One-Part Alkali-Activated Wood Biomass Binders for Cemented Paste Backfill

Abstract

This study developed a one-part alkali-activated slag/wood biomass fly ash (WBFA) binder (AAS) for preparing cemented paste backfill (CPB) as an alternative to traditional cement. Through multi-scale characterizations (XRD, FTIR, TGA, rheological testing, and MIP) and performance analyses, the regulation mechanisms of slag/WBFA ratios on hydration behavior, microstructure, and mechanical properties were systematically revealed. Results demonstrate that high slag proportions significantly enhance slurry rheology and mechanical strength, primarily through slag hydration generating dense gel networks of hydration products and promoting particle aggregation via reduced zeta potential. Although inert components in WBFA inhibit early hydration, the long-term reactivity of slag effectively counteracts these negative effects, achieving comparable 28-day compressive strength between slag/WBFA-based CPB (4.11 MPa) and cement-based CPB (4.16 MPa). Microstructural analyses indicate that the disordered gels in AAS systems exhibit silicon–oxygen bond polymerization degrees (950 cm⁻¹) comparable to cement, while WBFA regulates Ca/Si ratios to induce bridging site formation (900 cm⁻¹), significantly reducing porosity and enhancing structural compactness. This research provides theoretical support and process optimization strategies for developing low-cost, high-performance mine filling materials using industrial solid wastes, advancing sustainable green mining practices.

1. Introduction

Cemented paste backfill (CPB), a critical technology in the mining industry, is widely utilized in underground mining operations for resource extraction and tailings management [1,2]. By blending mine tailings with cementitious materials such as cement to form slurries that are subsequently injected into underground voids, CPB technology not only mitigates surface subsidence but also prevents resource wastage during mining activities [3,4]. The application of CPB facilitates rational resource utilization and provides effective environmental protection solutions for mining production [5,6]. Recent advancements in CPB research have focused on optimizing its multi-field performance (thermo–hydro–mechanical–chemical interactions) under varying curing conditions, such as humidity and temperature, to enhance long-term stability in complex underground environments [7,8,9]. In addition, with the growing global demand for sustainable development in the mining sector, CPB-related technological research and applications have garnered significant attention [10,11]. Notably, CPB demonstrates substantial potential and value in tailings treatment and improving mining environments.

However, despite the broad application prospects of CPB technology in mining operations, the use of cement—its primary binder—poses significant environmental challenges. Cement production requires substantial energy consumption and is associated with high carbon dioxide emissions, contributing critically to global climate change [12]. According to climate change studies, cement manufacturing accounts for approximately 5%–8% of global CO₂ emissions [13]. This substantial environmental impact has spurred interest in academia and industry in developing cement alternatives, particularly sustainable substitutes that reduce carbon footprints, lower energy demands, and maintain economic viability [14]. Consequently, the urgent demand for low-carbon, environmentally friendly, and sustainable cement alternatives has laid the groundwork for exploring novel binding materials.

Wood biomass fly ash (WBFA), an emerging alternative material, has attracted growing research interest due to its abundant availability and eco-friendly properties [15]. Global WBFA production, estimated at 18–150 million tons annually based on biomass energy utilization data and typical ash yield rates (1%–5% of feedstock mass), includes China’s contribution of approximately 4–6 million tons per year derived from its 40 GW biomass power capacity and sector-specific ash generation patterns [16]. Generated as a combustion residue from wood biomass, WBFA possesses unique resource advantages and can be processed for application across multiple fields [17]. Its prospects are particularly promising in the construction industry, especially in cement and concrete production [18]. Studies indicate that WBFA can partially replace cement, reducing its consumption and thereby lowering environmental impacts during production [19]. WBFA not only enhances the workability of concrete but also improves its durability and impermeability. Recent research further demonstrates that WBFA effectively increases the compressive strength of concrete and refines its microstructure [20,21,22,23]. For instance, Gabrijel et al. [18] demonstrated that WBFA with a high CaO composition significantly mitigated the drying shrinkage of concrete, achieving a reduction of up to 65% over a one-year period. Similarly, Berra et al. [24] discovered that when WBFA was used as a partial substitute for cement (with substitution rates ranging from 15% to 30%), the workability of concrete improved significantly, as evidenced by a reduction in the required water-reducing agent by approximately 33% compared to conventional concrete. These findings align with broader trends in sustainable material science, where industrial by-products are increasingly valorized to address resource scarcity and carbon emissions [25]. Consequently, significant progress has been made in applying WBFA as an environmentally friendly substitute in construction. Today, WBFA is increasingly utilized in various environmental management processes, including in wastewater treatment, soil remediation, and carbon sequestration [26,27,28,29]. These applications highlight its versatility beyond construction, as well as its role in contributing to sustainability and reducing environmental impacts. However, despite its demonstrated potential in this sector, broader utilization of WBFA still faces challenges, including optimizing its efficient utilization, reducing production costs, and expanding application domains, necessitating further research and exploration [30].

Given the environmental advantages of WBFA and its application potential in the construction sector, exploring its integration into CPB technology presents a critical research question. Recent advancements in CPB binder design, such as the use of recycled polymer fibers to enhance rheological properties and reduce pipeline wear, provide a precedent for innovative material substitutions [31,32]. Successfully employing WBFA as an alternative binder in CPB could not only address carbon emission issues associated with cement production but also promote broader applications of WBFA and resolve its utilization challenges. However, no published studies to date have investigated WBFA applications in CPB systems, establishing this research direction as a novel and scientifically significant contribution.

In this study, a “one-part” technique was used to prepare binders using WBFA and blast-furnace slag, which is more applicable in cast-in-situ projects. Additionally, X-ray diffraction (XRD) and thermogravimetric analysis (TGA) were used to identify the phase assemblage of binders. Fourier-transform infrared spectroscopy (FTIR) was conducted to characterize the structural information, especially in gels. Finally, CPB specimens were fabricated using the developed binder, followed by a systematic evaluation of their engineering properties, encompassing flowability, rheological behavior, uniaxial compressive strength (UCS), and microstructural characteristics.

2. Materials and Experiments

2.1. Raw Materials

The 42.5 Ordinary Portland cement was provided by Anhui Conch Cement Company (China). Ground granulated blast-furnace slag was supplied by a steel mill in China. The wood biomass fly ash (WBFA) was obtained from a plant in China. Figure 1 displays the SEM micrograph of WBFA. The image reveals a highly heterogeneous particle morphology, characterized by coexisting irregular angular fragments and porous aggregates, indicative of pyrolysis and agglomeration during combustion. The particles exhibit rough surfaces with localized sintered regions and undulating textures. Fine secondary particles are adhered to larger particle surfaces, suggesting interactions between high-temperature molten phases and incompletely combusted ash components. The chemical composition of three raw materials is presented in

Table 1. Chemical composition of cement, slag, and fly ash.

	CaO	SiO2	Al2O3	MgO	Fe2O3	Na2O	K2O	SO3	Others
Cement	64.5	20.1	5.4	-	3.2	0.6	-	2.9	3.3
Slag	40.2	33.8	13.6	8.4	0.4	0.2	0.5	1.5	1.4
WBFA	17.2	50.9	12.5	3.2	7.7	1.7	0.8	3.8	2.2

. Laboratory tap water was used as mixing water for sample preparation. The particle size distribution and XRD pattern of WBFA are shown in Figure 2 and Figure 3, respectively. It can be seen that quartz is the main mineral, with small amounts of hematite, anhydrite, and anorthite.

In addition, in order to prepare CPB, the raw materials also include tailings. The tailings used here came from a mine in northeastern China, and their main components include SiO₂ (60.3%), Fe₂O₃ (16.6%), Al₂O₃ (4.9%), etc. The fine content (<20 μm) of the tailings being 19%, they can be regarded as coarse tailings [33]. Detailed information on the particle size of the tailings can be seen in Figure 2.

2.2. Sample Preparation

The mixture design for pastes is presented in

Table 2. Mixture design of cement–slag (CS) pastes and AAS pastes. Unit: g.

	Cement	Slag	WBFA	Na2SiO3∙5H2O	Water
CS	30	70	0	0	40
S80	0	80	20	34	40
S60	0	60	40	34	40
S40	0	40	60	34	40

. The binder-to-water ratio was constant at 0.4, and the Na₂O-to-binder ratio in AAS pastes was about 5% by mass. In this study, the so-called “one-part” technique was applied for the preparation of AAS pastes [34]. The label “CS” refers to the cement–slag paste, whereas the labels “S80”, “S60”, and “S40” represent the AAS pastes with 80%, 60%, and 40% slag incorporation in the precursor by mass. CS and AAS pastes were prepared at room temperature (20 °C). The raw materials (cement, slag, WBFA, and sodium silicate pellets) were first mixed for 2 min to ensure a homogeneous distribution in the dry state. Water was then gradually added to the binder materials. The slurry was mixed at low speed for 1 min, followed by another 1 min at high speed. The fresh slurries were cast into small polythene bottles. Unlike cementitious materials, AAS pastes are sensitive to curing under high-humidity conditions [35,36]. Therefore, both the CS and AAS pastes were cured under sealed conditions for consistency. After curing for designated days, the paste was crushed into pieces and immersed in isopropanol to halt the hydration of the cement or the reaction of slag and WBFA. The paste pieces were then ground into fine powder in isopropanol and dried in a vacuum oven at 20 °C for 3 days. The well-dried powdered samples were employed for further characterization.

For the CPB sample, the solid content and binder dosage were fixed at 75% and 10%, respectively. The solid content corresponds to 1278.4 kg/m³, and the binder dosage is 127.8 kg/m³. After the required materials were prepared, the dry materials were mixed for 3 min, and then water was added and stirred for 5 min to prepare the CPB sample. After the fresh samples were cured for a specific period of time, UCS tests and mercury intrusion porosimetry (MIP) tests were performed. It is worth noting that the cylindrical CPB specimens with dimensions of 5 cm × 10 cm (diameter × height) were subjected to UCS testing. Furthermore, in accordance with the Chinese national standard GB/T 39489-2020, the target compressive strength for CPB specimens should range from 0.2 MPa to 5 MPa.

2.3. Experimental Methods

2.3.1. XRD, TGA, and FTIR Tests

XRD tests were carried out to determine the phase assemblage of the hydrates and reaction product in the CS and AAS pastes. The samples were scanned between 10° and 70° at a rate of 5°/min. TGA was performed to measure the mass loss of the reaction product with the increase in temperature. The reaction-stoppage powdered samples were heated from 40 °C to 1000 °C at a rate of 10 °C/min in an argon atmosphere. FTIR measurements were carried out to specifically examine the structure of gels in different pastes. The samples were scanned across a range of 600 to 2000 cm⁻¹ with a resolution of 2 cm⁻¹.

2.3.2. Flow Spread and Rheology Tests

The flow spread test was conducted to evaluate the workability of the slurry. A miniature slump cone with dimensions of 50 mm top diameter, 100 mm bottom diameter, and 150 mm height was employed. During the experiment, the mixture was first poured into the slump cone and properly compacted. Subsequently, the mold was removed to observe the free-flow pattern of the material. The flow spread value was quantified by measuring the horizontal expansion radius after mold removal, which served as an indicator of fluidity. A larger spread radius typically corresponds to better flow characteristics, while a smaller radius suggests inferior fluidity of the material [37].

The fluidity test measures the free-flow behavior of the CPB material by assessing its ability to spread once poured from a slump cone. This test provides a basic indication of the slurry’s workability and how easily it can be transported and placed in underground voids [38]. On the other hand, the rheological test provides a more detailed and comprehensive analysis by measuring the material’s resistance to flow under different shear rates [39]. The rheological measurements were conducted using a Brookfield RSR-CC rheometer equipped with a vane rotor (20 mm diameter, 40 mm length). The testing protocol followed the shear procedure outlined in reference [39]: initial shearing at 100 s⁻¹ for 60 s, followed by a 15 s rest period. Subsequently, a ramp-up phase was executed over 60 s, during which the shear rate increased linearly from 0 s⁻¹ to 100 s⁻¹, immediately followed by a ramp-down phase returning from 100 s⁻¹ to 0 s⁻¹. Pre-shearing helps to stabilize the sample by homogenizing the mixture and eliminating any shear-induced aggregates or inconsistencies that could otherwise affect the accuracy of the results [40]. Rheological parameters of the material were derived by fitting the Bingham model (Equation (1)) to the data collected during the ramp-down phase, enabling systematic analysis of its rheological properties.

$$\tau ={\tau }_0+\eta \dot{\gamma }$$

(1)

where $\tau$ and ${\tau }_0$ represent the shear stress and (dynamic) yield stress, respectively, $\eta$ denotes the plastic viscosity, and $\dot{\gamma }$ corresponds to the shear rate.

2.3.3. Zeta Potential Test

Electrochemical interactions between particles and particle–fluid interfaces in suspensions significantly influence the rheological properties of the system, which originates from the regulatory mechanism of colloidal particle aggregation states on macroscopic flow behavior [41]. In this study, zeta potential characterization was performed using a Malvern Zetasizer Nano ZS90 analyzer (Malvern Panalytical Ltd., Malvern, United Kingdom). Suspensions were prepared with a precise solid-to-liquid ratio of 0.1 g/L [42]. To ensure data reliability, triplicate zeta potential measurements were conducted for each sample.

2.3.4. UCS Test

After the CPB samples were cured for 3 days and 28 days under sealed conditions, the UCS test was performed using a Humboldt HM-5030 (Humboldt Mfg. Co., Norridge, IL, USA). The size of the sample was a cylinder with a diameter of 5 cm and a height of 10 cm. During the test, the selected deformation rate was 1 mm/min [43]. All the proportions were tested three times, and the average value was taken as the final strength value.

2.3.5. MIP Test

MIP was employed to determine the pore size distribution of the samples. Small specimen fragments were selected for analysis using an Auto-Pore IV 9510 instrument (Micromeritics Instrument Corporation, Norcross, GA, USA) with a measurable pore diameter range of 0.003 μm to 1000 μm. This technique characterizes pore structures by forcing mercury into material voids under incrementally applied pressures. Mercury remains non-intrusive until sufficient pressure overcomes the capillary resistance of specific pores. Stepwise pressure increases enable quantification of mercury intrusion volumes, which correlate with pore size ranges. Experimental procedures included initial drying and mass measurement of samples, placement into the instrument’s sample chamber, and systematic recording of mercury volume changes at each pressure increment. Pressure intrusion data were subsequently analyzed to derive critical parameters such as pore size distribution, pore volume, and specific surface area.

3. Results and Discussion

3.1. Characterizations of Pastes

3.1.1. XRD

Figure 4 shows the XRD pattern of CS and AAS pastes cured for 7 days and 60 days. As for the two CS pastes, gels and ettringite are the two main hydrates, without significant changes between 7 days and 60 days [44]. Portlandite, a primary hydrate during cement hydration, is rarely found in the two CS pastes. On one hand, a 70% dosage of slag in the CS paste would consume the portlandite during cement hydration due to the pozzolanic reaction. On the other hand, there might remain small amounts or poorly crystallized portlandite in CS pastes, whose intensity is relatively weaker than that of the other hydrates. Blite (C₂S, dicalcium silicate) is the remaining phase in the clinker of CS pastes, which is hardly reacted during cement hydration. As for the AAS pastes, the XRD pattern is dominated by the peak of quartz, due to the presence of WBFA. This is consistent with the results in [45]. The intensity of quartz increases with the increase of WBFA dosages. Ettringite is absent in AAS pastes; instead, the gel is the only detectable reaction product. However, in a pure slag system [35,46,47], hydrotalcite is also a significant reaction product. The absence of hydrotalcite is probably due to the introduction of WBFA, making the reaction product less ordered.

3.1.2. TGA

Figure 5 shows the TG and DTG curves of CS and AAS pastes cured for 7 and 60 days. As shown in Figure 5A, the weight loss of CS paste is higher than that of the AAS pastes, which indicates a higher content of hydrates in the cement system. In addition, the weight loss increases with the increase of the curing age, indicating the continuous reaction of pastes. Figure 5B shows the DTG curve of CS and AAS pastes. In the cement paste, the DTG curve is dominated by three peaks: the one between 40 °C and 250 °C (attributed to water loss from C–(A–)S–H gels (Equation (2)) [48]), the one between 400 °C and 500 °C (attributed to water loss from portlandite (Equation (3)) [49]) and the one between 600 °C and 800 °C (attributed to CO₂ loss from carbonates [50]). The carbonate might be due to the carbonation of portlandite during storage. However, these two phases are not evident in the XRD result, probably because the reflections are minimal. The DTG curves of AAS pastes are less characteristic than those of CS pastes. The intensity of gel peak in AAS pastes is lower than that in CS pastes. Additionally, weight loss between 300 °C and 400 °C is also identified, which can be attributed to the presence of hydrotalcite-like phases.

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

(2)

$${\mathrm{C}\mathrm{a}\mathrm{C}\mathrm{O}}_3\to \mathrm{C}\mathrm{a}\mathrm{O}+{\mathrm{C}\mathrm{O}}_2\uparrow$$

(3)

3.1.3. FTIR

Figure 6 shows the FTIR pattern of CS and AAS pastes cured for 7 days and 60 days. It can be seen that four patterns are dominated by the peak near 950 cm⁻¹, which is assigned to the asymmetric stretching vibration of Si–O bonds in the Q² sites of gel chains [51]. Typically, the peak with a higher wavenumber indicates the gel with a higher degree of polymerization. The polymerization degree of gels is comparable in the CS and AAS pastes. Furthermore, small shoulders are observed on the low-frequency side of Q² peaks (about 900 cm⁻¹) in AAS pastes. This observation is in agreement with [52]. According to [53], this shoulder can be attributed to the stretching vibration of Si–O in the bridging site of the gel, especially with a lower Ca/Si ratio. Moreover, the reflection near 830 cm⁻¹ is associated with the stretching vibration of Si–O bonds in the Q¹ sites of gels [54]. The presence of Q¹ sites is more pronounced in the cement paste compared to the AAS pastes.

3.2. Characterizations of CPB

3.2.1. Flow Spread and Yield Stress

Figure 7A shows the results of the flow spread and yield stress of CPB prepared with different binders. It is evident that for CPB prepared with AAS materials, as the WBFA content decreases (i.e., the slag content increases), the flow spread of the CPB decreases from 235 mm to 181 mm, while the yield stress increases from 56.3 Pa to 93.9 Pa. This phenomenon can be explained from several aspects. First, slag generates a large amount of calcium silicate hydrate gel during the hydration process. These hydration products form a more tightly knit network structure, thereby increasing the viscosity and internal friction of the slurry, leading to reduced flowability [55,56]. Additionally, WBFA contains relatively more low-reactivity mineral components, such as quartz. When the WBFA content is high, fewer hydration products are formed in the slurry, resulting in a looser structure. Therefore, as the WBFA content decreases and the proportion of slag increases, more hydration products are formed, and the slurry structure becomes more compact, leading to a reduction in the flow spread and an increase in the yield stress. This explanation is supported by the zeta potential results (Figure 7B). As seen from the figure, the zeta potentials for S40-CPB, S60-CPB, and S80-CPB are −26.8 mV, −22.1 mV, and −18.9 mV, respectively. Zeta potential reflects the charge on the surface of the slurry particles and the repulsive forces between the particles [57]. When the proportion of slag increases (i.e., WBFA content decreases), the generation of hydration products decreases, leading to a reduction in the charge density on the surface of the slurry particles, thus weakening the electrostatic repulsive forces between the particles. A lower zeta potential means that the attractive forces between the particles are stronger, leading to easier particle aggregation, which further enhances the structural integrity of the slurry, consequently reducing flowability and increasing yield stress [3].

It should be noted that the flowability of CS-CPB is worse than that of S40-CPB and S60-CPB, but better than that of S80-CPB. This is because, on the one hand, slag has a higher negative zeta potential compared to cement [58]; on the other hand, the alkali activator further reduces the zeta potential between slag and WBFA [41]. Therefore, the CPB samples prepared with AAS have a higher negative zeta potential than CS-CPB (Figure 7B), which is why the flowability of S40-CPB and S60-CPB is superior to that of CS-CPB. However, for S80-CPB, the slag content is too high, leading to the formation of more hydration products, which makes the internal structure of the slurry more compact (i.e., the packing density of the system increases), increasing internal friction and reducing flowability [59,60].

3.2.2. UCS

UCS tests were conducted on CPB samples prepared with different cementitious material systems (CS-CPB, S40-CPB, S60-CPB, and S80-CPB) at 3 days and 28 days, with the results shown in Figure 8. The experimental results indicate that the UCS of CPB samples from different systems shows significant differences at both 3 days and 28 days. The CS-CPB sample exhibited significantly higher strength at both curing times compared to the other systems, while the strength gradually increased with the reduction of WBFA content (S40-CPB, S60-CPB, S80-CPB), particularly for the S80-CPB sample, whose final strength approached that of CS-CPB.

For the 3-day UCS, the CS-CPB sample exhibited a higher compressive strength (1.46 MPa), a phenomenon attributed to the fast hydration reaction rate of cement [61]. The cement–water reaction generates a large amount of hydration products (such as C–S–H and CH), which can quickly fill the pores of the CPB and form a dense network in the microstructure, significantly enhancing the early strength of CPB [38]. In contrast, the strength of CPB from the slag–WBFA system (S40-CPB, S60-CPB, and S80-CPB) was lower, especially for S40-CPB (0.88 MPa). The fundamental cause of this phenomenon lies in the slow rate of slag hydration. The silicate and aluminate components in slag, under the influence of alkaline activators, form hydration products like C–S–H and C–A–S–H, but this reaction is much slower than the cement hydration reaction [62]. The slag hydration process is influenced by factors such as the fineness of the slag and the concentration of alkaline activators, so it did not significantly complete hydration within 3 days, resulting in a lower strength [63]. Notably, the addition of WBFA significantly suppressed the early strength of the slag system. The low-reactivity minerals (such as quartz, feldspar, etc.) abundant in WBFA hardly participate in the hydration reaction. These inert components reduce the concentration of reactive components in the slurry, thereby slowing down the slag hydration rate [64]. Higher WBFA content further dilutes the active components in the slag, suppressing the formation of effective hydration products, which significantly reduces the early strength of the samples.

After 28 days of curing, the UCS of CPB from the slag–WBFA system significantly increased, especially for S80-CPB (4.11 MPa), whose strength was nearly equivalent to that of CS-CPB (4.16 MPa). This indicates that with the extension of curing time, slag gradually generates more hydration products during the hydration process, significantly enhancing the cementing properties and mechanical performance of the samples. Particularly for the S80-CPB sample, due to the higher slag content, a large number of hydration products were generated during the hydration reaction, greatly improving the strength of the sample [65]. However, the suppressive effect of WBFA on the hydration process remained significant during the long-term curing process. The strength of S40-CPB (2.17 MPa) was still lower than that of other slag–WBFA system samples, indicating that the incorporation of WBFA somewhat limited the full progression of the hydration reaction. Although the slag hydration reaction was gradually completed under 28 days of curing, the low-reactivity components in WBFA still had an adverse impact on the hydration process, limiting the formation of hydration products. The high content of minerals such as quartz in WBFA results in relatively fewer active components in the hydration reaction, thereby affecting the density of the final hydration products and limiting the improvement in strength.

3.2.3. Microstructure

Figure 9 shows the MIP results of S40-CPB and S80-CPB at 28 days. The total porosity of S40-CPB and S80-CPB are 39.34% and 33.11%, respectively. This indicates that S40-CPB has a higher overall porosity compared to S80-CPB, which reflects a more porous and less compact structure. The capillary pore (100–1000 nm) and large pore (>1000 nm) are identified as the primary factors adversely affecting the strength properties of CPB [32]. Upon further examination of the pore distribution, the cumulative percentage of large pore and capillary pore in S40-CPB are 2.88% and 22.17%, respectively. In comparison, S80-CPB has a slightly lower percentage of macropores (2.74%) and capillary pores (17.86%). These results suggest that S80-CPB has a more uniform and denser pore structure.

The higher porosity and the larger proportion of large pores and capillary pores in S40-CPB lead to a weaker microstructure with less effective particle bonding and lower compressive strength. In contrast, the S80-CPB sample contains a higher slag content, which results in the formation of more hydration products and a denser hydration product network. This denser network of hydration products contributes to better particle interactions and more compact pore distribution, thereby improving the overall structure and enhancing compressive strength.

4. Conclusions

This study systematically examined the hydration behavior of the alkali-activated slag–WBFA system and its influence on CPB performance, utilizing comprehensive characterization techniques including XRD, FTIR, TGA, rheological testing, and SMIP analyses. The study elucidates the regulatory mechanisms by which slag–WBFA composition ratios govern hydration product formation, microstructural evolution, and macro-mechanical properties of the cementitious matrix. Based on these experimental findings, the following conclusions can be drawn:

• The hydration products of the alkali-activated slag–wood biomass ash system were dominated by disordered gels, without forming crystalline phases such as ettringite or hydrotalcite. However, the Si–O bond vibration peak (950 cm⁻¹) in these gels exhibited a polymerization degree comparable to those in cement-based systems. WBFA reduced the Ca/Si ratio, facilitating the formation of bridging sites (900 cm⁻¹) within the gel matrix and thereby enhancing microstructural densification.
• Increasing slag content (with reduced WBFA proportion) decreased the flow spread of CPB while elevating yield stress. This behavior arises from slag hydration products forming dense network structures, which increase internal friction within the slurry. Concurrently, higher slag proportions reduced particle surface zeta potential, weakening electrostatic repulsion and enhancing particle aggregation, further diminishing fluidity.
• Inert components in WBFA (e.g., quartz) suppressed early hydration reactions, but long-term strength was compensated by sustained slag hydration. High-WBFA CPB (e.g., S40-CPB) exhibited a 3-day compressive strength of only 0.88 MPa, significantly lower than cement-based CPB (CS-CPB: 1.46 MPa), primarily due to WBFA diluting reactive components and retarding slag hydration. However, after 28 days, high-slag systems (e.g., S80-CPB) achieved a strength of 4.11 MPa, comparable to CS-CPB (4.16 MPa), demonstrating that long-term hydration products from slag effectively offset WBFA’s inhibitory effects.
• High-slag S80-CPB, enriched with hydration products, formed tightly interconnected networks and exhibited significantly lower porosity than high-WBFA S40-CPB, particularly showing uniform densification in the micropore range. In contrast, S40-CPB’s loose structure and unreacted WBFA particles resulted in elevated porosity, compromising mechanical strength. These findings highlight that optimizing slag-to-WBFA ratios to regulate microstructure is critical for enhancing CPB performance.

While this study provides a comprehensive investigation into the use of WBFA and slag as binders in CPB, several areas remain open for future research. For example, the long-term durability of CPB materials incorporating WBFA warrants further exploration, especially under challenging environmental conditions such as high humidity, freeze–thaw cycles, and varying pH levels. Future studies should focus on the degradation mechanisms of these materials over extended periods to assess their long-term performance.