resistant Performance

Figure 14 illustrates the variation in the BPN value of the bitumen mixture utilizing IWAAs as a function of the carbonation time. It can be seen that BPN value gradually increased with the extension of the carbonation time. For example, the increase in the BPN value was 1.67% (S1), 10.00% (S2), 15.00% (S3), and 18.33% (S4) compared with the uncarbonated, respectively, implying that the skid resistance was strengthened to some extent. This is due to the fact that with the progress in the CO₂ uptake, f-CaO, f-MgO, and Ca(OH)₂ compounds inside the IWAAs might have a chemical reaction to generate the carbonate crystalline phase with characteristics of abrasion resistance and cube-like blockiness and boost the enhancement of the surface-microstructural densification, angularity, and micro-texture of the IWAAs for aggregates, coupled with excellent interfacial adhesion, which would upgrade the aggregates’ resistance to abrasion loss and the frictional impact of external loading [1], as illustrated in Figure 15. Additionally, the variation rate in the skid resistance gently slowed down with the CO₂ sequestration time, which is consistent with the mentioned microscopic observation and performance characterization of IWAAs, which may be closely associated with the carbonation efficiency reported in Section 3.1. Namely, carbonation crystal phases could exert a replenishing and densification effect and downgrade the porous interconnectivity on the surface, which might effectively hinder the further ingress and diffusion of CO₂ into IWAAs and cause the reduction in CO₂ uptake.

4. Conclusions

This study investigated the performance variation rule and enhancement mechanism of the IWAAs–bituminous mixture in relation to the physical property, mineral phase composition, and microstructural morphology of the prepared IWAAs, as well as the road performance of the bituminous mixture. The main conclusions of this study are summarized as follows:

(1)

With the progress in the CO₂ uptake, the Ca-based (e.g., f-CaO and Portlandite) and Mg-based (e.g., periclase and brucite) compounds, as well as the hydration products inside IWAAs, might have a chemical reaction with CO₂ to generate stabilized carbonation crystals, effectively boosting the enhancement of the microstructural densification and homogenization. Herein, the 12 h carbonation time could improve both the anti-crushing properties and the dimensional stability by 82.21% and 41.48%, respectively.

(2)

Based on the mineral composition analysis, the carbonation efficiency of the IWAAs progressively decreased with the extension of the carbonation time, since the carbonated phase generated from the mineralization reaction would exert a replenishing and densification effect on the surface and hinder the CO₂ uptake and diffusion into IWAAs during the further carbonation.

(3)

The high-temperature rut resistance, moisture damage resistance, and skid resistance gradually increased with prolonging the carbonation time from 6 h to 48 h for the IWAAs–bituminous mixture, where the optimal carbonation time was 12 h, and the corresponding increases were 31.92%, 5.59%, and 10.00%, respectively. The generated carbonate crystalline phase with abrasion resistance and irregular morphology could boost the enhancement of the microstructural densification and homogenization, together with excellent interfacial adhesion to bitumen due to its alkalinity.

(4)

After comprehensive consideration of the carbonation efficiency, the properties’ growth rates, and the limits values of the specification for IWAAs, as well as the IWAAs–bituminous mixture, the 12 h carbonation time is recommended during the preparation and carbonation process of IWAAs.