Sustainable Cementitious Materials: Strength and Microstructural Characteristics of Calcium Carbide Residue-Activated Ground Granulated Blast Furnace Slag–Fly Ash Composites

Abstract

This study developed a sustainable low-carbon cementitious material using calcium carbide residue (CCR) as an alkali activator, combined with ground granulated blast furnace slag (GGBS) and fly ash (FA) to form a composite. The objective was to optimize the CCR dosage and the GGBS-to-FA ratio to enhance the unconfined compressive strength (UCS) of the composite, providing a viable alternative to traditional Portland cement while promoting solid waste recycling. Experiments were conducted with a water-to-binder ratio of 0.55, using six GGBS-to-FA ratios (0:10, 2:8, 4:6, 6:4, 8:2, and 10:0) and CCR contents ranging from 2% to 12%. Results indicated optimal performance at a GGBS-to-FA ratio of 8:2 and an 8% CCR dosage, achieving a peak UCS of 18.04 MPa at 28 days, with 79.88% of this strength reached within just 3 days. pH testing showed that with 8% CCR, pH gradually decreased over the curing period but increased with higher GGBS content, indicating enhanced reactivity. Microstructural analyses (XRD and SEM-EDS) confirmed the formation of hydration products like C-(A)-S-H, significantly improving density and strength. This study shows CCR’s potential as an effective and environmentally friendly activator, advancing low-carbon building materials and resource recycling in construction.

1. Introduction

The rapid development of China’s economy has significantly increased its energy demand. As the world’s largest energy producer, consumer, and greenhouse gas emitter, China faces tremendous pressure in terms of energy resources and environmental sustainability [1]. By the end of 2019, China consumed 1.91 billion tons of coal, accounting for 51.7% of global consumption. Simultaneously, its CO₂ emissions reached 10.04 billion tons, contributing 30% of the global total. These figures underscore the substantial role of China’s coal consumption in driving CO₂ emissions, with far-reaching implications for global climate change and environmental challenges [2]. To advance low-carbon transitions and achieve sustainable development, China has established ambitious “dual carbon” goals: to peak carbon emissions by 2030 and achieve carbon neutrality by 2060 [3]. These objectives pose significant challenges for the civil engineering sector, necessitating the exploration of innovative pathways to enhance energy efficiency and reduce emissions. A pivotal strategy involves decreasing reliance on traditional high-energy-consumption materials such as cement. Furthermore, repurposing industrial waste to develop low-carbon, environmentally friendly materials and minimizing energy-intensive processes aligns with the principles of green, clinker-free, and sustainable construction practices [4].

Alkali-activated binders represent an emerging class of environmentally friendly materials characterized by high compressive strength, excellent chemical stability, and outstanding heat resistance, making them highly suitable for applications in civil engineering [5,6,7,8,9,10]. In comparison to traditional Portland cement, alkali-activated materials offer distinct advantages. For example, they utilize abundant industrial by-products as raw materials, thereby reducing the depletion of natural resources, and their production process eliminates the need for energy-intensive high-temperature calcination, leading to a substantial reduction in carbon emissions. These benefits position alkali-activated binders as an ideal alternative to Portland cement, garnering significant attention as a research focus in both academic and industrial fields [5,11,12].

Ground granulated blast furnace slag (GGBS), a by-product of the ironmaking process, demonstrates excellent cementitious properties and is typically activated by alkaline substances such as lime (CaO or Ca(OH)₂). With the rapid development of China’s steel industry, the annual production of slag has reached 240 million tons [13]. Similarly, fly ash, primarily derived from residual coal combustion in thermal power plants, has seen an annual output approaching 900 million tons by 2020 [14]. The abundant availability of GGBS and fly ash provides a rich source of raw materials for research on alkali-activated materials.

In the research and application of alkali-activated binders, calcium hydroxide (Ca(OH)₂) has been widely used as an alkali activator due to its low cost and good durability [15,16]. Andal Mudimby suggested that lime improves the workability of alkali-activated materials (AACs), and a small amount of lime can enhance their compressive strength [17]. Compared to NaOH, Ca(OH)₂ offers the advantage of a more controlled activation process for GGBS. However, its activation efficiency is relatively low, and it is typically used in combination with other activators. Additionally, Yang et al. found that adding Ca(OH)₂ in amounts exceeding 10% of the solid mass significantly reduces the compressive strength of the material [16]. Further investigation is needed to understand the stability and strength development of binders based on Ca(OH)₂-activated GGBS and FA. Other commonly used alkali activators, such as NaOH, water glass, and Na₂CO₃, have demonstrated remarkable effectiveness in activating GGBS and FA [18,19]. However, these activators are often associated with significant drawbacks, including environmental pollution, high cost, and poor transportation safety, which limit their feasibility for large-scale application [20]. In this context, finding an alkali activator that is low-carbon, environmentally friendly, cost-effective, and easy to transport has become a critical research focus. Calcium carbide residue (CCR) has recently attracted attention due to its unique physicochemical properties and environmental advantages, positioning it as a promising alternative in this field.

In recent years, calcium carbide residue (CCR) has gained attention as a potential low-carbon and environmentally friendly alkali activator [21,22,23]. CCR is an industrial by-product generated during acetylene production, with an annual output in China exceeding 400 million tons [24,25]. With calcium hydroxide (Ca(OH)₂) content of up to 85%, CCR exhibits strong alkalinity (pH > 12) [26]. However, the open-air disposal of CCR not only occupies valuable land resources but also poses significant environmental risks, as rainwater leaching can contaminate soil and water systems, severely impacting the ecosystem [27]. Studies have demonstrated that CCR can serve as an alkali activator for GGBS-based soil stabilization [28,29]. It effectively stimulates the hydration reactions of GGBS and FA, forming calcium aluminate hydrate (CAH) and calcium silicate hydrate (CSH) phases, which significantly enhance material strength [30,31,32]. Additionally, CCR can act as a calcium source to further improve the strength of cementitious materials [33,34,35,36]. Replacing traditional limestone with CCR could potentially avoid up to 20 million tons of CO₂ emissions annually [25].

Nevertheless, current research on the interactions within the composite systems of CCR, GGBS, and FA, as well as the underlying micro-mechanisms, remains insufficient. In particular, systematic studies on the optimization of CCR dosage, the coordination and enhancement of hydration reactions among different components, and the mechanisms for improving strength are lacking. Moreover, to further investigate and address the issue of strength reduction caused by excessive Ca(OH)₂, we incorporated FA into the GGBS-based system. This study aims to explore the strength characteristics and microstructural features of GGBS-FA blends under varying CCR activation ratios.

In summary, this study focuses on the following key aspects. (1) The effect of CCR content on the unconfined compressive strength (UCS) of GGBS-FA composite binders with varying ratios. Through systematic experiments, the optimal mass ratio of FA to GGBS and the ideal CCR dosage were identified. (2) Strength development under different curing durations. The compressive strength evolution of an 8% CCR-activated GGBS-FA (8:2) composite was analyzed over various curing periods (3, 7, 14, and 28 days), revealing the material’s strength development trends. (3) Mechanisms of hydration reactions and microstructural evolution. Advanced characterization techniques, including X-ray diffraction (XRD) and scanning electron microscopy coupled with energy-dispersive spectroscopy (SEM-EDS), were employed to investigate the hydration products, providing deeper insights into the activation mechanisms of CCR in GGBS-FA composite binders. This study not only contributes to the theoretical understanding of the resource utilization of industrial by-products but also offers practical guidance for developing low-carbon eco-friendly construction materials. By addressing critical challenges in material innovation, this research holds promise for facilitating the building industry’s progress toward achieving the “dual carbon” goals.

2. Materials and Methods

2.1. Materials

2.1.1. Ground Granulated Blast Furnace Slag

The ground granulated blast furnace slag (GGBS) used in this study was procured from Longze Water Purification Materials Co., Ltd., located in Gongyi City, Henan Province. The material appears white in color. The physical and chemical properties of GGBS are presented in Table 1, while its particle size distribution curve is shown in Figure 1. The primary chemical composition of the GGBS is listed in Table 2, and its XRD pattern is depicted in Figure 2. In an alkaline environment, the glassy structure of GGBS is prone to degradation, progressively decomposing to form hydration products, such as calcium silicate hydrate (C-S-H) and calcium aluminum silicate hydrate (C-(A)-S-H). These hydration products contribute to the enhanced mechanical performance of the material [37].

2.1.2. Fly Ash

The fly ash (FA) used in this study was supplied by Longze Water Purification Materials Co., Ltd., located in Gongyi City, Henan Province. The material is characterized by a gray-black appearance. The physical and chemical properties of FA are summarized in Table 1, while its particle size distribution curve is presented in Figure 1. The primary chemical composition of the FA is detailed in Table 2. As indicated in Table 2, the Al₂O₃ content of the FA exceeds 30%, categorizing it as high-alumina FA. Under alkaline activation, this material is more prone to forming hydration products such as calcium aluminum silicate hydrate (C-(A)-S-H), which significantly enhances the structural performance of the material. According to the Chinese national standard “Fly Ash Used in Cement and Concrete” (GB/T 1596-2017), Grade I FA must have a combined SiO₂ + Al₂O₃ + Fe₂O₃ content of no less than 50% and a SO₃ content of no more than 5%. In this study, the FA has a SiO₂ + Al₂O₃ + Fe₂O₃ content of 89.26%, far exceeding the 50% requirement, and a SO₃ content of 0.72%, well below the 5% limit. Thus, the FA used in this research meets the criteria for Grade I FA. X-ray diffraction (XRD) analysis revealed that the FA is primarily composed of three mineral phases, quartz, mullite, and rutile, as illustrated in Figure 3.

2.1.3. Calcium Carbide Residue

The calcium carbide residue (CCR) used in this study was sourced from Shandong Wanhua Chemical Chlor-Alkali Thermal Power Co., Ltd. The material is gray in color and was sieved through a 0.075 mm mesh prior to use. The physical and chemical properties of CCR are presented in Table 1, and its particle size distribution curve is shown in Figure 1. The main chemical components of CCR are listed in Table 2, while its XRD pattern is illustrated in Figure 4. The primary mineral phases identified in CCR are calcium hydroxide and calcite. Upon dissolution in water, CCR exhibits strong alkalinity, with a pH greater than 12 (solid-to-liquid ratio of 1:10 based on mass).

2.2. Methods

2.2.1. Test Procedure

This study aimed to examine the effects of varying CCR dosages on the mechanical properties of composite binders with different GGBS-to-FA ratios. GGBS and FA were used as the primary materials, with CCR serving as an external alkali activator. Prior to the commencement of this study, preliminary experiments were conducted to evaluate the workability and strength of materials under different water-to-binder (w/b) ratios. The results indicated that a w/b ratio of 0.55 offered the best balance between workability and strength. Consequently, this ratio was maintained throughout the study to ensure consistency and comparability. Drawing on relevant studies by other researchers [37,38], the CCR dosages were set at 2%, 4%, 6%, 8%, 10%, and 12% of the total mass of GGBS and FA. The mass ratios of GGBS to FA were adjusted to 0:10, 2:8, 4:6, 6:4, 8:2, and 10:0, respectively. The specimens were cured for 3, 7, 14, and 28 days to evaluate the effects of curing time on mechanical performance. A total of 39 mix designs were prepared, and the detailed mix proportions are listed in Table 3.

2.2.2. Sample Preparation and Test Methods

Following the proportions outlined in Table 3, the required amounts of CCR, GGBS, and FA were accurately weighed and thoroughly mixed before adding water. The mixture was then placed into a mixer, with the speed set to 30 rpm, and stirred for 3 min. After mixing, the blend was poured into standard molds with dimensions of 40 × 40 × 40 mm and compacted using vibration. Once the specimens were molded, they were covered with plastic film and left to cure at room temperature for 24 h before demolding. After demolding, the specimens were further cured at room temperature for periods of 3, 7, 14, and 28 days. The experimental procedure is illustrated in Figure 5, and the detailed testing process is described as follows.

1. The unconfined compressive strength (UCS) test was performed on specimens cured for 3, 7, 14, and 28 days, with a loading rate of 1 mm/min. Each group comprised three parallel specimens, and each specimen was tested three times under consistent experimental conditions. The UCS values from all tests were recorded, and the average of these values was taken as the representative UCS for the group. All tests were conducted using a universal testing machine manufactured by Shenzhen Suns Technology Stock Co., Ltd., Shenzhen, China.

2. pH measurement. After the UCS tests, representative sample fragments were immersed in anhydrous ethanol for 24 h to halt the hydration process. The fragments were then dried at 60 °C for 24 h and subsequently ground into powder. According to the Chinese standard “Geotechnical Testing Method” (GB/T 50123—2019), 5 g of the powder was mixed with 50 g of deionized water and stirred at 180 rpm for 30 min. After standing for 30 min, the pH of the suspension was measured using a pH meter with an accuracy of 0.01. The pH meter used was the PH-100A model, produced by Zhejiang Lichen Instrument Technology Co., Ltd., Shaoxing, China.

3. X-ray diffraction (XRD) analysis: The powder obtained in step (2) was passed through a 0.075 mm (200 mesh) sieve for X-ray diffraction (XRD) analysis. The analysis was carried out using an Empyrean X-ray diffractometer (PANalytical, Almelo, The Netherlands). The operating conditions were as follows: a tube voltage of 40 kV, a tube current of 40 mA, a wavelength λ = 0.15406 nm, a 2θ scanning range from 5° to 80°, a step size of 0.01° per step, and a scanning speed of 10°/min.

4. SEM-EDS analysis: Dried fragments from step (2) were processed into approximately 5 mm flake-shaped particles for analysis. Scanning electron microscopy (SEM), combined with energy-dispersive X-ray spectroscopy (EDS), was used to examine the microstructure and determine the elemental composition. The analysis was conducted using a Tescan Mira 4 field-emission scanning electron microscope (Brno, Czech Republic), operating at an accelerating voltage of 15 kV and a magnification of 2000×. The energy-dispersive spectrometer (EDS) used in this study was an Ultim Max 65 manufactured by Oxford Instruments, Oxford, UK.

3. Results and Discussion

3.1. Effect of CCR Doping and GGBS-FA Ratio on the Strength of Materials

Figure 6 presents the unconfined compressive strength (UCS) results of the composite cementitious material cured for 3 days. The results indicate that the ratio of GGBS to FA significantly affects the strength of the composite material. As shown in Figure 6a, when the CCR content is below 8%, the strength of the composite cementitious material increases significantly with the increase in GGBS content (and the corresponding decrease in FA content) [39]. This is because GGBS, compared to FA, exhibits higher reactivity under the activation of CCR, providing abundant calcium elements and additional nucleation sites [40,41,42], which further promote the formation of C-(A)-S-H gel [43], thereby enhancing the hardening properties of the material. Notably, when the GGBS-FA ratio is 10:0 and the CCR content is 2%, 4%, and 6%, the compressive strengths of the samples reach maximum values of 8.31 MPa, 11.58 MPa, and 14.31 MPa, respectively.

Moreover, it is evident from the figure that as the FA content increases, the UCS of the composite material gradually decreases, particularly when the GGBS-FA ratio is 0:10, and the compressive strength reaches its lowest values of 0.25 MPa, 0.26 MPa, and 0.29 MPa, respectively. This reduction in strength is attributed to the lower reactivity of the material when the GGBS content is reduced, resulting in a decrease in the amount of C-(A)-S-H gel produced during the hydration process, which in turn lowers the overall strength of the material.

As shown in Figure 6b, with the increasing dosage of CCR (8%, 10%, and 12%), the unconfined compressive strength (UCS) of the composite cementitious materials increases with a higher GGBS content. The UCS reaches its maximum values when the GGBS/FA ratio is 8:2, with values of 14.41, 14.98, and 15.20 MPa, respectively, which are higher compared to other GGBS-FA ratios. This behavior can be explained as follows: at lower CCR dosages (CCR < 8%), the limited OH⁻ provided by CCR results in a relatively slow hydration reaction, leading to fewer hydration products. In contrast, when the CCR dosage is ≥ 8%, the abundant OH⁻ enhances the alkalinity of the G10F0 system, effectively activating the hydration of GGBS. Additionally, the high calcium content in GGBS (as shown in Table 2) promotes a rapid hydration reaction within the composite material. This leads to the formation of significant amounts of hydration products, such as C-(A)-S-H gel, with an ideal Ca/Al molar ratio of 1.5–2.5. However, in the G10F0 system, the calculated Ca/Al molar ratio is 4.42, which is much higher than the ideal range. This excess hydration of CaO generates excessive heat, causing the formation of microcracks during the setting process (as shown in Figure 7), which damages the structural integrity and ultimately reduces both the UCS and overall stability of the composite material [44].

In contrast, the inclusion of FA moderates the hydration reaction rate, thereby enhancing the toughness of the composite material. Samples containing FA exhibit no significant cracking during the curing process. As demonstrated in Figure 6b, for CCR content values of 8%, 10%, and 12%, the UCS of samples with a GGBS-to-FA ratio of 8:2 is consistently higher than that of samples with a GGBS-to-FA ratio of 10:0.

In summary, low CCR content is insufficient in activating the performance of the blended material, while an excessively high CCR content overly activates the 100% GGBS system, resulting in material defects. To fully harness the material’s potential while enhancing the toughness of the cementitious system, the addition of FA is necessary when CCR content is ≥8%, as indicated by the above analysis. Thus, the optimal CCR content is determined to be ≥8%, with the ideal GGBS-to-FA ratio being 8:2. This ensures an adequate supply of OH⁻ from the alkali activator while minimizing defects that could compromise mechanical properties. The FA content is fixed at 20% of the blended system, as higher FA levels reduce hydration efficiency and adversely affect early-age performance. This observation is consistent with the findings of Luo [45].

3.2. Optimal Ratio of CCR

Figure 8 illustrates the effect of varying CCR dosages on the unconfined compressive strength (UCS) of the composite cementitious material with a GGBS-FA ratio of 8:2. The test results show that as the CCR dosage increases, the UCS of the composite material gradually rises, eventually reaching a plateau. This indicates that while the increase in CCR dosage provides abundant OH⁻ ions to activate the composite material, the reactive components in GGBS and FA that can be activated by the alkali are limited. Therefore, the continued increase in CCR dosage does not result in a proportional and unlimited growth in the strength of the composite material. Once the CCR dosage exceeds a certain threshold, the rate of strength increase gradually decreases, as shown in Table 4.

In practical engineering applications, it is necessary to balance material dosage, strength, and strength growth rate to enhance material utilization efficiency. Based on the above analysis, this study proposes a comprehensive optimization method to maximize the activation of the GGBS/FA = 8:2 composite. The specific steps are as follows:

(1) The strength growth rate for each dosage point is calculated based on the experimental data. Based on Equation (1) and the data from Figure 8, the strength growth rate (λ) for different CCR dosages is determined, with the results shown in Table 4.

(1)$\lambda ={\displaystyle \frac{S_i-S_{i-1}}{C_i-C_{i-1}}}$

S_i represents the strength value at the i−th dosage point, in MPa; C_i denotes the corresponding dosage, in %.

(2) The average strength $\overline{S}$ of all the test samples is determined. The calculation is performed using Equation (2), and the results are presented in Table 4.

(2)$\overline{S}={\displaystyle \frac{1}{n}}{\displaystyle \sum _{i=1}^n{S}_i}$

In this equation, n represents the total number of dosage levels, with n = 6.

(3) Dosage levels with strength values exceeding the average are selected to establish a candidate range. As shown in Table 4, the dosages where the strength values surpassed the average are 8%, 10%, and 12%.

(4) From the candidate range identified in Step (3), the dosage with the highest growth rate is selected as the optimal dosage. According to Table 4, the growth rate at 8% is 93.95%, which is higher than the rates observed at 10% and 12%. Therefore, 8% is selected as the optimal CCR dosage for the GGBS-FA ratio of 8:2.

3.3. Effect of Maintenance Time on the Strength of Composite Cementitious Materials

Figure 9 illustrates the unconfined compressive strength (UCS) of composite cementitious materials activated with 8% CCR, containing varying ratios of GGBS and FA, at curing periods of 3, 7, 14, and 28 days. As shown in Figure 9, the compressive strength of the composite materials increases progressively with longer curing times. Taking G8F2 as an example, the UCS values at 3, 7, 14, and 28 days are 14.41, 16.76, 17.61, and 18.04 MPa, respectively. Compared to the samples cured for 3 days, the strength values of the samples cured for 7, 14, and 28 days increase by 16.31%, 22.21%, and 25.19%, respectively.

From these data, it can be observed that the compressive strength of the specimens does not show a significant increase with extended curing time. Additionally, the compressive strength of the composite cementitious material at 3 days reaches 79.88% of its 28-day strength, indicating excellent early strength performance and suggesting promising potential for engineering applications. This early strength characteristic implies that the composite material can reduce construction time and costs in practical engineering projects. Moreover, it is noteworthy that throughout the entire curing period, the compressive strength of the G8F2 samples consistently exceeds that of samples with other ratios. This further demonstrates that the combined use of GGBS and FA can optimize the material properties, achieving an optimal performance state.

3.4. Effect of GGBS-FA Ratio and Curing Time on pH

Figure 10 shows the pH variation in cementitious materials with different GGBS-FA ratios activated by 8% CCR over different curing periods. As illustrated in the figure, for a fixed curing period, the pH value of the composite cementitious material gradually increases with a higher GGBS content (and lower FA content). Using a curing time of 3 days as an example, as the GGBS content in the system increases from 0% to 100%, the pH of the composite system rises from 12.34 to 12.70, representing a 2.91% increase. This suggests that the hydration reaction of GGBS, activated by CCR, releases a greater amount of OH⁻, thereby elevating the system’s pH. The GGBS used in this study contains 35.30% CaO, which provides an abundant source of CaO for the alkali activation reaction, leading to the formation of more C-(A)-S-H gel and the release of additional OH⁻. Moreover, the experimental results indirectly confirm the synergistic effect between CCR and GGBS during the hydration process, which enhances the system’s alkalinity by generating more OH⁻.

With a constant GGBS/FA ratio, the pH values of the composite cementitious materials gradually decrease as the curing time is extended. For instance, in the G8F2 system, the pH values at 3, 7, 14, and 28 days of curing are 12.60, 12.46, 12.31, and 12.29, respectively. The pH at 3 days is notably higher than at 7, 14, and 28 days. This decline occurs because, as curing progresses, OH⁻ ions are gradually consumed in activating the reactions of FA and GGBS, which promotes the formation of the C-(A)-S-H gel. While this process enhances the material’s mechanical strength, it also results in a steady decrease in the system’s pH.

It should be noted that although the results in Section 3.1 indicate that the compressive strength of G8F2 is greater than that of G10F0, this does not contradict the finding in this section that the pH value of G8F2 is lower than that of G10F0. This is because a high GGBS content results in the release of more OH⁻ ions, thereby increasing the pH of the composite material. However, compressive strength is not solely dependent on pH; it is also affected by factors such as the formation of hydration products, reaction activity, and the densification of the internal structure. Although the pH of G10F0 is higher, the addition of FA in G8F2 improves the toughness and internal structure of the material, ultimately resulting in a higher compressive strength compared to G10F0.

3.5. XRD and SEM-EDS Analysis

Scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and X-ray diffraction (XRD) analyses were conducted on samples with GGBS-FA ratios of 0:10, 4:6, 8:2, and 10:0 after a curing period of 28 days. These analyses aim to reveal the microstructure, elemental distribution, and crystalline phase composition of GGBS-FA composites activated by CCR, providing a deeper understanding of their reaction mechanisms.

As shown in Figure 11a, the SEM image of the G0F10 sample reveals a relatively loose particle arrangement with poor structural integrity, a limited amount of gel products, and a significant presence of unreacted FA particles. In conjunction with the UCS strength test results from Section 3.1, this indirectly confirms that an excessive amount of FA adversely affects the UCS of the composite cementitious material [46,47]. This deficiency primarily arises from two factors: first, the inherently low reactivity of FA, and, second, the lack of sufficient calcium sources in the system, which leads to a slow hydration reaction and hinders the formation of C-(A)-S-H and other flocculent or clustered gel products, ultimately resulting in insufficient strength development in the sample. Additionally, an energy-dispersive spectroscopy (EDS) mapping analysis was performed on region A in Figure 11a, with the elemental composition shown in Figure 11b. The data indicate that the calcium content in the system is relatively low, at only 1.96%, while the aluminum and silicon contents are 13.74% and 14.96%, respectively. Moreover, small amounts of iron (0.46%) and titanium (0.38%) are also detected.

As illustrated in Figure 11c, the XRD analysis of the G0F10 sample identifies six minerals: quartz, mullite, calcite, calcium aluminate, hematite, and rutile. As illustrated in Figure 11c, the XRD analysis of the G0F10 sample identifies six mineral phases: quartz, mullite, calcite, calcium aluminate, hematite, and rutile. Quartz, mullite, hematite, and rutile originate from the mineral phases of FA, with hematite and rutile being non-reactive in the geopolymerization process [27]. Mullite is primarily observed at 2θ angles of 16.39°, 35.2°, and 40.8°, while quartz is predominantly detected at 2θ angles of 20.82° and 26.22°. Calcite mainly originates from the carbonation reaction of CCR, and its low peak intensity, as seen in the figure, aligns with the low calcium content observed in the EDS analysis. A small amount of calcium aluminate is formed following the reaction between calcium elements from CCR and aluminum elements from FA [48].

As illustrated in Figure 12, the microstructural analysis of the G4F6 sample reveals significant improvements. Figure 11a shows that gel-like substances fill the voids between particles, resulting in a marked increase in material density compared to G0F10. The EDS data in Figure 12b indicate that with the increased GGBS content, the calcium concentration in the G4F6 system reaches 8.62%, representing a substantial increase relative to G0F10. In contrast, the concentrations of aluminum and silicon decrease to 8.88% and 9.93%, respectively. Additionally, 0.17% sodium was detected. The XRD analysis in Figure 12c identifies the formation of C-(A)-S-H gel, with diffraction peaks observed at 2θ angles of 6.66°, 20.80°, 36.94°, and 49.43°. The formation of C-(A)-S-H gel suggests an enhancement in the system’s hydration reaction, which is closely related to the incorporation of GGBS. The additional calcium provided by GGBS significantly boosts the hydration activity and promotes the formation of hydration products. Lloyd et al. [49], through microscopic analysis, demonstrated that the calcium source in alkali-activated FA-GGBS systems is critical for the development of early strength.

The analysis of Figure 13a,c reveals that the G8F2 sample, compared to G0F10 and G4F6, exhibits distinct formations of plate-like calcium hydroxide, needle-shaped ettringite (Ca₆Al₂(SO₄)₃(OH)₁₂·26H₂O), and C-(A)-S-H gel. The formation of these mineral phases significantly enhances the material’s compactness, reducing the number of pores and defects. Furthermore, the gel substances fill the surface and most of the pores in the composite binder, creating a continuous and uniform microstructure. This is consistent with the findings of Kumar et al. [10], who suggested that this may be attributed to the formation of gel phases, which result in a denser microstructure.

Ettringite is formed through the reaction of Al₂O₃, calcium sources, and sulfate ions (SO₄²⁻, hydrated from SO₃), derived from the GGBS-FA system, under alkaline conditions, as described in Equation (3). The diffraction peaks corresponding to ettringite are observed at 2θ angles of 6.66°, 20.80°, 36.94°, and 49.43°. Additionally, the EDS data from Figure 13b indicate the presence of 1.32% sulfur (S) in the G8F2 system, further confirming the formation of ettringite. This finding underscores the critical role of sulfur, specifically in the form of sulfate ions, in the ettringite formation process, highlighting its essential contribution to the material’s microstructural development.

(3)$6\mathrm{CaO}·{\mathrm{Al}}_2{\mathrm{O}}_3·3{\mathrm{SO}}_3{+26\mathrm{H}}_2\mathrm{O}\to {\mathrm{Ca}}_6{\mathrm{Al}}_2\left({\mathrm{SO}}_4\right)3{\left(\mathrm{OH}\right)}_{12}·{26\mathrm{H}}_2\mathrm{O}$

As shown in Figure 14a, compared to G0F10, G4F6, and G8F2, the pores in the G10F0 sample are largely filled with gel-like substances, significantly enhancing the density of the material, with almost no visible pores or defects on the sample surface. Relevant studies [50,51] have shown that an increased calcium source enhances the solubility of silicon and aluminum, leading to the formation of C-(A)-S-H products and contributing to a denser and more compact microstructure.

It can be seen from Figure 14b that the Si element in the material is significantly reduced, which is reflected in the XRD pattern by a notable decrease in the quartz peak, as shown in Figure 14b. Calcite and calcium hydroxide become the main components of the material, accompanied by a small amount of C-(A)-S-H gel and ettringite formation. Furthermore, with the increase in GGBS content, the concentrations of sulfur (S) and sodium (Na) also increase, reaching 1.77% and 0.80%, respectively. This suggests that the S and Na elements primarily originate from GGBS. Additionally, the presence of a small amount of Al₂O₃ in GGBS provides the necessary conditions for the formation of C-(A)-S-H gel.

Additionally, EDS data for G4F6, G8F2, and G10F0 samples indicate the presence of trace amounts of sodium (Na). However, no significant Na-(A)-S-H gel is observed in the mineral phases of these samples. This phenomenon may be attributed to the following reason: in alkaline-activated systems, when sodium-based activators (such as NaOH or Na₂SiO₃) are used to activate low-calcium materials, Na⁺ ions replace Ca²⁺ ions as the primary cation and combine with dissolved Si⁴⁺ and Al³⁺ to form a three-dimensional network structure, resulting in the generation of Na-(A)-S-H gel. However, in the material system of this study, primarily high-calcium materials (such as CCR and GGBS) are utilized, with GGBS containing only trace amounts of Na⁺. Furthermore, the high calcium content in the system hinders the dominance of Na⁺ in the gel formation process. Consequently, the predominant gel phase generated is C-(A)-S-H rather than Na-(A)-S-H.

3.6. The Mechanism of Action of CCR-FA-GGBS Binder

Based on the microstructural analysis and experimental results, the internal interactions within the composite binder system can be divided into three stages, as illustrated in the chemical reaction schematic (Figure 15).

• (1). Alkaline activation stage. The primary component of CCR, calcium oxide (CaO), undergoes hydration to produce calcium hydroxide (Ca(OH)₂), which significantly raises the pH of the system. This high alkalinity is critical for activating GGBS and FA, facilitating the dissolution of their components and creating favorable conditions for the formation of calcium silicate hydrate (C-S-H) and calcium aluminum silicate hydrate (C-(A)-S-H) gels. These hydration products fill voids and increase density, thereby improving the overall strength and stability of the composite material (Figure 14). Previous studies have demonstrated that such alkaline conditions are essential for optimizing the reactivity of industrial by-products, such as slag and FA, in cement-based systems [52].

• (2). Hydrolysis and dissolution stage. At this stage, the high concentration of hydroxide ions (OH⁻) disrupts the Si-O-Si and Al-O-Al bonds in slag and FA. This disruption promotes the dissolution of silicate and aluminate phases, releasing calcium (Ca²⁺), magnesium (Mg²⁺), silicate (SiO₃²⁻), and aluminate ([Al(OH)]₄⁻) ions, as shown in the equations provided in the manuscript. These dissolved ions are critical for the subsequent formation of hydration products. This process is consistent with findings reported by An et al. [53], who observed similar dissolution phenomena in alkali-activated slag systems, which enhanced gel formation

• (3). Gel reorganization stage. Calcium ions (Ca²⁺) react with active silicate (SiO₃²⁻) and aluminate ([Al(OH)]₄⁻) ions to form amorphous CSH and C-(A)-S-H gels. Initially, these gels exhibit a loose structure but provide strong adhesion and stability to the material. As hydration progresses, silicon (Si) sites in the CSH structure are gradually replaced by aluminum (Al), resulting in the formation of more stable and complex C-(A)-S-H gels. This process fills internal voids within the material, increasing its density and strength. Additionally, the carbonation of residual Ca(OH)₂ further enhances the material’s density and stability through reactions (e.g., Equation (10)), leading to the formation of calcite (CaCO₃). This secondary consolidation process significantly improves the material’s long-term durability and resistance to chemical degradation, as reported in previous studies [54,55].

These stages collectively explain the mechanisms behind the strength and durability of the composite binder. The high alkalinity provided by CCR, combined with the synergistic activation of GGBS and FA, leads to the formation of stable gel phases that contribute to the enhanced mechanical performance of the material.

(4)${\mathrm{CaO}+\mathrm{H}}_2\mathrm{O}\to {\mathrm{Ca}\left(\mathrm{OH}\right)}_2\to {\mathrm{Ca}}^{2+}{+\mathrm{OH}}^{-}$

(5) ${\mathrm{Ca}}_2{\mathrm{Al}}_2{\mathrm{SiO}}_7{+\mathrm{OH}}^{-}{+\mathrm{H}}_2\mathrm{O}\to {2\mathrm{Ca}}^{2+}{+2\left[\mathrm{Al}\right(\mathrm{OH})}_4{]}^{-}{+(\mathrm{SiO}}_3{)}^{2-}$

(6) ${\mathrm{Ca}}_2{\mathrm{MgSi}}_2{\mathrm{O}}_7{+\mathrm{OH}}^{-}+{\mathrm{H}}_2\mathrm{O}\to {2\mathrm{Ca}}^{2+}{+\mathrm{Mg}}^{2+}{+2(\mathrm{SiO}}_3{)}^{2-}$

(7) ${\mathrm{SiO}}_2{+2\mathrm{OH}}^{-}\to {{(\mathrm{SiO}}_3)}^{2-}{+\mathrm{H}}_2\mathrm{O}$

(8) ${\mathrm{Al}}_2{\mathrm{O}}_3{+2\mathrm{OH}}^{-}{+3\mathrm{H}}_2\mathrm{O}\to 2{{\left[\mathrm{Al}\right(\mathrm{OH})}_4]}^{-}$

(9) ${\mathrm{Ca}}^{2+}{+(\mathrm{SiO}}_3{)}^{2-}{+\left[\mathrm{Al}\right(\mathrm{OH})}_4{]}^{-}{+\mathrm{H}}_2\mathrm{O}\to \mathrm{C-(A)-S-H}$

(10) ${\mathrm{Ca}\left(\mathrm{OH}\right)}_2{+\mathrm{CO}}_2\to {\mathrm{CaCO}}_3{+\mathrm{H}}_2\mathrm{O}$

4. Conclusions

This study utilized CCR as an alkali activator to develop a low-carbon binder based on GGBS and FA through systematic experiments and optimization methods. The research aims to explore the potential of resource utilization for industrial by-products and provide an alternative to traditional cement. The main conclusions are as follows:

• (1). Through systematic experiments and optimization analysis, this study determined that the optimal mass ratio of GGBS to FA was 8:2, with the optimal CCR dosage being 8%. These parameters provide reliable design guidelines for improving material performance.

• (2). Under the conditions of 8% CCR dosage and a GGBS/FA ratio of 8:2, the high GGBS content in the alkaline environment produced abundant calcium aluminum silicate hydrate (C-(A)-S-H) gel, which significantly enhanced the structural stability and durability of the material. SEM-EDS and XRD analyses revealed that these hydration products effectively filled the micropores, reduced porosity, and significantly improved compressive strength.

• (3). The SO₃ content in GGBS provided an ample source of sulfate ions for ettringite formation. Through reactions with the calcium source from CCR and the aluminum source from FA, the resulting ettringite not only improved the early strength of the material but also further optimized the microstructure.

• (4). This study innovatively utilized CCR as an alkali activator, achieving efficient utilization of industrial by-products. Compared to traditional chemical activators, CCR offers a new pathway for developing low-carbon, efficient, and cost-effective materials, demonstrating broad application prospects for sustainable construction materials.

Author Contributions

Investigation, methodology, data curation, writing—original draft, formal analysis, and visualization: X.L.; conceptualization, validation, resources, writing—review and editing, and funding acquisition: G.X.; investigation, data curation, and writing—review and editing: D.Y.; investigation, supervision, and writing—review and editing: L.D.; investigation and writing—review and editing: A.T. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data supporting the findings of this study are available upon request.

Conflicts of Interest

The authors declare that they have no relevant conflicts of interest that could influence the outcomes of this study.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures and Tables

Figure 1. Particle size grading curves of CCR, GGBS, and FA.

Figure 2. XRD spectrum of GGBS.

Figure 3. XRD spectrum of FA.

Figure 4. XRD spectrum of CCR.

Figure 5. The whole process of the test.

Figure 6. UCS of specimens with different GGBS/FA ratios at different CCR doping levels. (a) CCR = 2, 4, and 6%; (b) CCR = 8, 10, and 12%.

Figure 7. Specimens with 8% CCR, with a GGBS/FA ratio of 10:0.

Figure 8. The effect of CCR dosage on the UCS of G8F2.

Figure 9. Effect of maintenance time on UCS of composite cementitious materials.

Figure 10. Effect of FA-GGBS ratio and curing time on pH value.

Figure 11. Microscopic test results of G0F10: (a) morphology, (b) energy spectrum of region A; and (c) XRD.

Figure 12. Microscopic test results of G4F6: (a) SEM morphology, (b) energy spectrum of region B, and (c) XRD.

Figure 13. Microscopic test results of G8F2: (a) SEM morphology, (b) energy spectrum of region C, and (c) XRD.

Figure 14. Microscopic test results of G10F0: (a) SEM morphology, (b) energy spectrum of region D, and (c) XRD.

Figure 15. Chemical reaction process of CCR to stimulate GGBS-FA cementitious material.

Table 1

Physical and chemical properties of GGBFS, FA, and CCR.

Material	Density/(g·cm−3)	Specific surfacearea/(m2·kg−1)	Activity Index/%
GGBS	2.93	3112.67	115
FA	2.34	2090.67	85
CCR	2.24	966.33	-

Table 2

Chemical composition of GGBS, FA, and CCR.

Materials	GGBS/%	FA/%	CCR/%
SiO2	34.50	53.96	1.55
Al2O3	16.70	31.14	1.96
Fe2O3	1.50	4.16	0.48
CaO	35.30	4.01	67.87
MgO	5.01	1.01	0
SO3	1.24	0.72	0.31
K2O	--	2.03	--
TiO2	--	1.13	--
Na2O	--	0.88	--
P2O5	--	0.67	--
Loss on ignition (LOI)	0.96	--	25.85
Moisture content	0.20	--	0.50

Table 3

Mix proportion for the experiment.

Groups	CCR/%	GGBS/%	FA/%	Water-Cement Ratio	Curing Time/d
A	2, 4, 6, 8, 10, 12	0	100	0.55	3
		20	80
		40	60
		60	40
		80	20
		100	0
B	8	80	20	0.55	7, 14, 28

Table 4

Strength growth rate and average strength.

CCR/%	$\mathit{\lambda }$/%	$\overline{\mathit{S}}$/Mpa
2	380.50	12.55
4	148.35
6	97.50
8	93.95
10	28.55
12	11.00

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