Reaction Sequence Coordination in Ternary Solid-Waste Systems for Low-Carbon Cementitious Materials

Abstract

Using solid waste as supplementary cementitious materials (SCMs) is an effective strategy for promoting low-carbon construction development. However, single or binary systems often exhibit mismatched reaction kinetics, thereby limiting their performance at high cement replacement rates. This study focuses on a novel low-carbon concrete designed based on reaction sequence coordination, containing recycled brick powder (RBP), ground granulated blast-furnace slag (GGBS), and self-combusting coal gangue (SCCG). The effects of RBP, GGBS, and SCCG on the hydration process and microstructure of the novel low-carbon concrete with different replacement levels have been studied by testing compressive strength, workability, and durability and observing microstructural changes. The results showed that an optimized ternary composition with an RBP:GGBS:SCCG ratio of 4:3:1 achieves a cement replacement level of 30% while exhibiting a 28-day compressive strength of 38.26 MPa, representing a 14.2% increase compared with plain cement mortar. Microstructural analyses indicate that this enhanced performance results from a time-dependent reaction sequence, in which GGBS contributes predominantly at early ages by supplying calcium, whereas RBP and SCCG mainly participate through delayed pozzolanic reactions and pore refinement at later ages. Consequently, the optimized ternary mortar exhibits a water absorption of 11.12% and a 27.2% reduction in electrical flux. This study aims to provide practical strategies for enhancing the performance of low-carbon cementitious materials through a reaction sequence coordination design approach, thereby improving the utilization efficiency of solid waste in the production of low-carbon building materials.

1. Introduction

Cement is one of the most widely used materials in the global construction industry. However, this production process is accompanied by substantial carbon emissions, posing significant challenges to the green transition and sustainable development of the construction industry [1,2,3]. Therefore, developing cement substitutes has become a key strategy for the construction industry to achieve decarbonization [4,5,6]. Among numerous alternatives, construction and demolition (C&D) waste has garnered particular attention due to its recyclability, reusability, and substantial generation volume [7,8,9]. Recent studies have explored multiple application pathways of C&D waste in construction materials [10]. For instance, recycled brick powder shows great promise for use in water-based coatings due to its low water absorption and high hardness [11]. Recycled aggregates modified with nano-titanium dioxide (including recycled glass, clay bricks and concrete sand) can improve the rheological properties of concrete [12,13,14]. Recycled aggregates made from construction and demolition waste exhibit significantly enhanced sulphate resistance following carbonation or nano-silica impregnation treatment [15]. Using C&D wastes as cement substitutes not only reduces cement consumption but also provides a sustainable solution to alleviate the growing pressure of waste accumulation [16,17,18].

Recycled brick powder (RBP), derived from C&D waste, has been widely investigated as a potential supplementary cementitious material because of its high SiO₂ and Al₂O₃ contents, pozzolanic reactivity, and micro-filling effect [19,20,21]. Numerous studies have shown that RBP can participate in secondary hydration reactions by consuming Ca(OH)₂ and forming additional cementitious gels, thereby contributing to strength development and pore structure refinement at later ages [22,23,24]. In addition to RPB, Other clay-based materials also exhibit similar pozzolanic activity [25,26,27]. For example, when untreated kaolin and claystone are calcined at 800 °C and used as partial substitutes for cement, their performance levels can reach 106% to 132% [28]. Similarly, when lime-rich calcined clay is used as a partial substitute for cement, the mechanical properties and durability it exhibits are comparable to those of ordinary mortar [29]. Nevertheless, the relatively low early-age reactivity of RBP significantly restricts its application at high cement replacement levels. When used as the sole cement substitute, excessive incorporation of RBP often results in insufficient early strength, increased porosity, and deteriorated durability performance [30,31,32].

To address these limitations, binary systems incorporating RBP with more reactive supplementary cementitious materials, particularly ground granulated blast-furnace slag (GGBS), have been widely investigated. The addition of GGBS can partially compensate for the delayed reactivity of RBP by supplying calcium-rich hydration products at early ages, thereby enhancing early strength development and matrix continuity [33,34]. Therefore, RBP–GGBS systems generally exhibit superior early-age mechanical performance compared with mixtures containing RBP alone.

Despite these improvements, most published studies still limit the cement replacement rate in such binary systems to relatively low levels to maintain stable mechanical properties and durability. At higher substitution rates, the reduction in cement content, coupled with the imbalance between calcium ion availability and the reactivity of aluminosilicates, may impede sustained hydration and pore refinement, thereby limiting the achievable reduction in cement consumption and the associated carbon emissions [35,36]. Alternatively, binary systems combining RBP with aluminosilicate-rich materials, such as metakaolin or coal gangue, have been reported to improve later-age pozzolanic activity and durability-related properties [37,38,39]. However, due to the limited early-age calcium supply and the inherently slower reaction kinetics of these materials, such systems often exhibit insufficient early framework formation, leading to reduced early strength and compromised workability. Overall, existing single or binary solid-waste-based cementitious systems generally face the challenge of achieving a balance between early-stage performance, long-term durability, and achievable cement replacement rates. These observations show that the key limitation of cementitious materials with high solid waste content lies not only in the inherent reactivity of individual supplementary cementitious materials (SCMs), but more critically in the lack of effective coordination between their hydration and pozzolanic reactions over time. In multicomponent cementitious systems, macroscopic performance is primarily governed by the time-scale-dependent synergy among early-age hydration, calcium availability, delayed aluminosilicate reactions, and progressive pore structure refinement. Simply increasing the replacement level or indiscriminately combining multiple solid wastes does not necessarily lead to synergistic performance enhancement.

Based on this perspective, a reaction-sequence-coordinated design concept for multicomponent cementitious systems is proposed in this study. In the proposed ternary system, GGBS is introduced to ensure sufficient early-age calcium availability and matrix continuity, thereby supporting initial hydration and early strength development. Recycled brick powder and self-combusting coal gangue (SCCG) are incorporated as comparatively low-reactivity components that preferentially contribute at later curing stages through delayed pozzolanic reactions and microstructural refinement. In addition, the porous morphology of SCCG may provide internal space and moisture reservoirs for hydration product growth, thereby promoting sustained reactions and interfacial densification.

Although ternary composite systems have been extensively studied, most existing research has focused primarily on optimizing mixing ratios or combining materials with complementary properties. In contrast, this study proposes a fundamentally different approach that emphasizes coordinating the reaction sequences among the multiple components. Accordingly, this study aims not merely to present another ternary solid-waste cementitious system with improved properties, but to investigate whether coordinating the functional roles and reaction kinetics of different SCMs across curing stages can alleviate the trade-off between early-age strength, durability, and cement replacement level. By systematically comparing the mechanical performance, workability, durability, and microstructural evolution of single, binary, and ternary mortar systems incorporating RBP, GGBS, and SCCG, this work aims to elucidate how early-age calcium supply, delayed aluminosilicate reactivity, and progressive pore refinement interact to govern mechanical performance and durability. The results provide a mechanistic basis for rational design of multicomponent solid waste cementitious materials in low-carbon construction.

2. Materials and Methods

2.1. Materials

2.1.1. Source of Raw Materials

The materials used in this study included Portland cement (PC, Grade 42.5), recycled brick powder (RBP), ground granulated blast furnace slag (GGBS), and self-combusting coal gangue (SCCG). The PC, conforming to Chinese National Standard GB 175-2007 [40] (Specifies technical requirements for general-purpose Portland cement, including fineness, setting time, stability, and strength grades), was sourced from a local supplier in Shenyang, China. Natural Yellow River sand, employed as fine aggregate, was provided by Weifang Jiujian Building Materials Co., Ltd. (Weifang, China). RBP was obtained from recycled waste bricks collected in Huanggu District, Shenyang, and processed into powder using a Laboratory-grade jaw crusher (China). GGBS was procured from a manufacturer in Gongyi City, Henan, China, while SCCG was supplied by a company in Chaoyang City, Liaoning, China. To enhance their reactivity, RBP, GGBS, and SCCG were mechanically activated for 15 min in an XQM-4 vertical planetary ball mill (China). Tap water was used for all mixtures.

2.1.2. Physical Properties of Raw Materials

The microstructures of recycled brick powder (RBP), ground granulated blast furnace slag (GGBS), and self-combusting coal gangue (SCCG) were examined by scanning electron microscopy (SEM), as presented in Figure 1. RBP particles showed a rough surface morphology, characterized by angular and fragment-like shapes with visible fracture surfaces, which can be attributed to the mechanical grinding process. GGBS particles mostly exhibited irregular polyhedral forms with smooth surfaces and fine sizes. In contrast, SCCG displayed a relatively rough and porous surface, containing distributed crystalline and amorphous mineral phases. The median particle size (D50) was determined to be 12.735 μm for RBP, 10.87 μm for GGBS, and 15.35 μm for SCCG (Figure 2). The corresponding specific surface areas were 207 m²/kg, 1198 m²/kg, and 2525 m²/kg, respectively.

2.1.3. Crystal Phases of Raw Materials

The mineral phase compositions of RBP, GGBS, and SCCG were characterized by XRD (Figure 3). RBP is predominantly quartz (SiO₂), GGBS is primarily amorphous with a broad hump characteristic of slag, and SCCG contains quartz (SiO₂), mica (KAl₂(AlSi₃O₁₀)(OH)₂), calcite (CaCO₃), and kaolinite (Al₂Si₂O₅(OH)₄). See

Table 1. Chemical and physical properties of PC, RBP, GGBS and SCCG.

Properties		PC	RBP	GGBS	SCCG
Chemical composition (wt.%)	SiO2	24.1	53.8	33.9	34.5
	Al2O3	6.77	20.31	18.2	17.7
	Fe2O3	2.58	10.01	1.11	1.03
	CaO	57.23	2.44	33.68	3.4
	MgO	2.91	1.63	5.79	6.01
	SO3	1.87	4.44	1.32	1.64
	LOI	2.01	-	-	-
Specific surface area (m2/kg)	-	2576.12	207	1198	2525

for the chemical composition.

The physical and chemical properties of raw materials directly influence the mixing, hydration and hardening processes. The rough, angular edges of RBP increase internal friction within the mixture; whilst this reduces workability, it improves inter-particle bonding. The smooth surface of GGBS facilitates flow, but its high specific surface area (1198 m²/kg) increases water demand. The porous structure of SCCG acts as a water reservoir, promoting late-stage hydration, but it often increases water absorption and reduces early strength. In terms of chemical composition, the higher CaO content in GGBS (33.68 wt.%) provides an early source of calcium, whilst the higher Al₂O₃ content in RBP (20.31 wt.%) and SCCG (18.2 wt.%) contributes to secondary pozzolanic reactions. The SiO₂ in RBP (53.8 wt.%) ensures pozzolanic activity. Although the high specific surface area of SCCG (2525 m²/kg) enhances reaction rates, it also increases water demand. In terms of phase behavior, the high crystallinity of quartz in RBP restricts early-stage reactions but enhances long-term stability. The amorphous structure of GGBS enhances hydration activity. The coexistence of crystalline and amorphous phases in SCCG confers moderate pozzolanic activity, whilst its porous structure, beneficial for internal curing, also increases water demand.

2.2. Experimental Design

2.2.1. Specimen Preparation

Waste clay bricks were first crushed into powder using a jaw crusher. The obtained brick powder was subsequently mixed with other solid waste materials, namely ground granulated blast furnace slag (GGBS) powder and self-combusting coal gangue (SCCG) powder. The mixed materials were dried in a constant-temperature oven at 105 ± 5 °C for 48 h to eliminate residual moisture. After drying, the mixture was further ground using an XQM-4 planetary ball mill (China)and sieved through a vibrating screen. The fraction with a particle size smaller than 75 μm was collected and designated as recycled brick powder (RBP) for subsequent use.

Cubic mortar specimens with a side length of 70 mm were prepared in accordance with the Chinese standard JGJ/T 70-2009 [41]. Cylindrical specimens with a diameter of 100 mm and a height of 50 mm were fabricated following GB/T 50082-2009 [42]. All specimens were cast and cured under standard laboratory conditions at room temperature.

2.2.2. Mix Design

A mortar prepared with ordinary Portland cement only was used as the control group and designated as BG-0. In the experimental groups, cement was partially replaced by RBP, GGBS, and SCCG. The experimental program was designed in three sequential stages. First, to evaluate the effects of different solid waste combinations, the 7-day and 28-day compressive strengths were measured for a single RBP system (BG-1), binary systems incorporating RBP and GGBS (BG-2) or RBP and SCCG (BG-3), as well as a series of ternary systems (BG-4 to BG-15). Second, with the total cement replacement level fixed at 30% by mass, the influence of RBP dosage was investigated by varying its proportion to 33%, 40%, 50%, and 60% of the total supplementary cementitious materials. Finally, based on the optimal ternary composition identified in the previous stage, the effect of the overall cement replacement level was examined at replacement ratios of 25%, 30%, 35%, and 40%. The detailed mix proportions of all mortar groups are summarized in

Table 2. Mixing ratio for test blocks (total binder mass = 540 g, water-to-binder ratio = 0.5).

Groups	RBP:GGBS:SCCG	PC (g)	RBP (g)	GGBS (g)	SCCG (g)	Fine Aggregate (g)
BG-0	--	540	0	0	0	1125
BG-1	--	378	162	0	0	1125
BG-2	--	378	81	81	0	1125
BG-3	--	378	81	0	81	1125
BG-4	2:1:1	378	81 (50%)	40.5	40.5	1125
BG-5	5:3:2	378	81	48.5	32.5	1125
BG-6	3:2:1	378	81	54	27	1125
BG-7	4:3:1	378	81	60.75	20.25	1125
BG-8	2:1:2	378	65	32.5	65	1125
BG-9	4:3:3	378	65 (40%)	48.5	48.5	1125
BG-10	2:2:1	378	65	65	32.5	1125
BG-11	4:5:1	378	65	81	16	1125
BG-12	3:1:1	378	97 (60%)	32.5	32.5	1125
BG-13	6:3:1	378	97	48.5	16.5	1125
BG-14	1:1:1	378	54 (33%)	54	54	1125
BG-15	3:4:2	378	54	65	32.5	1125
BG-16	4:3:1	405	67.5	50.625	16.875	1125
BG-17	4:3:1	351	94.5	70.875	23.625	1125
BG-18	4:3:1	324	108	81	27	1125

. It should be noted that the total binder mass (PC + SCMs) was kept constant at 540 g for all mixtures. The cement replacement ratio was defined as the mass percentage of supplementary cementitious materials (RBP, GGBS, and/or SCCG) relative to the total binder. Accordingly, a 30% replacement corresponds to 162 g of SCMs replacing an equivalent mass of cement. For binary systems (BG-2 and BG-3), the total SCM content was also fixed at 30% of the binder mass, while the internal proportions of RBP, GGBS, and SCCG were adjusted according to the specific mix design. The water-to-binder ratio was maintained at 0.5 for all mixtures, ensuring comparability among different groups.

2.2.3. Test Method

Compressive Strength Test

The compressive strength of mortar specimens was tested following the Chinese standard JGJ/T 70-2009 [41] (Test methods for basic properties of building mortar, including test procedures for consistency, segregation, density, compressive strength, and other parameters), using a continuous loading procedure until failure. Mortar specimens with dimensions of 70 mm × 70 mm × 70 mm were used for the compressive strength test. All specimens were cured at (20 ± 2) °C and a relative humidity of ≥95% until the specified curing ages (7 days and 28 days). Three parallel specimens were prepared for each mix design for the compressive strength test. The compressive strength test was conducted using a computer-controlled electro-hydraulic servo press (GYE 300B, China) with a loading rate of 2.4 kN/s. The test results were reported to an accuracy of 0.1 MPa.

Consistency Test

The workability of fresh mortar was evaluated by measuring its consistency (flow diameter) using a flow table test, as specified in JGJ/T70-2009 [41].

Water Absorption Test

Following 28 days of standard curing, specimens were oven-dried, cooled, and then placed on supports in a shallow tray. Distilled water was introduced to a depth of 35 mm below the specimen’s bottom surface. After being covered and maintained under constant conditions for 48 ± 0.5 h, the specimens were removed, surface moisture was wiped off, and they were immediately weighed to calculate the water absorption rate, following the procedure outlined in JGJ/T 70-2009 [41].

Electrical Flux Test

The resistance to chloride ion penetration was evaluated by measuring the total charge passed (electrical flux) through saturated specimens over a 6 h period, conducted in compliance with the Chinese standard GB/T 50082-2009 [42] (Specifies test methods for the long-term and durability properties of concrete).

X-Ray Diffraction Analysis

The mineralogical composition of raw materials (Figure 3) and the phase evolution of hydration products were analyzed using a Bruker D8 Advance X-ray diffractometer with Cu-Kα radiation. Scans were performed over a 2θ range of 5° to 70° at a step size of 0.02°.

Scanning Electron Microscopy Analysis

The microstructural development and morphology of hydration products, as well as the grain morphologies of raw materials (Figure 1), were examined using a ZEISS Gemini 300 field-emission scanning electron microscope. Samples were sputter-coated with gold prior to observation to ensure conductivity.

DTG-TG Analysis

The thermal decomposition behavior of hydration products was analyzed using a TA Instruments TGA 550 thermogravimetric analyzer. Powder samples extracted from 28-day pastes (systems BG-1, BG-2, BG-3, and BG-7) were heated from 30 °C to 800 °C at a rate of 10 °C/min under a nitrogen atmosphere.

Particle Size Distribution Measurement

The particle size distribution of the raw materials used in the test was measured using a Malvern Mastersizer 2000 laser particle size analyzer. The specific surface areas were determined according to GB/T 8074-2008 [43] (Method for determining the specific surface area of cement (Behring method)).

Chemical Composition Analysis by XRF

In this experiment, the chemical composition analysis was carried out using a Paralytical Axios X-ray fluorescence spectrometer.

2.2.4. Statistical Analysis

Three parallel specimens were prepared for the compressive strength test, and the results are presented as a bar chart showing the average values. For other tests, such as water absorption and electrical conductivity, one specimen was prepared for each test. The results are presented as measured values, and standard deviations are not calculated.

3. Results and Discussion

3.1. Mechanical Performance and Mix Design Optimization

The mechanical performance and mix design optimization of mortar systems incorporating different solid-waste combinations were evaluated based on compressive strength development.

Table 3. The compressive strength of the test.

Groups	RBP:GGBS:SCCG	Substitution Rate	7-Day Compressive Strength (MPa)	28-Day Compressive Strength (MPa)
BG-0	--	--	27.73	33.5
BG-1	--	30%	19.8	24.8
BG-2	--	30%	27.68	35.6
BG-3	--	30%	17.84	27.82
BG-4	2:1:1	30%	21.65	34.12
BG-5	5:3:2	30%	22.05	33.16
BG-6	3:2:1	30%	25.66	33.64
BG-7	4:3:1	30%	27.02	38.26
BG-8	2:1:2	30%	23.01	32.79
BG-9	4:3:3	30%	24.37	34.41
BG-10	2:2:1	30%	25.20	32.11
BG-11	4:5:1	30%	26.93	31.51
BG-12	3:1:1	30%	22.39	32.52
BG-13	6:3:1	30%	23.37	29.68
BG-14	1:1:1	30%	22.84	33.17
BG-15	3:4:2	30%	23.85	30.84
BG-16	4:3:1	25%	25.54	37.8
BG-17	4:3:1	35%	24.6	33.1
BG-18	4:3:1	40%	22.7	31.98

summarizes the mix proportions of all investigated mortar systems together with their compressive strength results. The internal ratios of recycled brick powder (RBP), ground granulated blast furnace slag (GGBS), and self-combusting coal gangue (SCCG), together with the cement substitution rate, are presented along with the corresponding 7-day and 28-day compressive strength values. Each reported compressive strength value represents the average of three parallel specimens tested for the same mixture. These results provide the basis for subsequent performance comparison and mix design optimization of the ternary cementitious system. The selected mixtures were chosen to represent key compositions that exhibited distinct performance characteristics, allowing for a more focused analysis of the underlying mechanisms.

3.1.1. Effect of Single and Binary Systems

The compressive strength results of mortars incorporating single and binary supplementary cementitious materials are summarized in Figure 4. The mortar containing only recycled brick powder (RBP, BG-1) exhibited a pronounced reduction in strength, with its 28-day compressive strength reaching only 74% of that of the plain cement mortar (BG-0). This behavior is primarily governed by the inherently low early-age reactivity of RBP, which limits its contribution to early hydration and leads to an insufficiently developed initial microstructure.

In contrast, the binary system incorporating RBP and ground granulated blast-furnace slag (GGBS, BG-2) showed a substantial improvement in compressive strength at both 7 and 28 days, even slightly exceeding the control mixture. This indicates that early-age strength development is governed by the dominant contribution of GGBS, which supplies calcium-rich hydration products and partially compensates for the delayed pozzolanic activity of RBP.

The RBP with self-combusting coal gangue (SCCG) binary system (BG-3) exhibited a different strength evolution pattern. Although its early-age strength remained relatively low, the 28-day compressive strength was higher than that of the RBP-only system. This improvement is closely associated with the porous morphology and mineral composition of SCCG, which provide internal space and favorable nucleation sites for secondary hydration products at later curing stages. However, compared with GGBS, the activation and reaction rate of SCCG are relatively slow, limiting its contribution to early-age strength development.

Overall, the results suggest that macroscopic strength development in these systems is governed by a time-dependent distribution of contributions, in which GGBS dominates early-age framework formation, whereas RBP and SCCG contribute more significantly at later ages. However, none of the binary systems simultaneously achieve high early strength and optimized later-age performance, highlighting the necessity of a ternary design to fully exploit the complementary functions of the three solid wastes.

3.1.2. Optimization of Ternary Composition

To achieve a balanced mechanical performance across different curing ages, the ternary system was systematically optimized by adjusting the relative proportions of recycled brick powder (RBP), ground granulated blast furnace slag (GGBS), and self-combusting coal gangue (SCCG), with emphasis on coordinating their contributions at different curing stages. As shown in Figure 5, when the GGBS-to-SCCG ratio was fixed at 1:1, increasing the RBP content resulted in a non-monotonic variation in compressive strength. The highest 28-day compressive strength (34.41 MPa) was obtained at an RBP content of 40%, while a comparable strength level was maintained at 50% RBP. This indicates that an appropriate RBP dosage window exists at approximately 40–50%, within which the later-age pozzolanic contribution of RBP can be effectively mobilized without excessively impairing early-age strength development.

Based on this dosage window, the ratio between GGBS and SCCG was further optimized. As illustrated in Figure 6, at a relatively high RBP content (50%), increasing the proportion of GGBS significantly enhanced early-age strength and simultaneously improved 28-day performance. This behavior highlights the important role of GGBS in supplying calcium ions and accelerating early hydration, thereby establishing a favorable alkaline environment that supports subsequent pozzolanic reactions. In contrast, when the RBP content was reduced to 40% (Figure 7), an excessive GGBS proportion led to a decline in 28-day strength, suggesting that an imbalance between calcium availability and reactive aluminosilicate phases can restrict the sustained development of later-age pozzolanic reactions.

Supplementary experiments with fixed SCCG or GGBS contents (Figure 8 and Figure 9) further confirm that a balanced proportion among the three components is essential for coordinating strength development at different curing ages. An excessive GGBS content promotes early-age strength but may suppress later-age pozzolanic activity, whereas overly high RBP or SCCG contents tend to delay early hydration and weaken the initial microstructural framework.

Among all tested mixtures, the ternary blend with an RBP:GGBS:SCCG ratio of 4:3:1 consistently exhibited the most favorable mechanical performance, achieving a 28-day compressive strength of 38.26 MPa, which exceeded that of the plain cement mortar. This optimized composition represents a balanced reaction window, in which early-age hydration promoted by GGBS is effectively coordinated, in a time-dependent manner, with the sustained pozzolanic reactivity of RBP and SCCG at later ages. Such balance avoids both insufficient early framework formation and premature depletion of reactive phases, enabling a stable and continuous strength development process. Consequently, the superior performance of the 4:3:1 mixture arises from an effective coordination of functional contributions among the three solid-waste components rather than from the dominance of any single constituent.

3.1.3. Effect of Cement Replacement Level

With the optimal ternary ratio of RBP:GGBS:SCCG = 4:3:1 fixed, the influence of cement replacement level on compressive strength was evaluated at replacement rates of 25%, 30%, 35%, and 40% (Figure 10). Both 7-day and 28-day compressive strengths exhibited a non-monotonic trend, initially increasing and then decreasing with increasing replacement level.

The highest compressive strengths were achieved at a cement replacement level of 30%, with 7-day and 28-day strengths of 27.02 MPa and 38.26 MPa, corresponding to 97% and 114.2% of the plain cement mortar, respectively. At a lower replacement level (25%), the limited dosage of supplementary cementitious materials reduced particle packing efficiency and restricted the development of secondary hydration products. In contrast, at higher replacement levels (≥35%), the substantial reduction in cement content weakened the primary hydration framework, while the delayed pozzolanic reactions of the solid waste components were insufficient to fully compensate, within the relevant curing period, for the loss of cement hydration products.

These results indicate that a 30% cement replacement level provides a favorable balance between cement hydration and pozzolanic reactions in the ternary system, enabling stable mechanical performance while substantially increasing the utilization of solid waste materials.

3.2. Workability and Construction Feasibility

Workability is a critical parameter governing the practical applicability of cementitious materials. While the preceding section identified the optimal ternary composition in terms of mechanical performance, it is necessary to verify whether this optimized mixture can simultaneously satisfy construction requirements. This section therefore evaluates the flowability of fresh mortars incorporating RBP, GGBS, and SCCG, with particular emphasis on the effects of RBP dosage and cement replacement level.

The consistency results of ternary mortars with varying RBP contents are summarized in

Table 4. Fluidity parameters of different mineral admixture systems.

Groups	RBP:GGBS:SCCG	Cement Replacement Rate	Water-Binder Ratio	Consistency (mm)
B-0	--	--	0.5	74
BG-14	1:1:1	30%	0.5	73
BG-9	4:3:3	30%	0.5	72
BG-4	2:1:1	30%	0.5	70
BG-12	3:1:1	30%	0.5	65
BG-7	4:3:1	30%	0.5	68
BG-16	4:3:1	25%	0.5	69
BG-17	4:3:1	35%	0.5	63
BG-18	4:3:1	40%	0.5	62

and Figure 11. When the GGBS-to-SCCG ratio was fixed at 1:1 and the water-to-binder ratio remained constant, the mortar consistency gradually decreased with increasing RBP content. Specifically, the flow diameter declined slightly from 73 mm at 33% RBP (by mass of total binder) to 70 mm at 50% RBP, followed by a more pronounced reduction to 65 mm at 60% RBP.

These results indicate the existence of a relatively tolerant RBP dosage window (≤50%) in terms of workability. At moderate dosages, the increased water demand associated with the high specific surface area of RBP remains limited, allowing sufficient free water to maintain particle mobility. However, when the RBP content exceeds this threshold, cumulative water absorption and increased interparticle friction markedly reduce flowability.

Notably, the mixture with optimal mechanical performance (BG-7, 50% RBP by mass of total binder) maintained a consistency of 68 mm, which falls within the acceptable range of 60–80 mm specified in JGJ/T 70-2009 [41]. This suggests that a relatively high RBP utilization level can be achieved without significantly compromising workability, provided that its dosage is properly controlled.

Figure 12 illustrates the influence of cement replacement level on the consistency of mortars prepared with the optimal ternary ratio of RBP:GGBS:SCCG = 4:3:1. When the replacement rate increased from 25% to 30%, only a slight reduction in flowability was observed. In contrast, further increasing the replacement level resulted in a marked decrease in consistency, reaching 63 mm and 62 mm at replacement rates of 35% and 40%, respectively.

This deterioration in workability can be associated with several coupled factors. Increasing the proportion of solid waste admixtures raises the overall water demand of the system, while the reduced cement content lowers the fraction of fine cement particles that normally enhance lubrication. In addition, the angular morphology and rough surface texture of RBP and SCCG hinder particle rearrangement during flow.

Overall, the results indicate that a cement replacement level of 30% represents a practical upper limit for maintaining satisfactory workability under the present mix conditions. Importantly, this threshold coincides with the optimal replacement level identified for compressive strength, demonstrating a good consistency between mechanical optimization and construction feasibility. The optimized ternary mixture (BG-7) therefore achieves a favorable balance between strength performance and workability, confirming its suitability for practical construction applications. In this section, the RBP content refers to its mass fraction relative to the total binder content.

3.3. Durability Performance of Ternary Mortars

Durability governs the long-term service performance of cementitious materials, particularly under aggressive environments. In this study, water absorption and chloride ion penetration resistance were selected as representative indicators to evaluate the durability of ternary mortars incorporating RBP, GGBS, and SCCG. These parameters are closely related to pore connectivity and microstructural compactness, thereby providing insight into the microstructural characteristics associated with the proposed synergistic design.

3.3.1. Water Absorption Behavior

The water absorption results of different mortar systems are summarized in

Table 5. Water absorption of different mortar specimens.

Groups	RBP:GGBS:SCCG	Cement Replacement Ratio	Dry Mass (g)	Pre-Drying Mass (g)	Water Absorption Rate (%)
B-0	--	--	675.0	750.1	11.22
BG-14	1:1:1	30%	677.3	756.3	11.71
BG-9	4:3:3	30%	680.3	760.9	11.85
BG-4	2:1:1	30%	681.5	762.8	11.93
BG-12	3:1:1	30%	685.7	768.3	12.04
BG-7	4:3:1	30%	685.6	762.4	11.12
BG-16	4:3:1	25%	683.3	758.4	10.99
BG-17	4:3:1	35%	682.2	760.2	11.43
BG-18	4:3:1	40%	680.5	760.0	11.68

and Figure 13. To clarify the influence of mix parameters, two experimental series were considered. The first examined the effect of RBP content at a fixed total replacement level of 30%, while the second evaluated the influence of cement replacement level at the optimal ternary ratio of RBP:GGBS:SCCG = 4:3:1.

As shown in Figure 13a, when the GGBS-to-SCCG ratio was fixed at 1:1, increasing the RBP content from 33% to 60% resulted in a gradual increase in water absorption from 11.71% to 12.04%. Even the lowest value in this series exceeded that of the plain cement control (11.22%). This trend indicates that excessive RBP introduces additional porosity due to its inherently porous morphology and limited early-age reactivity, leading to a higher proportion of residual particles that readily retain moisture.

In contrast, the optimized ternary mixture (BG-7, RBP:GGBS:SCCG = 4:3:1) exhibited a markedly lower water absorption of 11.12%, representing an absolute reduction of 0.8 percentage points compared with the control. This improvement reflects the formation of a denser microstructure through complementary hydration and pozzolanic reactions. In this system, early-age hydration products associated with GGBS contribute to the formation of a continuous matrix, while the subsequent pozzolanic reactions of RBP and SCCG are conducive to progressive filling of capillary pores, thereby mitigating the intrinsic porosity of the individual components.

When the cement replacement level was varied at the fixed 4:3:1 ratio (Figure 13b), water absorption first decreased and then increased, with the minimum value occurring at a replacement level of 30%. Beyond this threshold, water absorption rose noticeably. This behavior mirrors the compressive strength development trend, confirming that excessive cement reduction weakens the primary hydration framework and increases open porosity, which cannot be fully compensated by secondary hydration products.

Overall, the water absorption results indicate that durability-related performance in ternary mortars is sensitive to both the internal proportion of solid waste components and the total cement replacement level. A replacement level of 30% represents a favorable balance between matrix formation and pore refinement under the present curing conditions, consistent with the trends observed in mechanical performance.

3.3.2. Resistance to Chloride Ion Penetration

The resistance to chloride ion penetration, quantified by electrical flux, is presented in

Table 6. Chloride ion penetration results of mortar specimens.

Groups	RBP:GGBS:SCCG	Electrical Flux (C)	Ratio	Change Rate (%)
B-0	--	4570	1.00	--
BG-14	1:1:1	3781	0.82	17.2
BG-9	4:3:3	3425	0.74	25.1
BG-4	2:1:1	3895	0.85	14.7
BG-12	3:1:1	4023	0.88	12
BG-7	4:3:1	3324	0.73	27.2
BG-16	4:3:1	3052	0.66	33.2
BG-17	4:3:1	3650	0.80	20.1
BG-18	4:3:1	3933	0.86	13.9

and Figure 14. Consistent with the water absorption tests, the effects of RBP content and cement replacement level were systematically evaluated to assess the durability performance of the ternary mortars.

As shown in Figure 14a, at a fixed cement replacement level of 30%, the incorporation of ternary admixtures substantially reduced the electrical flux compared with the plain cement mortar. With increasing RBP content, the electrical flux initially decreased and then slightly increased. However, all values remained substantially lower than those of the control mixture. Among the tested compositions, the optimized ternary mixture exhibited the lowest electrical flux, indicating an enhanced resistance to chloride ion transport.

Figure 14b further illustrates the influence of cement replacement level at the optimal ternary ratio of RBP:GGBS:SCCG = 4:3:1. The electrical flux decreased sharply as the replacement level increased from 25% to 30%, reaching a minimum value of 3324 C. This corresponds to a reduction of 27.2% relative to the control. Further increasing the replacement level resulted in a gradual rebound in electrical flux, which is consistent with the deterioration of matrix integrity observed at excessive cement reduction.

The enhanced resistance to chloride ion penetration can be associated with two complementary mechanisms. First, the combined pozzolanic reactions of RBP, GGBS, and SCCG consume Ca(OH)₂ and generate additional C–S–H and C–A–S–H gels. These hydration products refine the pore structure and reduce pore connectivity. Second, the alumina-rich phases in RBP contribute to chemical chloride binding, such as the formation of Friedel’s salt. This process further restricts ion migration. Together, these effects significantly hinder chloride transport through the hardened matrix.

The durability performance of the optimized ternary mortar demonstrates that the synergistic incorporation of RBP, GGBS, and SCCG is associated with an improvement in microstructural compactness rather than a simple dilution effect. The optimal mixture (BG-7) simultaneously achieved a low water absorption of 11.12% and a 27.2% reduction in electrical flux compared with the plain cement mortar. These results support its superior resistance to fluid ingress and ion penetration.

Importantly, the durability enhancement follows the same optimal parameter window identified for mechanical performance and workability, namely a ternary ratio of 4:3:1 and a cement replacement level of 30%. This consistency highlights the robustness of the proposed design strategy and suggests that the observed improvements are associated with coordinated microstructural development rather than isolated test conditions.

3.4. Synergistic Hydration and Microstructural Evolution Mechanisms

The enhanced macroscopic performance of the optimized ternary mortar system is closely associated with the synergistic hydration behavior and microstructural evolution arising from the combined incorporation of recycled brick powder (RBP), ground granulated blast furnace slag (GGBS), and self-combusting coal gangue (SCCG). To elucidate the underlying mechanisms, XRD, SEM, and TG–DTG analyses were conducted on representative mortar systems, including single, binary, and ternary admixture formulations. These techniques provide complementary evidence linking hydration kinetics, phase assemblage, and microstructural densification.

3.4.1. XRD Analysis

Figure 15a–d presents the XRD patterns of hydrated pastes at 28 days for BG-1 (RBP), BG-2 (RBP + GGBS), BG-3 (RBP + SCCG), and BG-7 (ternary system). All samples exhibit typical hydration products of cement-based materials, including portlandite (Ca(OH)₂), ettringite (AFt), and poorly crystalline calcium silicate hydrate (C–S–H). The presence of amorphous hydration products is reflected by the broad diffuse hump in the range of 20–35° (2θ).

Compared with the RBP-only system (BG-1), the binary system incorporating GGBS (BG-2) shows a noticeable reduction in Ca(OH)₂ peak intensity. This observation suggests an increased consumption of Ca(OH)₂ associated with the relatively higher reactivity of GGBS, which is conducive to slag hydration and subsequent pozzolanic reactions. In contrast, the RBP + SCCG system (BG-3) retains relatively stronger Ca(OH)₂ reflections, suggesting that SCCG contributes more gradually to secondary hydration and exhibits a lower reaction rate when acting alone.

Notably, the ternary system (BG-7) exhibits the weakest Ca(OH)₂ diffraction among all mixtures, accompanied by a pronounced enhancement of the amorphous hump. This behavior suggests an increased extent of secondary hydration and the formation of greater amounts of C–S–H and C–A–S–H gels. The simultaneous attenuation of crystalline Ca(OH)₂ and enrichment of amorphous hydration products support the view that the ternary blend facilitates a more sustained pozzolanic reaction compared with the single or binary systems. Such enhanced formation of amorphous phases provides a microstructural basis for the improved strength development and durability performance observed at the macroscopic level.

3.4.2. SEM Analysis

Figure 16 presents SEM micrographs of hydrated paste samples at 28 days corresponding to BG-1 (RBP), BG-2 (RBP + GGBS), BG-3 (RBP + SCCG), and the optimized ternary system BG-7.

SEM micrographs further illustrate the microstructural differences induced by varying admixture combinations. As shown in (BG-1, Figure 16a), the RBP-only system displays a relatively loose microstructure with unreacted brick particles embedded in the matrix and a noticeable presence of capillary pores. A large amount of CH is present, and the degree of secondary hydration is low. The bonding between hydration products and RBP particles remains limited at 28 days, which is consistent with the relatively slow reaction kinetics of RBP.

In the binary RBP + GGBS system (BG-2, Figure 16b), a denser matrix is observed. However, localized voids and partially reacted particles are still present, indicating incomplete microstructural homogenization.

The RBP + SCCG system (BG-3, Figure 16c) exhibits hydration products preferentially nucleating within the porous SCCG particles. While this internal growth may contribute to local densification, the overall matrix remains insufficiently developed, which can be associated with limited early-age gel formation.

In contrast, the ternary system (BG-7, Figure 16d) exhibits a relatively compact and homogeneous microstructure. Interconnected C–S–H and C–A–S–H gels are observed filling the interstitial spaces between particles, while SCCG pores appear to provide internal space that may facilitate moisture retention and nucleation of secondary hydration products. The coordinated interaction among the three components is associated with enhanced particle packing, reduced pore connectivity, and a more compact interfacial morphology, which is consistent with the enhanced mechanical and durability performance observed at the macroscopic level.

3.4.3. TG–DTG Analysis

Thermogravimetric and differential thermogravimetric (TG–DTG) analyses were conducted on paste samples cured for 28 days using a TA TGA 550 analyzer (China). The investigated systems included the single-admixture system BG-1, the binary systems BG-2 and BG-3, and the optimized ternary system BG-7. The corresponding TG and DTG curves are shown in Figure 17.

The Ca(OH)₂ content of the hydrated paste was quantified based on TG–DTG analysis by evaluating the mass losses occurring at specific temperature intervals associated with Ca(OH)₂ dihydroxylation and CaCO₃ decarbonation. According to stoichiometric relationships, the Ca(OH)₂ content (CH) was calculated as:

$$CH={WL}_{\mathrm{C}\mathrm{H}}\times {\displaystyle \frac{74}{18}}+{WL}_{\mathrm{C}\mathrm{a}\mathrm{C}{\mathrm{O}}_3}\times {\displaystyle \frac{100}{44}}$$

where WL_CH and ${WL}_{\mathrm{C}\mathrm{a}\mathrm{C}{\mathrm{O}}_3}$ represent the mass losses corresponding to Ca(OH)₂ dehydroxylation and CaCO₃ decomposition, respectively. The calculated data is summarized in

Table 7. Thermogravimetric parameters of hydrated paste specimens at 28 days.

Groups	Age	CH Content (%)	CaCO3 Content (%)	CH Content (%)
BG-1	28 d	1.93	8.34	26.88
BG-2	28 d	1.89	6.68	22.93
BG-3	28 d	1.58	4.62	16.89
BG-7	28 d	1.24	5.08	16.63

.

As shown in Figure 17 and Table 7, the paste containing only RBP (BG-1) exhibited a Ca(OH)₂ content of 26.88%. When RBP was combined with SCCG in the binary system BG-3, the Ca(OH)₂ content decreased markedly to 16.89%, corresponding to an absolute reduction of 11.79 percentage points. This reduction reflects the enhanced consumption of Ca(OH)₂ due to secondary hydration reactions promoted by the presence of SCCG at later curing stages.

In the binary system incorporating RBP and GGBS (BG-2), the Ca(OH)₂ content was 22.93%. This value is lower than that of BG-1 but higher than that of BG-3. The relatively higher Ca(OH)₂ content in BG-2 may be related to the high early hydration activity of GGBS, which is activated in the alkaline environment provided by cement hydration and participates in secondary hydration reactions, while its subsequent pozzolanic reaction with RBP contributes to partial portlandite consumption. As a result, the Ca(OH)₂ content in BG-2 remains at an intermediate level.

For the ternary system BG-7 with an RBP:GGBS:SCCG ratio of 4:3:1, the Ca(OH)₂ content further decreased to 16.63%, representing reductions of 11.97 and 6.30 percentage points relative to BG-1 and BG-2, respectively. This system exhibits the lowest Ca(OH)₂ content among all tested mixtures, indicating a relatively high degree of portlandite consumption. Such behavior suggests that the combined hydration and pozzolanic reactions in the ternary blend are associated with a more effective utilization of available calcium, which is consistent with the denser microstructural features and improved macroscopic performance observed in the ternary system.

3.5. Ternary Synergistic Mechanism

The superior macroscopic performance of the optimized ternary mortar system cannot be attributed solely to the additive effects of recycled brick powder (RBP), ground granulated blast furnace slag (GGBS), and self-combusting coal gangue (SCCG). Rather, it is more reasonably interpreted as arising from a time-dependent coordination of hydration and pozzolanic reactions, governed by the complementary physicochemical characteristics and intrinsic reaction kinetics of the three solid-waste components.

This coordinated interaction is manifested in the evolution of hydration products, pore structure, and matrix continuity, enabling a balance between early-age strength development and sustained microstructural densification at elevated cement replacement levels.

At early curing stages, GGBS plays a dominant role in establishing the primary load-bearing framework of the matrix. Owing to its latent hydraulic activity and relatively high CaO content, GGBS is readily activated in the alkaline environment generated by cement hydration, leading to the preferential formation of calcium-rich C–S–H gels. These early hydration products effectively compensate for the limited early-age reactivity of RBP and SCCG, thereby ensuring matrix continuity and initial mechanical integrity. During this stage, RBP and SCCG mainly contribute through physical filling and nucleation effects, while their chemical reactivity remains comparatively limited.

With ongoing hydration, the contributions of RBP and SCCG become increasingly pronounced. The reactive SiO₂ and Al₂O₃ phases in RBP gradually participate in pozzolanic reactions, consuming Ca(OH)₂ released from cement hydration and GGBS dissolution. This delayed reactivity promotes the formation of secondary C–S–H and C–A–S–H gels, which progressively fill capillary pores and refine the pore structure. Compared with systems dominated by highly reactive supplementary cementitious materials, the relatively slower reaction kinetics of RBP may help sustain pozzolanic activity over extended curing periods by avoiding premature depletion of calcium hydroxide.

SCCG contributes to microstructural development through a combination of physical and chemical mechanisms. Its porous morphology provides internal space for hydration product growth and may function as an internal curing reservoir, facilitating moisture redistribution within the matrix. In addition, the heterogeneous mineral composition and rough surface texture of SCCG particles offer favorable nucleation sites for hydration products, promoting localized densification at the particle–matrix interface. Although the intrinsic pozzolanic reactivity of SCCG is lower than that of GGBS, its gradual participation in secondary reactions further enhances pore filling and interfacial bonding at later ages.

Accordingly, the synergistic behavior of the ternary system is better understood in terms of temporal matching of reaction rates rather than maximization of individual reactivity. Early-age framework formation dominated by GGBS ensures initial structural stability, while the delayed yet sustained pozzolanic reactions of RBP and SCCG drive progressive microstructural densification. This coordinated reaction sequence mitigates the limitations commonly observed in binary systems, where excessive early reactivity may suppress long-term pozzolanic potential or insufficient calcium availability may delay framework formation.

Notably, the optimized RBP:GGBS:SCCG ratio of 4:3:1 appears to establish a balanced reaction window in which calcium supply, aluminosilicate availability, and internal curing capacity are effectively coordinated. Excessive GGBS content may accelerate early hydration but constrain later-age pozzolanic reactions due to rapid calcium consumption, whereas overly high RBP or SCCG contents tend to delay early framework establishment. The selected ratio alleviates these competing effects, allowing each component to contribute without inducing adverse interactions.

Consequently, the enhanced compressive strength, reduced water absorption, and improved resistance to chloride ion penetration observed in the optimized ternary mortar are consistent with a refined and progressively densified microstructure, rather than with dilution or packing effects alone. The reaction-sequence-coordinated ternary design therefore provides a rational basis for developing multicomponent solid-waste cementitious systems capable of achieving high cement replacement levels without compromising mechanical performance or durability.

4. Discussion

The results indicate that the improved performance of the ternary system is primarily governed by the coordinated interaction among RBP, GGBS, and SCCG, which exhibit distinct reaction characteristics at different stages. GGBS, with its latent hydraulic properties, reacts rapidly in the presence of calcium hydroxide and alkaline conditions, contributing to early-stage strength development and the formation of a primary structural framework. In contrast, RBP and SCCG, which are rich in aluminosilicate phases, mainly participate in later-stage pozzolanic reactions, consuming Ca(OH)₂ and generating additional C–S–H and C–A–S–H gels.

The reduction in Ca(OH)₂ content to 16.63% in the optimized system further confirms the enhanced pozzolanic activity, which contributes to the refinement of the pore structure and the densification of the matrix. The porous structure of SCCG may also facilitate internal curing and provide nucleation sites for hydration products, further promoting microstructural development.

Compared with binary systems reported in the literature, where either early strength (in slag-rich systems) or later-age performance (in aluminosilicate-rich systems) is improved, the present ternary system achieves a more balanced performance. Previous studies have shown that RBP–GGBS systems can enhance early strength due to increased calcium availability but often require relatively low replacement levels to maintain durability [33,34,35]. Similarly, systems incorporating materials such as metakaolin or coal gangue tend to improve later-age properties but suffer from insufficient early framework formation [37,38,39].

In contrast, the ternary system proposed in this study enables a more effective coordination of reaction kinetics, resulting in improved compressive strength and reduced permeability at a relatively high replacement level of 30%. The observed increase of approximately 14% in compressive strength and the reduction in chloride ion transmittance by about 27% demonstrate the advantage of this approach over conventional systems.

These findings suggest that the coordination of reaction sequences is a key factor in designing high-performance cementitious materials with high solid waste content. This mechanism-oriented approach provides new insights beyond traditional mix proportion optimization and may be extended to other multicomponent systems.

Among the tested mixtures, the ternary composition with an RBP:GGBS:SCCG ratio of 4:3:1 exhibited the most favorable overall performance in terms of compressive strength, workability, and durability. It should be noted that this ratio is identified as optimal within the experimental range investigated, rather than through a full factorial or statistical optimization approach. The superiority of this composition is consistently supported by multiple performance indicators and is further corroborated by microstructural analysis, suggesting that the observed trend is not incidental but associated with the coordinated reaction behavior of the three components. Further statistical optimization using factorial design or response surface methodology could be conducted in future studies to quantitatively validate the optimal composition.

This study has several limitations. The properties of construction and demolition waste (CDW), including RBP and SCCG, may vary significantly depending on their source, composition, and processing conditions, which could affect the reproducibility and generalizability of the results. In addition, the experimental investigation was conducted under controlled laboratory conditions with a limited range of replacement levels and mixture proportions. Long-term durability aspects, such as freeze–thaw resistance and performance under variable environmental conditions, were not evaluated in this study. Therefore, further research is required to assess the robustness of the proposed ternary system under different raw material sources and practical engineering conditions.

5. Conclusions

Based on experimental results and microstructural analyses, the following conclusions can be drawn:

(1)

Mortars containing only RBP or those based on a binary system exhibit inadequate performance, with reduced early-age strength and limited gains in later-age strength. In contrast, the RBP–GGBS–SCCG ternary system achieves a balanced improvement in strength, workability and durability.

(2)

At a cement replacement rate of 30%, the optimal ratio of RBP:GGBS:SCCG is 4:3:1, with this system demonstrating the best performance. The 28-day compressive strength reached 38.26 MPa, representing an increase of approximately 14% compared to the control mortar.

(3)

The optimized ternary system significantly improved durability. Water absorption was reduced to 11.12%, and chloride ion transmittance decreased by approximately 27%, indicating reduced permeability and a denser pore structure.

(4)

Microstructural analyses revealed that GGBS primarily promotes early-stage hydration, whilst RBP and SCCG enhance late-stage reactions. The porous structure of SCCG facilitates the growth of hydration products and the refinement of pores.

(5)

The Ca(OH)₂ content in the ternary system decreased to 16.63%. Compared with the unitary and binary systems, this represents a decrease of 11.97 and 6.30 percentage points, respectively.

Overall, the improved performance of the ternary system is attributed to the coordination of reaction sequences among RBP, GGBS, and SCCG, where early-stage framework formation and delayed pozzolanic reactions act synergistically. This provides a practical strategy for designing low-carbon cementitious materials with balanced mechanical properties and durability.