A Comparative Study of Waste Red-Clay Brick Powder (WRCBP) and Fly Ash (FA) as Precursors for Geopolymer Production

Abstract

Utilizing waste red-clay brick powder (WRCBP) as a precursor for manufacturing geopolymers is increasingly popular due to its environmental and economic benefits. However, the geopolymerization of this waste remains insufficiently explored. This study evaluates the differences in physical–mechanical properties and microstructural evolution of WRCBP- and fly ash (FA)-based geopolymers to determine the reactivity of WRCBP. Mineral admixtures, including granulated blast furnace slag (GF) and metakaolin (MT), were incorporated with WRCBP to fabricate geopolymer pastes, while FA was used in parallel for comparison. The effects of activator modulus (1.2 and 1.4 for Na₂SiO₃) and curing conditions (65 °C and 90 °C) on the mechanical and microstructural performance of the prepared pastes were investigated through water demand analysis, compressive strength testing, mercury intrusion porosimetry (MIP), and scanning electron microscopy (SEM). The results indicate that WRCBP-based pastes achieved a comparable compressive strength (39.8 MPa) under appropriate alkali-activated and curing conditions relative to FA-based pastes (42.5 MPa). The modulus of the alkaline activator exerted a greater influence on strength development than the raw material composition. For both WRCBP- and FA-based pastes, 65 °C was identified as a more suitable curing temperature. Moreover, compared with FA-based pastes, pastes produced using WRCBP provide enhanced social and economic benefits. Overall, this study confirms that high-performance binders can be engineered by incorporating WRCBP, thereby supporting the development of sustainable low-carbon construction materials.

1. Introduction

In recent years, rapid urbanization in China has generated a large quantity of construction and demolition (C&D) wastes, and their production is anticipated to continue rising in the future [1,2]. Notably, waste red-clay brick (WRCB) constitutes a major portion of such demolition waste. It has been reported that WRCB accounts for 50–70% of total construction waste in China [3], and over 20 billion m³ of this waste has been produced in the past five decades due to intensive construction and demolition of brick structures [4]. Nevertheless, the majority of WRCB is still disposed of via landfilling, which can trigger a series of detrimental environmental impacts, including the occupation of vast land resources and pollution of water and soil environments [5,6]. Consequently, it is of great importance to explore efficient and environmentally friendly strategies for handling these brick wastes.

WRCB predominantly comprises SiO₂ and Al₂O₃, which jointly account for over 80% of its chemical constituents, along with minor components such as Fe₂O₃, TiO₂, CaO, and MgO [7]. Therefore, WRCB can be recycled into construction materials through multiple pathways, including its use as aggregates, supplementary cementitious materials (SCMs), or raw feed for cement production. Previous studies have indicated that recycled concrete containing 10% recycled clay brick aggregate exhibited superior mechanical properties compared with the reference group [8]. Moreover, the incorporation of 10–20 wt% recycled brick aggregates into concrete with steam curing can boost 28-day compressive strength by 2.5–11.3% and improve impermeability by mitigating heat damage [9].

Beyond recycling as aggregate, Si-rich WRCB also possesses gelation potential, enabling its use as an SCM similar to fly ash and slag to partially replace cement. Naceri et al. [10] reported that ground WRCB powder effectively improved the compressive strength of mortars when replacing 10 wt% of cement. Similar findings were reported by Şenol et al. [11], who attributed the enhancement primarily to a higher degree of clinker hydration and reduced porosity compared with reference specimens. Shao et al. [12] observed that mortars containing WRCBP generally showed reduced early-age compressive strength, with the reduction proportional to the replacement level; however, after 90 days of hydration, blended mortars with 20% WRCBP attained the highest strength of 62.2 MPa due to pozzolanic reactions forming C-A-H and C-A-S-H gels, which generated a denser microstructure. Additionally, the incorporation of WRCBP improves the durability of blended mortars [13,14].

Recently, WRCB has gained increasing interest for manufacturing geopolymers due to its environmental advantages. According to Sedira et al. [15], alkali-activated WRCBP materials showed excellent strength and workable performance. Tuyan et al. [16] investigated the effects of alkali activator concentration and curing conditions on the consistency and strength of WRCBP-based geopolymer composites, and reported a maximum compressive strength of 36.2 MPa after 5 days of curing. Reig et al. [17] optimized the geopolymerization parameters of WRCBP by adjusting alkali activator type and concentration; the optimal products were obtained using 5 mol/kg NaOH after 7 days, reaching nearly 30 MPa, which could be further increased to 50 MPa by tuning the water-to-binder ratio, binder-to-sand ratio, and SiO₂/Na₂O ratio. Hwang et al. [18] explored the potential reuse of WRCBP and waste ceramic powder (WCP) as high-strength alkali-activated pastes that cure well at ambient temperatures. Fly ash (FA) and granulated blast furnace slag (GF) were used as blending materials, while Na₂SiO₃ and NaOH served as alkali activators. The test results showed that the fresh AAP mixtures were highly workable, and the hardened samples achieved compressive strengths ranging from 36 to 70 MPa.

Meanwhile, the research and application of FA as a precursor for geopolymers have gradually matured in recent years. Wielgus et al. [19] demonstrated that fluidized bed combustion FA is a promising and environmentally friendly raw material for geopolymers, offering good mechanical properties and low density, with high compressive strength achievable under ambient curing. Chen et al. [20] optimized FA–slag-based geopolymer pastes using 17 mixtures, identifying alkali equivalent, activator modulus, and slag replacement ratio as the main influential factors. De Matos Riscado et al. [21] validated a dosage methodology for FA-based geopolymers based on key compositional parameters—water-to-binder ratio, aggregate-to-binder ratio, alkaline solution modulus, and silica modulus—achieving a maximum compressive strength of 50.19 MPa. Hamed et al. [22] concluded that FA–GF-based geopolymers offer a sustainable and applicable alternative to cement, improving the strength of high-plasticity clay soils while reducing permeability. However, the cost of FA has gradually increased due to its excellent performance and rising demand [23], making it increasingly urgent to seek alternative materials.

The present study focuses on analyzing and characterizing geopolymers fabricated with WRCBP, with GF and Metakaolin (MT) serving as mineral admixtures. FA was employed as a reference material to evaluate the relative performance of the two modified geopolymer systems. The mix ratio of raw constituents, activator modulus and alkali concentration, and the influence of curing conditions were examined in detail. Physical and mechanical parameters, including water demand, porosity, and compressive strength under varying mixture designs, were analyzed. Mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM) were employed to elucidate the microstructure of hydration products.

2. Materials and Methods

2.1. Materials

The WRCB was acquired from DingHao Building Materials Company in Shenzhen, China. Using a ball mill operating at 400 r/min for 40 min, the material was ground and subsequently sieved to obtain two particle size fractions of less than 45 μm and 75 μm. Ahmed et al. [24] emphasized the significant influence of particle size on the properties of geopolymers synthesized using WRCBP, reporting a substantial enhancement in compressive strength when fractions with D₅₀ < 15 μm were incorporated. Komljenović et al. [25] investigated alkali-activated FA geopolymers and found that compressive strength was predominantly dependent on the content of fine FA particles (<43 μm), with the highest strength observed in mixtures containing the greatest proportion of fines. Similarly, Chen et al. [26] reported that smaller colloidal particle volumes correlate with higher reaction activity.

Given its chemical similarity to WRCBP, FA was employed as a reference material to enable a comprehensive comparison of the two geopolymer systems. The particle morphologies of WRCBP and FA are shown in Figure 1a and Figure 1b, respectively. Particle size characteristics for both WRCBP and FA used in this study are listed in

Table 1. Characteristic parameters of WRCBP and FA particles.

Parameter	Fine WRCBP	Coarse WRCBP	FA
Median size/μm	2.342	8.612	27.365
Volume average size/μm	4.101	14.310	40.538
Area average size/μm	1.194	2.862	6.674
Specific surface area/m2/g	0.895	0.373	0.160

, as determined using an X-ray fluorescence spectrometer (XRF, S4 Explorer, Bruker, Germany).

GF and MT are blended materials with potential hydraulic properties. In this experiment, they were incorporated as admixtures into the WRCBP-based cementitious system to address the shortcomings of simplex WRCBP pastes, namely their low calcium content, high water demand, and inferior early strength. In addition, the alkaline constituents in GF provide auxiliary activation for the overall cementitious system, thereby enhancing performance. The chemical compositions of the raw source materials are presented in

Table 2. Chemical compositions of the raw source materials (%).

Composition	WRCBP	FA	MT	GF
CaO	11.6	13.2	3.1	19.2
SiO2	50.2	53.5	51.3	44.5
Al2O3	21.8	23.9	38.9	22.7
Fe2O3	5.3	4.6	2.5	0.9
SO3	4.5	0	0.6	1.5
MgO	0	2.7	0.5	5.3
K2O	0	0	0.5	0.4
Na2O	3.8	0.6	0.3	0.3
ZnO	0	0	0.1	0.1
TiO2	0	0	0.4	2.5
MnO	0	0	0	0.5
LOI	2.6	1.2	1.8	2.1
others	0.2	0.3	0	0

.

Pelletized NaOH with a purity of 96% was procured from Bowen Technology Company in Shenzhen, China, while Na₂SiO₃ was obtained from Hengli Chemical Company in Tongxiang, China. The initial modulus of the Na₂SiO₃ solution was 2.3 and was adjusted to 1.2 and 1.4 using NaOH; the solution is a colorless, transparent liquid.

Table 3. Chemical parameters of three different moduli of Na₂SiO_3.

Modulus	Na2O Content (wt%)	SiO2 Content (wt%)	H2O Content (wt%)
2.3	13.2%	29.6%	57.2%
1.2	25.3%	29.6%	45.1%
1.4	21.7%	29.6%	48.7%

presents the technical specifications of the Na₂SiO₃ solutions at the three different moduli. Ordinary municipal tap water was used for mixing.

2.2. Fabrication of Modified Paste

The production of pastes involved a series of carefully controlled steps. Initially, WRCBP or FA was proportionally combined with GF and MT and mixed until homogeneity was achieved. A stirring duration of 3 min was adopted to ensure thorough blending and adequate dispersion of the powders [24]. Following this, the prepared alkali-activating solution was added to the mixed raw materials and stirred for an additional 4 min to form a uniform paste.

The freshly prepared pastes were then cast into 40 × 40 × 160 mm³ molds. Subsequently, the specimens were placed on a vibrating table and vibrated for 2 min to eliminate entrapped air. Each specimen was then covered with a polyethylene film to minimize evaporation of free water and transferred to a thermostatic chamber set at either 65 °C or 90 °C for 24 h of curing [16,17,27]. After the initial curing stage, the specimens were demolded and moved to a curing chamber with a temperature of 20 ± 2 °C and a relative humidity of about 90% until the designated test ages of 7 and 28 days.

Figure 2 presents the images of WRCBP- and FA-based pastes after 7 days of autoclave curing. Table 4 lists the material compositions of the prepared WRCBP- and FA-based pastes. To maximize WRCBP utilization, the mass ratios of WRCBP/GF/MT were set at 7/2/1 and 6/3/1. The reference ratio of alkaline solution to raw materials was set at 0.35 approximately [28,29]. The target flowability of the modified pastes was maintained at 140 ± 10 mm by controlling the addition of external water. The values 6% and 8% refer to the ratio of Na₂O in the alkali activator to the total mass of the gelatinous material under the corresponding activator moduli (Table 3), with the calculation method provided in Equation (1).

$$\mathbf{X}={\displaystyle \frac{({\mathrm{M}}_{{\mathrm{N}\mathrm{a}}_2\mathrm{S}\mathrm{i}{\mathrm{O}}_3}+{\mathrm{M}}_{\mathrm{N}\mathrm{a}\mathrm{O}\mathrm{H}})\times {\mathrm{W}}_{{\mathrm{N}\mathrm{a}}_2\mathrm{O}}}{{\mathrm{M}}_{\mathrm{W}\mathrm{R}\mathrm{C}\mathrm{B}\mathrm{P}}+{\mathrm{M}}_{\mathrm{G}\mathrm{F}}+{\mathrm{M}}_{\mathrm{M}\mathrm{T}}}}$$

(1)

X—represents the content of Na₂O in the alkali activator to the total mass of the gelatinous material (6%, 8%);

• ${\mathbf{W}}_{{\mathbf{N}\mathbf{a}}_2\mathbf{O}}$—represents the content of Na₂O in the alkaline activator under the current moduli (as shown in Table 3).

Figure 2. (a) Prepared WRCBP-based specimens and (b) FA-based specimens.

Figure 2. (a) Prepared WRCBP-based specimens and (b) FA-based specimens.

Table 4. Raw material ratio of studied pastes (g).

Alkali-Activator	WRCBP	FA	GF	MT	Na2SiO3	NaOH	Water
1.4–8%	910	0	260	130	401.4	77.9	112
1.4–6%	910	0	260	130	301.0	58.4	145
1.2–8%	910	0	260	130	344.3	66.8	130
1.2–6%	910	0	260	130	258.2	50.1	168
1.4–8%	780	0	390	130	401.4	77.9	117
1.4–6%	780	0	390	130	301.0	58.4	149
1.2–8%	780	0	390	130	344.3	66.8	135
1.2–6%	780	0	390	130	258.2	50.1	174
1.4–8%	0	910	260	130	401.4	77.9	35
1.4–6%	0	910	260	130	301.0	58.4	60
1.2–8%	0	910	260	130	344.3	66.8	52
1.2–6%	0	910	260	130	258.2	50.1	70
1.4–8%	0	780	390	130	401.4	77.9	45
1.4–6%	0	780	390	130	301.0	58.4	71
1.2–8%	0	780	390	130	344.3	66.8	63
1.2–6%	0	780	390	130	258.2	50.1	80

Table 4. Raw material ratio of studied pastes (g).

Alkali-Activator	WRCBP	FA	GF	MT	Na2SiO3	NaOH	Water
1.4–8%	910	0	260	130	401.4	77.9	112
1.4–6%	910	0	260	130	301.0	58.4	145
1.2–8%	910	0	260	130	344.3	66.8	130
1.2–6%	910	0	260	130	258.2	50.1	168
1.4–8%	780	0	390	130	401.4	77.9	117
1.4–6%	780	0	390	130	301.0	58.4	149
1.2–8%	780	0	390	130	344.3	66.8	135
1.2–6%	780	0	390	130	258.2	50.1	174
1.4–8%	0	910	260	130	401.4	77.9	35
1.4–6%	0	910	260	130	301.0	58.4	60
1.2–8%	0	910	260	130	344.3	66.8	52
1.2–6%	0	910	260	130	258.2	50.1	70
1.4–8%	0	780	390	130	401.4	77.9	45
1.4–6%	0	780	390	130	301.0	58.4	71
1.2–8%	0	780	390	130	344.3	66.8	63
1.2–6%	0	780	390	130	258.2	50.1	80

2.3. Testing and Characterization Method

The flowability of fresh pastes were tested by micro slump test according to ASTM C1437 [30]. The blended pastes were poured into a truncated cone mold with an upper diameter of 36 mm, a height of 60 mm, and a lower diameter of 60 mm. Removed additional pastes around the top and bottom of the cone and then lifted the truncated cone mold vertically and the pastes were allowed to flow freely on the glass plate for 30 s. Finally, the flowability of pastes were determined by averaging the maximum diameter of flow in two mutually perpendicular directions. Three parallel experiments were conducted, and the average of the measured values were taken as the experimental results.

The compressive strength tests were conducted using a constant-load cement compression-resistance testing machine. Prism specimens of 40 × 40 × 160 mm were used, and the final results were calculated as the average of three specimens. During testing, the loading rate was maintained at 2.4 kN/s.

The microstructural characteristics of the WRCBP- and FA-based pastes at 7 and 28 days were examined by SEM combined with EDS using a Quanta™ 250 FEG (FEI company, Hillsboro, OR, USA). Specimens were collected after compressive strength testing and cut into small cubes of about 15 × 15 × 15 mm. These specimens were completely soaked in anhydrous ethanol for 3 days, followed by vacuum-dried at 60 °C. Before analysis, performed a 30 s Au coating treatment on the fresh fracture surface of the specimens.

The pore structures of the WRCBP- and FA-based pastes at 28 days were quantified via mercury intrusion porosimetry (MIP) using an Autopore 9510 (Micromeritics Company, Atlanta, GA, USA). The mercury employed during testing had a density of 13.5335 g/mL, a contact angle of 130°, and an interfacial tension of 0.485 N/m.

3. Results and Discussion

3.1. Water Demand Analysis of Modified Paste

The consumption of mixing water in a given mix design has a significant impact, not only governing the workability of the fresh pastes but also influencing the strength and durability of the hardened composites. In this study, the target flowability of the modified pastes was maintained at 140 ± 10 mm to ensure consistent workability. Due to differences in particle size and morphology between WRCBP and FA, as well as variation in water-holding capacity within different alkaline solutions, distinct water dosages were required to achieve comparable flowability.

Figure 3 shows the water demand for the prepared WRCBP- and FA-based pastes with equivalent fluidity (a 40 × 40 × 160 mm³ mold), including both the silicate solution water content and added external water. WRCBP-based pastes consistently exhibited a higher water demand than FA-based pastes, which is attributable to the greater porosity and rougher surface texture of WRCBP. This observation aligns with the findings of Alghamdi et al. [31]. Kuri et al. [32] also reported that WRCBP has relatively higher inter-particle friction, which consequently reduces flowability. For WRCBP-based pastes, the average water demand per mold was 324.5 g, whereas FA-based pastes required 242.7 g, representing a reduction of about 33.7% under identical conditions. In addition, GF exhibited a coarser particle size compared with WRCBP and FA; therefore, increasing its proportion necessitated a corresponding rise in water dosage to attain the required fluidity.

3.2. Compressive Strength of the Samples

3.2.1. Effect of Activator Modulus and Na₂O Content

Figure 4 presents the compressive strength of WRCBP- and FA-based pastes under different alkali activators when the raw material composition was 7/2/1. When the activator modulus was adjusted from 1.4 to 1.2, both WRCBP- and FA-based pastes exhibited significant strength improvements. Specifically, the WRCBP-based paste showed a maximum increase of 8.2%, whereas the FA-based paste demonstrated a 4.6% improvement. However, when the Na₂O content of the activator decreased from 8% to 6%, the WRCBP-based paste exhibited a 4.3% reduction in strength, while the FA-based paste showed a notable enhancement of 12.1%. When the modulus of the alkali activator was 1.2 and the Na₂O content was 8%, the 28-day compressive strength of the WRCBP-based paste sample reached 39.8 MPa, slightly higher than the 39.0 MPa of the FA-based paste sample. At this juncture, the WRCBP-based paste sample achieved its optimal compressive strength. Conversely, when the modulus of the alkaline activator was 1.2 and the Na₂O content was 6%, the 28-day compressive strength of the WRCBP-based paste sample reached 38.4 MPa, while the FA-based paste sample reached 42.5 MPa. In this instance, the FA-based paste sample exhibited its optimal compressive strength, which was higher than that of the WRCBP-based paste sample. It is evident that the influence of Na₂O content on WRCBP- and FA-based pastes is inconsistent, which is mainly attributed to the difference in chemical composition between WRCBP and FA.

To provide a more intuitive description of the strength of WRCBP-based pastes, we compared the 28-day compressive strengths of WRCBP- and FA-based pastes under their respective optimal activation conditions. It can be seen that after 28 days of curing at 65 °C, the optimal strength of WRCBP-based paste reached 39.8 MPa (7/2/1, 1.2, 8%), while the optimal strength of FA-based paste reached 42.5 MPa (7/2/1, 1.2, 6%). This indicates that when using WRCBP 1:1 instead of FA, the optimal strength of the WRCBP-based paste can reach 93.6% of that of the FA-based paste under appropriate alkaline activation conditions.

These results confirm that both the activator modulus and Na₂O content exert a significant influence on geopolymer strength. An optimal combination of Na₂SiO₃ modulus and Na₂O dosage is required to promote effective geopolymerization. Excessive modulus or Na₂O content can lead to the rapid breakage of Si-O and Al-O bonds in the precursor. A large amount of gel generated in a short time will encapsulate the unreacted precursor and impede further reaction, which will have a negative impact on the development of geopolymer strength. This is consistent with the findings of De Castro Carvalho [13], who believed that rapid gel formation in an overly alkaline medium can block the contact between unreacted particles and the alkaline solution. Additionally, Law et al. [33] reported that the reaction products which formed rapidly in the highly alkaline medium may block the unreacted particles contacting with alkali solution.

3.2.2. Effect of Component Materials

Figure 5 presents the compressive strength of WRCBP- and FA-based pastes under different alkaline activator conditions with raw material compositions of 6/3/1 and 7/2/1, respectively, measured at 7 and 28 days post-curing. The results show that changing the raw material composition from 6/3/1 to 7/2/1 did not produce a pronounced variation in strength for either group. For the WRCBP-based pastes, the maximum strength change rate reached 3.3%, whereas for the FA-based pastes it reached 6.2%. The WRCBP-based pastes achieved an optimal 28-day strength of 39.8 MPa under the conditions 7/2/1, 1.2, and 8%, while the FA-based pastes reached an optimal 28-day strength of 45.3 MPa under 6/3/1, 1.2, and 6%. These findings are consistent with previous reports regarding the optimal activator modulus and Na₂O content [17].

Figure 6 illustrates the relative compressive strength values compared with the highest strength achieved by WRCBP- and FA-based pastes after 28 days of curing at 65 °C. As shown in the figure, the degree of influence exerted by the activator parameters differs from that of the raw material composition. The impact of activator parameters exhibits distinct regularity and is more pronounced than the influence associated with variations in raw material ratios. In the WRCBP (FA) group, differences in activator parameters produced compressive strength variations of up to 10.8% (15.2%), whereas differences in raw material composition produced variations of less than 3% (7%). These findings confirm that, in geopolymers preparation, activator parameters play a more critical role than raw material composition. We believe this is because during the geopolymerization process, alkali activators can activate the Si-Al precursors, leading to the rapid breakage of Si-O bonds and Al-O bonds. As a result, a large amount of Si⁴⁺ and Al³⁺ leaches into the alkali solution. On the other hand, the high-concentration Na⁺ provided by alkali activators is an important component of geopolymerization products. Therefore, alkali activators not only participate in the reaction but also act as catalysts, enabling them to play a more critical role in the reaction process compared to other factors. This conclusion is in agreement with the study by Hamed et al. [22], who reported that the modulus of the activator (NaOH) exerted the highest influence on both compressive strength and permeability of geopolymer specimens compared to the composition ratio of raw materials. Future work should therefore prioritize optimization of the activator system to further enhance alkali-activated performance.

3.2.3. Effect of Curing Temperature

Building on previous experiments, the optimal WRCBP- and FA-based pastes (WRCBP-based paste: 7/2/1, 1.2, 8%; FA-based paste: 6/3/1, 1.2, 6%) were selected to analyze strength development under different curing temperatures. The influence of temperature on mechanical performance followed the same trend for both materials. As shown in Figure 7, when the curing temperature increased from 65 °C to 90 °C, the 7-day compressive strength of the WRCBP- and FA-based pastes increased by 4.3% and 5.5%, respectively, whereas their 28-day strength decreased by 2.0% and 4.7%.

This decline is attributable to two primary factors. First, the higher curing temperature leads to partial moisture evaporation from the paste, restricting the growth space needed for hydrate development and impeding the hydration process. Second, elevated temperatures accelerate the initial dissolution-precipitation reaction of the precursor powders, resulting in rapid formation of thick gel layers that encapsulate unreacted particles and hinder further polymerization [17,34]. Similar phenomena were reported by Görhan et al. [35] and Noushini et al. [36], who observed a decline in compressive strength with continuous increases in curing temperature. Overall, alkali-activated WRCBP- and FA-based geopolymers perform more favorably when cured at 65 °C.

3.2.4. Effect of Particle Size

Existing research indicates that particle size is a crucial factor in the alkali-activation process, as sufficient reaction requires an adequately fine particle size. In this experiment, to assess the impact of raw material fineness on geopolymerization, the compressive strength of WRCBP-based pastes (7/2/1, 1.2, 8%, 65 °C) prepared with average particle sizes less than 45 μm and 75 μm was determined. As shown in Figure 8, for WRCBP with particle size <45 μm, the compressive strength reached 42.8 MPa and 47.5 MPa at 7 and 28 days, respectively, representing increases of 15.7% and 19.3% compared with WRCBP of <75 μm. These results demonstrate that WRCBP-based pastes with finer particles consistently exhibit superior compressive strength at all curing ages.

The experimental conclusion is consistent with the research findings obtained by other scholars. Adequate reaction depends on a certain degree of particle fineness; finer particles possess a larger specific surface area, enabling more effective contact with the alkaline solution and thereby enhancing dissolution and depolymerization of raw materials [24,25,26].

3.3. Pore Structure

MIP is a highly accurate technique for characterizing pore structures. In this experiment, the WRCBP- and FA-based pastes that exhibited the highest compressive strength after 28 days of curing were selected for MIP analysis.

Table 5. MIP results of the modified pastes.

Species	Total Intrusion Volume/mL/g	Porosity/%	Slurry Density/g/cm3	Average Pore Diameter/nm	Median Pore Diameter (Volume)/nm	Median Pore Diameter (Area)/nm
F-paste	0.1198	20.5377	1.7139	19.1500	16.5000	16.3000
R-paste	0.1457	23.3300	1.6015	27.1300	161.6000	9.1700

presents the MIP results for both paste types. For the FA-based paste, the total porosity was 20.53%, with an average pore diameter of 19.15 nm and a slurry density of 1.71 g/cm³. In comparison, the WRCBP-based pastes exhibited a total porosity of 23.33%, an average pore diameter of 27.13 nm, and a slurry density of 1.60 g/cm³. The FA-based pastes demonstrated lower total porosity and smaller average pore diameter while possessing a higher slurry density. Moreover, the volume and area median pore diameters of the FA-based pastes were consistent, whereas those of the WRCBP-based pastes differed significantly. This inconsistency suggests a more complex pore structure in the WRCBP-based pastes, containing a mixture of large and small pores with a discontinuous distribution, which leads to a substantial disparity in median pore size.

Figure 9 highlights notable differences in pore size distribution between the two pastes. For the WRCBP-based paste, 87.73% of the pores were distributed within the 10–100 nm range, with only 12.27% exceeding 100 nm and no pores smaller than 10 nm detected. In contrast, the pore size distribution of FA-based paste in the range of 1–100 nm accounted for 46.17%, and 53.82% for exceeding 100 nm, indicating a more uniform pore size distribution. We believe that this is mainly attributable to the disparity in particle morphology between WRCBP and FA. WRCBP was obtained through ball-milling and sieving. Its particle appearance is irregular, and the particle size is relatively dispersed. This leads to greater differences in the reaction degree among different particles during the geopolymerization process (compared to FA-based paste), resulting in a more dispersed pore-size distribution of WRCBP-based paste. This denser and more rational pore structure of the FA-based pastes helps explain its higher compressive strength. The observed characteristics of volume and area median pore sizes in both pastes further substantiate these findings.

3.4. SEM-EDS Analysis

The WRCBP- and FA-based pastes (WRCBP: 7/2/1, 1.2, 8%; FA-based paste: 6/3/1, 1.2, 6%) with optimal compressive strength were subjected to SEM observation at a curing temperature of 65 °C. As shown in Figure 10a,c, unreacted raw material particles were clearly observed in both pastes. These unreacted particles were encased by sodium–alumino–silicate–hydrate (N–A–S–H) gel produced during geopolymerization process [37,38,39,40,41], resulting in a heterogeneous microstructure. With longer curing time, as depicted in Figure 10b,d, the quantity of unreacted particles markedly decreased, while the formation of N–A–S–H gel increased substantially, leading to a denser and more compact structure. This confirms that geopolymerization is a comparatively long-term process and continues to contribute to strength development at later curing stages. Additionally, microcracks were observed around unreacted FA particles, attributable to shrinkage stress between the gel products and the FA particles during curing [42,43], similar phenomena were rarely observed in WRCBP-based paste samples. This may be because during the geopolymerization reaction, the initial reaction intensity of the WRCBP-based paste was not as high as that of the FA-based paste, resulting in no significant stress difference in the short term. By comparing Figure 10b,d, it is evident that the overall gel integrity of the FA-based alkali-activated paste is higher than that of the WRCBP-based alkali-activated paste, which is consistent with the porosity differences previously reported for the two materials.

Figure 11 presents the EDS results for WRCBP- and FA-based pastes after 28 days of curing. As shown, the elemental compositions of both pastes are fundamentally similar. The predominant elements detected were Si, Al, Ca, Na, C, and O, with Si, Al, Na and O serving as the principal constituents involved in alkali-activated bond formation. Their relatively high abundances support the occurrence of effective geopolymerization. The low Ca content in both pastes is related to the characteristics of the precursor raw materials and the alkali-activation system employed, which is consistent with the observation results of Komnitsas et al. [44], and also confirms the above description of gel composition. Furthermore, the comparable Si and Al contents in the two pastes provide a plausible explanation for their similar strength development.

4. Conclusions

In this study, GF and MT were incorporated into WRCBP at varying proportions to serve as binding materials for fabricating geopolymer pastes. FA was used in parallel with WRCBP for comparison, enabling assessment of the disparities between these two alkali-activated precursors. The major findings are summarized as follows:

• Under identical conditions, WRCBP-based pastes exhibited about 33.7% higher water demand compared with FA-based pastes due to the coarser surface texture and higher porosity of WRCBP.
• WRCBP-based pastes cured at 65 °C for 7 days using a 1.2 modulus Na₂SiO₃ solution with 8% Na₂O content achieved a compressive strength of 37.0 MPa, which increased to 39.8 MPa after 28 days. Under appropriate alkali-activated and curing conditions, WRCBP-based pastes attained 93.6% of the compressive strength of FA-based pastes.
• For both WRCBP- and FA-based pastes, an optimal combination of activator modulus and Na₂O content is required. Excessively high or low values adversely affect performance. A curing temperature of 65 °C was found optimal for both materials; 90 °C offered only marginal early-age benefits while impairing long-term strength.
• Particle size significantly influences alkali activation. WRCBP-based pastes with particles <45 μm consistently demonstrated higher compressive strength than those with particles <75 μm, due to the larger specific surface area enhancing dissolution and gel formation during geopolymerization.
• MIP analysis showed that FA-based pastes had lower porosity and smaller average pore size than WRCBP-based pastes. SEM analysis confirmed the presence of unreacted particles in both matrices but revealed a more homogeneous microstructure in the FA-based pastes.

Overall, WRCBP-based geopolymers can achieve sufficient mechanical strength, maintain adequate fluidity, and form compact microstructures when designed with appropriate activator and mixture parameters. This study demonstrates the feasibility of recycling WRCBP to manufacture geopolymers, thereby broadening the scope of sustainable, low-carbon construction materials. Future research should further examine the mechanical and durability performance of WRCBP-based geopolymer mortars.