Study on the Sustainability of Carbon Emission Reduction in China’s Cement Industry

Abstract

Recycled concrete fines (RCFs) have the potential to serve as a supplementary cementitious material (SCM) after carbonation. Traditionally, carbonation of RCFs results in calcium carbonate primarily in the form of calcite, which significantly limits the development of RCFs as an SCM. In this research, a wet grinding carbonation (WGC) technique was introduced to enhance the reactivity of RCFs. The research indicates that RCFs after WGC exhibit a finer particle size and a larger specific surface area. The carbonation products include calcite with smaller grains, metastable calcium carbonate, and nanoscale silica gel and Al-Si gel. When RCF-WGC is used as an SCM in ordinary Portland cement (OPC), it significantly promotes the hydration of the cement paste, as evidenced by the advancement and increased intensity of the exothermic peaks of aluminates and silicates. RCF-WGC can significantly enhance the compressive strength of hydrated samples, particularly at early ages. Specifically, at a curing age of 1 day, the compressive strength of WGC5, WGC10, and WGC20 samples increased by 9.9%, 22.5%, and 7.7%, respectively, compared to the Ref sample (0% RCF-WGC). At a curing age of 3 days, the compressive strength of the WGC5, WGC10, and WGC20 samples showed even more significant improvements, increasing by 20.8%, 21.9%, and 11.8%, respectively. The performance enhancement of the WGC samples is attributed to the chemical reactions involving nanoscale silica gel, Al-Si gel, and calcium carbonate in the RCFs. When RCF-WGC is used as an SCM to replace 5%, 10%, and 20% of cement, it can reduce carbon emissions by 27.5 kg/t, 55 kg/t, and 110 kg/t, respectively. Large-scale application of RCFs as a high-value SCM can significantly reduce the life-cycle carbon emissions of the cement industry, contributing to the achievement of carbon peaking in China’s cement sector.

1. Introduction

Cement and concrete materials, as fundamental building materials, are widely used in various infrastructure projects, ranging from urban buildings to bridges, highways, ports, and dams [1,2]. However, the production of cement generates a significant amount of CO₂, accounting for approximately 8% of anthropogenic CO₂ emissions [3,4,5]. Statistics show that global cement production has exceeded 4 billion tons [6,7]. As a major player in infrastructure development, China has a massive demand for cement as a basic building material, with its cement production exceeding 2 billion tons, accounting for more than 50% of the global production [8,9]. Cement production primarily involves three main stages: raw material grinding, clinker calcination, and cement grinding. Among these, the energy for the clinker calcination stage mainly comes from coal. In this stage, a mixture with limestone as the primary raw material is calcined into cement clinker [10], accounting for 70% to 80% of the total energy consumption of the entire process. During calcination, carbon emissions from coal combustion and decomposition account for approximately 40% and 60%, respectively. In recent years, a significant proportion of China’s cement production lines are still small-scale. A production line with a daily output of 2000 tons has an energy consumption per unit of output that is more than 20% higher than that of a production line with a daily output of 5000 tons. Currently, common carbon reduction strategies in the cement industry include phasing out outdated cement production lines, upgrading and retrofitting production lines to achieve industry-wide optimization and energy savings, eliminating high-energy-consumption and high-emission fossil fuel calcination equipment, and further reducing the costs of electric kilns and other equipment through large-scale development. Other strategies include developing low-carbon cements, such as those with lower calcination temperatures; developing CO₂ capture and storage technologies; enhancing the durability of concrete to indirectly reduce CO₂ emissions [11], and developing high-reactivity SCM to partially replace cement [12,13,14].

The accumulation of waste concrete from building demolitions poses a significant challenge to achieving carbon peaking in China’s construction industry. Recycling waste concrete can effectively address this issue [15,16]. Waste concrete can be categorized by particle size into coarse aggregates, fine aggregates, and micro-powder, with micro-powder accounting for 20–30% of the total [17]. Due to its high water absorption, micro-powder has limited sustainable use. Waste concrete micro-powder primarily contains partially unhydrated cement clinker and hydration products such as C-S-H and CH [18,19,20], which have RCF potential for carbon sequestration and capture. Previous studies have demonstrated that carbonating RCFs can produce value-added supplementary cementitious materials, providing a new pathway for the sustainable utilization of RCFs [21,22], thereby aiding in achieving carbon peaking in the cement industry [23]. In addition, carbonated RCFs as SCM can alleviate the alkali–silica reaction in concrete [24].

The current research on carbonating RCFs includes carbonation curing, semi-wet carbonation, and wet carbonation [25,26]. Carbonation curing involves introducing CO₂ during the mixing of the paste, which can generate calcium carbonate in situ, improving the performance of hydrated samples and sequestering CO₂. However, the amount of CO₂ sequestered through carbonation curing is limited. Semi-wet and wet carbonation of RCFs result in the formation of silica gel and calcium carbonate [27]. When carbonated RCFs are used as an SCM in OPC, it can enhance the performance of hydrated samples. However, the calcium carbonate produced through this method is primarily in the form of calcite. Calcite, as a stable form of calcium carbonate, exhibits low reactivity in cement [28].

To further enhance the reactivity of RCFs as an SCM, this research introduces a wet grinding-assisted carbonation (WGC) technique [29], which significantly increases the reactivity of RCFs. During the wet carbonation process, mechanical forces applied through grinding induce distortions in the crystal structure of the materials [22,28,30,31]. This results in the formation of metastable calcium carbonate with smaller grain sizes and smaller silica gel particles, thereby enhancing the reactivity of RCFs. In this research, RCFs subjected to WGC were used to partially replace cement at various substitution levels. The mechanical properties, hydration process, hydration products, and microstructural evolution of the hydrated samples were investigated to elucidate the mechanisms by which the performance of the hydrated samples is improved. Additionally, the carbon sequestration potential of RCFs as an SCM was calculated for sustainability assessment. This research provides new insights into the sustainable utilization of RCFs, contributing to the achievement of carbon peaking in the cement industry.

2. Materials and Methods

2.1. Raw Materials

Considering the complexity of the composition of real RCFs, this research utilized cement paste hydrated for 180 days (with a water-to-cement ratio of 0.45) and ground into micro-powder to simulate real RCFs. The oxide compositions of the cement and RCFs used in this research are shown in

Table 1. XRF results of OPC and RCFs (wt.%).

Materials	CaO	Al2O3	SiO2	SO3	Fe2O3	MgO	K2O	Others
OPC	62.32	5.24	18.76	4.67	3.32	1.21	0.76	3.72
RCF	65.43	5.56	17.95	3.28	3.71	1.34	0.57	2.16

.

2.2. Sample Preparation

The process of WGC treatment of RCFs is illustrated in Figure 1. In the laboratory, zirconia balls with a diameter of 3 mm were selected as the grinding medium. RCFs and water were added to the grinding jar at a solid-to-liquid ratio of 5. The grinding process was initiated at 400 rpm, while CO₂ (99.9%) was introduced. After 30 min of wet grinding, the slurry was poured out, and the solid deposits were collected by centrifugation and then placed in a freeze dryer for drying. The freeze-dried sample was designated as RCF-WGC.

Cement paste samples were prepared according to the mix proportions shown in

Table 2. Mix proportions of pastes (wt%).

Mixture	OPC	RCF-WGC	Water
Ref	120	/	60
WGC5	114	6	60
WGC10	108	12	60
WGC20	96	24	60

. OPC, RCF-WGC, and water were thoroughly mixed for 5 min and then poured into cubic molds with a side length of 2 cm. The samples were cured in a standard curing room until the specified age, after which they were sampled for microstructural analysis. Simultaneously, cement mortar samples were prepared with a cement-to-sand ratio of 1:2, following the mix proportions in Table 2. The mixed mortar was poured into cubic molds with a side length of 4 cm and cured in a standard curing room until the specified age, after which compressive strength tests were conducted.

2.3. Characterizations

2.3.1. Compressive Strength

After curing the samples in a standard curing room until the testing curing time, the samples were removed, and their surfaces were dried. Subsequently, the compressive strength tests were conducted on the samples. Compressive strength measurements were conducted following the GB/T 17671-2021 [32], maintaining a loading rate of 0.5 mm/min.

2.3.2. Particle Size Distribution and Specific Surface Area

The particle size distribution of the RCFs and RCF-WGC was analyzed using Malvern Mastersizer 3000, Malvern, UK. The specific surface area was analyzed with a Micromeritics ASAP 2020Plus, Norcross, GA, USA, an automated specific surface area and porosity analyzer. Nitrogen was used as the adsorption gas, and the samples underwent a 10 h degassing process before testing. The surface area was determined using the Brunauer–Emmett–Teller (BET) method.

2.3.3. Heat of Hydration

The hydration heat release of the samples over 48 h at a constant temperature of 25 °C was tested using the TAM Air C80, New Castle, DE, USA. This included measuring both the rate of heat release and the cumulative heat release. The test employed an external stirring method, where the paste was thoroughly mixed to ensure uniformity. The calculated mass of the paste was then weighed and poured into the test vial for measurement.

2.3.4. Phase Evolution

XRD analysis was conducted on the cement paste employing a Bruker D8 Advance instrument, manufactured in Karlsruhe, Germany. The identification of the hydration product species was facilitated by applying the EVA analytical software (version EVA 5.2, Bruker). Quantitative X-ray diffraction (QXRD) assessments were undertaken using the Rietveld refinement technique to ascertain the phase composition, utilizing the Topas 4.2 computational suite. The mass loss of the cement paste was evaluated through TGA, Thermo Plus EVO2 TG8121, Rigaku, Tokyo, Japan, with a 20 mg sample being heated from 30 °C to 1000 °C at a heating rate of 10 °C per minute.

2.3.5. Microstructure Evolution

The morphology of the hydrated samples was observed using a scanning electron microscope (Tescan VEGA 3, Brno, Czech Republic). The accelerating voltage was set to 20 kV. Prior to testing, conductive adhesive was applied to the sample stage, and the powder sample was affixed to the adhesive. The entire sample stage was then placed in a sputter coater for gold coating, which was conducted for 80 s.

3. Results and Analysis

3.1. Characterizations of RCFs After WGC

The changes in RCFs before and after WGC are illustrated in Figure 2. Figure 2a,b show the particle size distribution of RCFs. RCF-WGC exhibits a smaller particle size, with RCF-raw displaying three peaks and a median particle size (d₅₀) of 22.849 μm, while RCF-WGC shows a single peak shifted to the left, with a d₅₀ of 3.193 μm. This indicates that WGC leads to particle size refinement in RCFs, which enhances its reactivity as an SCM [31].

Figure 2c,d depict the pore structure changes in RCFs. Compared to RCF-WGC, RCF-raw exhibits a larger pore structure (>10 nm), whereas RCF-WGC shows a significant increase in nanopore structures (<10 nm). This suggests that WGC significantly optimizes the pore structure of RCFs, transforming larger pores into nanopores. The specific surface area of RCF-raw is 41.31 m²/g, while that of RCF-WGC is 94.4 m²/g. The increased specific surface area of RCF-WGC is primarily attributed to the formation of more amorphous silica gel and Al-Si gel [33].

Figure 2e,f present the TG-DTG analysis results of RCFs. RCF-raw exhibits three major weight loss regions corresponding to C-S-H, CH, and calcium carbonate. In contrast, RCF-WGC exhibits two distinct weight loss regions: a minor mass loss below 200 °C associated with moisture release, followed by the primary decomposition peak of calcium carbonate at 600–800 °C. This is because WGC induces the carbonation of hydration products like C-S-H and CH, resulting in the formation of calcium carbonate.

Figure 2g and

Table 3. QXRD data of phases in RCFs treated with different methods (wt%).

Sample	C2S	CH	CC	AFm	AFt	Amor	Crystal Size of Calcite
RCF-raw	8.2	14.3	12.5	3.9	2.9	58.3	259
RCF-WGC	/	/	51.2	/	/	48.8	249

display the QXRD results for RCF-raw and RCF-WGC. RCF-raw contains unhydrated clinker C₂S, hydration products CH, AFt, AFm, calcium carbonate, and amorphous substances, primarily C-S-H gel. After WGC, calcium carbonate and amorphous materials are produced, with the amorphous phase mainly consisting of non-crystalline silica and Al-Si gel. The calcite crystal size of RCF-raw is 259 nm, and the calcite crystal size of RCF-WGC is 249 nm, indicating that wet grinding carbonation treatment refines the crystal size.

3.2. Compressive Strength

The compressive strength data of samples at different curing ages are shown in Figure 3. At a curing age of 1 day, compared to the reference sample, the compressive strength of the WGC5, WGC10, and WGC20 samples increased by 9.9%, 22.5%, and 7.7%, respectively. At a curing age of 3 days, the compressive strength improvements were even more pronounced, with increases of 20.8%, 21.9%, and 11.8%, respectively. This indicates that RCF-WGC, as an SCM, can significantly enhance the compressive strength of samples in the early stages of hydration, offsetting the dilution effect caused by the reduced cement clinker content. This experimental phenomenon is primarily attributed to the physical and chemical changes in RCFs after WGC. The particle size of RCFs is refined, and the specific surface area is increased, providing more nucleation sites when used as an SCM [34]. The high-reactivity calcium carbonate and silica gel generated from RCFs after WGC participate in forming more C-(A)-S-H, and the combined physical and chemical effects of RCF-WGC accelerate the hydration process, producing more hydration products and densifying the matrix structure, thereby resulting in higher compressive strength. At a curing age of 28 days, the compressive strength of the WGC5, WGC10, and WGC20 samples increased by 5.0%, 7.7%, and 2.2%, respectively, compared to the Ref sample. This demonstrates that the enhancement effect of RCF-WGC on cement paste is not only evident in the early stages of hydration but also throughout the entire hydration period. Additionally, it is observed that a 10% replacement level of RCF-WGC yields the most significant enhancement effect. When the replacement level is 20%, the compressive strength is lower than that of WGC5 and WGC10, primarily due to the dilution effect of cement clinker. From the above analysis, it is evident that RCFs, after WGC, used as an SCM in OPC, can significantly improve the compressive strength of cement samples.

3.3. Hydration Heat

Figure 4 presents the heat evolution curves of the hydrated samples, including the rate of heat release and cumulative heat release. From Figure 4, it is evident that the addition of RCF-WGC significantly accelerates the hydration process of OPC, increasing the rate of heat release and shifting the positions of peaks 1, 2, and 3 to the left, with increased intensity. The changes in cumulative heat release further support this conclusion. Peak 1 corresponds to the dissolution phase of cement, where the dissolution of cement clinker and the release of ions occur, accompanied by exothermic hydration. The increase in the intensity of peak 1 upon the addition of RCF-WGC indicates that RCF-WGC accelerates the dissolution process of cement. Peak 2 is attributed to the hydration of C₃S, forming C-(A)-S-H and CH. The addition of RCF-WGC significantly increases the intensity of peak 2 and shifts its position to the left, indicating that RCF-WGC promotes the hydration process of cement clinker, particularly the nucleation and growth of C-(A)-S-H and CH. The silica gel and Al-Si gel in RCF-WGC react with CH to form additional C-(A)-S-H, providing nucleation sites for hydration products, consistent with previous research findings [35]. Peak 3 corresponds to the dissolution reaction of C₃A, forming AFt. RCF-WGC promotes the leftward shift in peak 3, indicating that RCF-WGC accelerates the reaction of aluminates. The high-reactivity calcium carbonate in RCF-WGC reacts with C₃A to form Mc and Hc, as confirmed by subsequent XRD analysis. The changes in the heat evolution curves demonstrate that the addition of RCF-WGC in this study significantly accelerates the early hydration reactions of OPC. This corresponds with the trend observed in compressive strength changes. The early acceleration effect of RCF-WGC on hydration is more pronounced than previously reported in the literature [36,37], indicating the higher reactivity of RCF-WGC.

3.4. Phase Evolution of Hydration Samples

Figure 5 shows the TG-DTG curves of the hydrated samples at 1 day and 3 days. At 1 day of hydration, the TG-DTG analysis reveals three main weight loss regions, corresponding to the decomposition of C-A-S-H, AFt, and AFm (first region); CH (second region); and calcium carbonate (third region). The addition of RCF-WGC increases the intensity of the first and second weight loss peaks, with the most pronounced effect observed at a 10% replacement level of RCF-WGC. This indicates that RCF-WGC significantly promotes the hydration reactions, resulting in the formation of more hydration products. The trend in the TG-DTG curves at 3 days of hydration is similar to that at 1 day, consistent with the previous analyses of compressive strength and heat evolution. In the early stages of hydration, RCF-WGC accelerates the cement hydration process, leading to the production of more hydration products, which densifies the matrix structure and consequently enhances the compressive strength of the samples.

Figure 6 presents the XRD patterns of the hydrated samples at 1 day and 28 days. The addition of RCF-WGC results in the detection of Mc and Hc in the XRD patterns. This is due to the reaction between C₃A and the calcium carbonate in RCF-WGC, forming Hc and Mc, as described by Equations 1 and 2. This finding is consistent with the heat evolution analysis. The XRD patterns indicate reduced CH diffraction intensity at 18° and 34° 2θ in RCF-WGC blends compared to the Ref, which suggests progressive CH consumption with higher RCF-WGC content. This is because the silica gel and Al-Si gel in RCF-WGC react with CH to form C-(A)-S-H. The newly formed C-(A)-S-H provides additional stable nucleation sites for the precipitation of hydration products, thereby promoting cement hydration. This is further supported by the changes in clinker content observed in the analysis.

$${\mathrm{C}}_3\mathrm{A}+\mathrm{C}\mathrm{C}+11\mathrm{H}\to \mathrm{M}\mathrm{c}$$

(1)

$${\mathrm{C}}_3\mathrm{A}+0.5\mathrm{C}\mathrm{C}+0.5\mathrm{C}\mathrm{H}+11.5\mathrm{H}\to \mathrm{H}\mathrm{c}$$

(2)

Figure 7 presents the quantitative results obtained from QXRD analysis of the hydrated samples. Specifically, it can be observed that the amount of unreacted clinker in the hydrated samples decreases as the RCF-WGC content increases, with values of 37.6%, 35.1%, and 33.8% compared to 37.6%, 35.1%, and 33.8%, respectively, for the samples with 5%, 10%, and 20% RCF-WGC replacement. This indicates that the addition of RCF-WGC promotes the hydration of the cement clinker. Furthermore, at a curing age of 28 days, the content of calcite in the WGC samples decreases, which can be attributed to the reaction between calcite and C₃A, forming Mc (monocarboaluminate) and Hc (hemicarboaluminate). This observation is consistent with the previous analyses of compressive strength and heat of hydration, where the addition of RCF-WGC was shown to significantly accelerate the early hydration reactions of OPC. This acceleration leads to the formation of more hydration products, which densifies the matrix structure and consequently enhances the compressive strength of the samples.

3.5. Pore Structure

Figure 8 illustrates the changes in the pore structure of hydrated samples at 1 day and 3 days, focusing on pores smaller than 200 nm. The addition of RCF-WGC results in a larger cumulative pore volume in the samples. Specifically, at a curing age of 1 day, the cumulative pore volume of the WGC5, WGC10, and WGC20 samples increased by 26.1%, 46.2%, and 107.3%, respectively, compared to the Ref sample. At a curing age of 3 days, the increases were 4.9%, 18.2%, and 59.8%, respectively. Additionally, it was observed that the average pore diameter of the hydrated samples decreased with increasing RCF-WGC content. This indicates that as the RCF-WGC content increases, more of the larger pore volumes are transformed into smaller pores. As discussed in previous analyses, this transformation is due to the participation of calcium carbonate, silica gel, and Al-Si gel from RCF-WGC in the hydration reactions, leading to the formation of more C-(A)-S-H, Mc, and Hc. The combined effect of these hydration products refines the pore structure.

3.6. Morphology

Figure 9a–c show the microstructure of RCFs after WGC. In Figure 9a, the surface of RCFs is covered with a substantial amount of calcium carbonate. Figure 9b,c reveal a network of gels on the RCF surface, including C-(A)-S-H, silica gel, and Al-Si gel. As discussed earlier, the high-reactivity calcium carbonate, silica gel, and Al-Si gel formed on the surface of RCF-WGC promote cement hydration by participating in the hydration reactions and generating more hydration products. Figure 9d shows the reference sample at a curing age of 1 day, where the surface appears smooth with few hydration products. Figure 9e,f depict the WGC10 sample, showing a surface covered with numerous products, including C-(A)-S-H, Mc, and Hc. This indicates that RCF-WGC significantly accelerates the hydration of the sample in the early stages, primarily due to the rapid reaction between the high-reactivity calcium carbonate in WGC and C₃A. Figure 9g shows the reference sample at a curing age of 28 days, while Figure 9h,i show the WGC10 sample at the same age. Compared to the Ref sample, the WGC10 sample surface exhibits more hydration products, including C-(A)-S-H and needle-like AFt. This further demonstrates the enhanced hydration and formation of hydration products facilitated by RCF-WGC throughout the hydration period.

3.7. Sustainability Analysis

Based on the TG data from Figure 2e, the CO₂ sequestration capacity of RCFs can be calculated using Equation (3), and the degree of carbonation can be determined using Equation (4).

$$\mathrm{C}{\mathrm{O}}_2 \mathrm{sequestration} (\mathrm{g}/\mathrm{kg})={\displaystyle \frac{\mathrm{C}{\mathrm{O}}_2(\mathrm{w}\mathrm{t}.\mathrm{\%})}{100-\mathrm{C}{\mathrm{O}}_2(\mathrm{w}\mathrm{t}.\mathrm{\%})}}\times 1000$$

(3)

CO₂ sequestration capacity (g/kg) refers to the mass (g) of CO₂ that can be fixed per kilogram of RCFs. $\mathrm{C}{\mathrm{O}}_2(\mathrm{w}\mathrm{t}.\mathrm{\%})$ is obtained by the decarbonation amount of CC in the corresponding decomposition temperature range.

$${\mathrm{\delta }}_{\mathrm{C}\mathrm{a}\mathrm{O}}(\mathrm{\%})={\displaystyle \frac{\frac{\mathrm{C}{\mathrm{O}}_2(\mathrm{w}\mathrm{t}.\mathrm{\%})}{100-\mathrm{C}{\mathrm{O}}_2(\mathrm{w}\mathrm{t}.\mathrm{\%})}\times \frac{1}{\mathrm{M}{\mathrm{W}}_{\mathrm{C}{\mathrm{O}}_2}(\mathrm{k}\mathrm{g}/\mathrm{m}\mathrm{o}\mathrm{l})}}{\mathrm{C}\mathrm{a}{\mathrm{O}}_{\mathrm{t}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}}(\mathrm{k}\mathrm{g}/\mathrm{k}\mathrm{g})/\mathrm{M}{\mathrm{W}}_{\mathrm{C}\mathrm{a}\mathrm{O}}(\mathrm{k}\mathrm{g}/\mathrm{m}\mathrm{o}\mathrm{l})}}\times 100$$

(4)

${\mathrm{\delta }}_{\mathrm{C}\mathrm{a}\mathrm{O}}(\mathrm{\%})$ represents the degree of carbonization, $\mathrm{M}{\mathrm{W}}_{\mathrm{C}{\mathrm{O}}_2}$ and $\mathrm{M}{\mathrm{W}}_{\mathrm{C}\mathrm{a}\mathrm{O}}$ represent the molar masses of CO₂ (44 g/mol) and CaO (56 g/mol), and is the CaO content in the RCFs.

The results were calculated based on Equations (3) and (4) and indicate that the CO₂ sequestration capacity of RCFs after WGC is 211.7 g/kg, with a carbonation degree of 41.2%. When RCFs are used as an SCM, they can sequester CO₂ to varying degrees. Compared to Ref, the WGC5, WGC10, and WGC20 samples sequester 10.6 kg/t, 21.17 kg/t, and 42.34 kg/t of CO₂, respectively. Considering that the production of 1 ton of cement typically emits approximately 0.55 tons of CO₂, using RCF-WGC as an SCM to replace 5%, 10%, and 20% of cement can reduce CO₂ emissions by 27.5 kg/t, 55 kg/t, and 110 kg/t, respectively. The total CO₂ emission reduction is equal to the sum of the CO₂ sequestrated by RCFs and the CO₂ emission reduction by reducing cement usage; the calculation results are shown in Figure 10. The large-scale application of RCFs as a high-value SCM can significantly reduce the carbon emissions of the cement industry over its entire lifecycle, contributing to the achievement of carbon peaking in China’s cement sector. This approach not only enhances the sustainability of cement production but also aligns with global efforts to mitigate climate change by reducing industrial carbon footprints. These results highlight the effectiveness of the wet grinding carbonation process in enhancing the CO₂ sequestration potential of RCFs. The significant sequestration capacity and carbonation degree suggest that RCF-WGC can play a valuable role in reducing carbon emissions in the cement industry by acting as a supplementary cementitious material with enhanced reactivity and sustainability benefits.

4. Conclusions

(1) RCF-WGC produces highly reactive calcium carbonate, silica gel, and Al-Si gel, and its reduced particle size results in a larger specific surface area.

(2) The incorporation of RCF-WGC significantly promotes cement hydration, primarily due to the active participation of its calcium carbonate, silica gel, and Al-Si gel components, which facilitate the formation of additional C-(A)-S-H, Mc, and Hc phases.

(3) RCF-WGC notably enhances the compressive strength of hydrated samples, especially in the early stages. Specifically, at a curing age of 1 day, the compressive strength of the WGC5, WGC10, and WGC20 samples increased by 9.9%, 22.5%, and 7.7%, respectively, compared to the reference sample (0% RCF-WGC). At 3 days, the increases were even more pronounced, at 20.8%, 21.9%, and 11.8%, respectively.

(4) The use of RCF-WGC effectively reduces CO₂ emissions and promotes the recycling of waste resources. Compared to conventional low-carbon SCMs such as fly ash and slag, RCF-WGC exhibits comparable or superior CO₂ sequestration capacity and pozzolanic activity, making it a promising alternative for further reducing the carbon footprint of cementitious materials throughout their lifecycle.