Reaction mechanism and compressive strength of geopolymer based on the synergistic excitation of red mud and rice husk ash

1 Introduction

The synergistic high-value utilization of industrial solid wastes is significant for the development of a green and low-carbon industry. The evaluation of green energy sources, including solid waste as a renewable resource, has been addressed through various multi-criteria decision-making frameworks. Bhowmik et al. (2024) proposed an extended fuzzy-TODIM approach that incorporates solid waste among the green energy sources evaluated, highlighting the importance of systematic assessment in promoting sustainable energy transitions (Zhang et al., 2025; Chen and Huang, 2024; Melikoglu, 2025; Benlioğlu et al., 2026).

The geopolymerization reaction is the core theoretical basis in the field of solid waste alkali excitation. Its essence is that aluminosilicate minerals undergo a process of “dissolution–polymerization–condensation” in an alkaline environment and generate inorganic polymers with a three-dimensional network structure. In this process, the chemical composition of the raw materials, especially the molar ratio of SiO₂ to Al₂O₃ (Si/Al ratio), plays a decisive role in the structure and properties of the final products. Lv et al. (2026) used molybdenum tailings, Bayer red mud, and carbide slag to prepare all-solid waste alkali-activated materials, which significantly improved the cementitious properties through the complementarity of active components. The reaction mechanism was C-(A)-S-H gel forming under alkali excitation and interweaving to form a dense geopolymer network structure. Jin et al. (2025) proposed that magnesium oxychloride cement (MOC) can effectively solidify Bayer red mud, make its compressive strength meet the requirements of road base materials, and significantly improve the durability of water immersion and dry–wet cycles. Wang et al. (2026a) reported that, in a ternary geopolymer system comprising molybdenum tailings (MT), red mud (RM), and ground granulated blast furnace slag (GGBS), RM can refine the pore structure when its content is below 21%. With a mass ratio of MT:RM:GGBS at 30:21:49, the compressive strength of MRSGM reached 89.0 MPa in 28 days. The synergistic mechanism was that C-(A)-S-H and honeycomb N-A-S-H gel were interwoven to fill the matrix and refine the pores, which greatly reduced total porosity. These classic research results at the mechanism level provide an important reference for exploring the synergistic reaction mechanism of an RM and rice husk ash (RHA) binary system.

There has been much recent research on RM- and RHA-based geopolymers, laying a foundation for their synergistic utilization. Qu (2025) described possible applications of RM in solid waste recycling, confirming its feasibility as Si-Al raw material for geopolymer. This was also affirmed by Guo et al. (2024b) and Wu et al. (2025a). To explore the mechanical properties and microstructure of geopolymer with RM, slag, or fly ash as raw materials, Wang et al. (2026b) and Liu et al. (2025) studied the gelation and modification effect of RM in multi-component systems. They revealed the activation effect of RM for tailings and the micro-mechanism of reaction in the ternary geopolymer system. Research has shown that RM can be used as a Si-Al material source and alkali regulator to produce geopolymer. The specific study was carried out by Mishra et al. (2025) on RHA. Alkaline activators were used to confirm that compressive strength as well as a microscopically compact structure can be enhanced in room temperature conditions for geopolymer cementitious material, as shown in Rani et al. (2025), Mulapeer and Omar (2025), and Chen et al. (2026). They found that the combination of RHA, fly ash, and slag could optimize the strength and durability of geopolymer concrete and mortar. The micro-mechanisms of RHA-based geopolymer for stabilizing polluted soils has been revealed, and its potential usage areas have expanded, as well as how RHA could improve geopolymer reaction.

As mentioned above, geopolymer is one of the most popular hotspots of current research on the synergistic utilization of solid wastes, so RM and RHA have been investigated synergistically by researchers. There has been some initial exploration in this respect. Guo et al. (2024a) synergized the two to prepare eco-geopolymers, confirming that the system can realize the complementarity of active components, effectively alleviate the dissolution of harmful components in RM while improving the macroscopic strength of the material, and achieve the dual goals of environmental benefits and performance improvement. Dang et al. (2023) reported that RM–RHA geopolymer composite can effectively remove methylene blue dye solution and that their sorption capacities correlated with microporous structures and crystalline phases. Chaturvedi et al. (2025) used the two as raw materials to synthesize zeolite adsorbents, further confirming that the synergistic reaction of the two can regulate the microstructure of the product and give the material the dual functions of heat preservation and adsorption. In addition, Hamada et al. (2025) and Nanda et al. (2024) systematically reviewed the effect of RHA on the properties of geopolymer and the synthesis process of activator.

In summary, although existing research has confirmed the feasibility and superiority of the preparation of geopolymer by the synergistic excitation of RM and RHA, basic problems such as the nature of their synergistic effect are still unclear, which has become the core bottleneck restricting the large-scale application of this kind of solid waste geopolymer. In order to make up for these research gaps, this study systematically examined the synergistic reaction mechanism of RM–RHA binary system geopolymer. The novelty of this research is threefold. Firstly, unlike previous studies that focused on macroscopic performance optimization, this study quantitatively establishes the relationship between RHA content (i.e., Si/Al ratio) and gel chemical evolution, revealing the structural threshold of excess SiO₂ to destroy the aluminosilicate network and clarified the composition range of the system to obtain best performance. Secondly, through the combined characterization of X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR) under multiple curing ages, a three-stage geological polymerization reaction model is proposed which clearly divides the three kinetic stages of alkali dissolution, oligomer gel formation, and condensation into a three-dimensional network, thus providing a systematic framework for understanding the reaction process of the system. Thirdly, the characteristics of N-A-S-H as the main gel phase and C-(N)-A-S-H as the secondary auxiliary phase in the low-calcium system are clarified, and the chemical complementarity mechanism of RM and RHA in the synergistic process is clarified. It is demonstrated that the synergistic effect of the two is not limited to the physical filling effect but optimizes the gel structure and polymerization degree through the complementarity of chemical components. The above findings not only deepen understanding of the synergistic excitation mechanism of RM–RHA but also provide a theoretical basis for the rational design of two types of bulk solid waste to prepare high-performance geopolymers.

2 Theoretical basis

The essence of geological polymerization is that aluminosilicate minerals undergo three continuous processes of dissolution–polymerization–condensation under alkaline excitation conditions to form inorganic polymers with a three-dimensional network structure. In this reaction, the chemical composition of the raw materials, especially the molar ratio of SiO₂ to Al₂O₃ (Si/Al ratio), is the key parameter that determines the structure and final mechanical properties of the reaction products. It is generally believed that when the Si/Al ratio is between 2.0 and 3.5, the system can form a relatively complete N-A-S-H three-dimensional network structure, thereby obtaining better mechanical properties.

Based on the above theory, we selected red mud (RM) and rice husk ash (RHA) as the raw materials for Si and Al. We did this because RM is characterized by high Al, high Fe, and low Si (Si/Al ≈ 0.61), and the reaction activity is extremely low when excited alone. RHA is rich in amorphous SiO₂ (content >92%), which can effectively compensate for the lack of Si components in the RM system. The Si/Al ratio of the system can be systematically adjusted by compounding the two in different proportions, allowing its influence on the reaction products and mechanical properties to be studied.

Our experimental method selection was as follows. (1) Compressive strength testing is the most direct and universal index for evaluating the macroscopic mechanical properties of geopolymers, and it is also the most critical reference parameter in engineering applications. (2) XRD analysis was used to characterize the phase composition of raw materials and reaction products, especially for monitoring the formation and evolution of the amorphous gel phase. (3) FT-IR analysis was used to identify the chemical bonding and functional group characteristics of the reaction products, which can effectively track the degree of polymerization of the Si-O-Si/Si-O-Al network structure. The combination of these three methods can comprehensively reveal the reaction mechanism of RM–RHA synergistic excitation of geopolymer from the three levels of macroscopic properties-phase structure-chemical bonding. The in-depth analysis of the fine identification of the gel phase and its microscopic morphology will be further completed in the follow-up study by means of SEM-EDS and ²⁹Si MAS-NMR.

3 Experiment

3.1 Experimental raw materials

3.1.1 Solid raw materials

Bayer red mud (RM) was provided by an alumina company in Shandong province for testing, after natural drying and crushing. It was ground using a 200-mesh standard sieve, removing large impurities. Rice husk ash (RHA) was obtained from a grain processing company in Hunan province. Husks were heat treated at 600 °C for 2 h; after that, they were ground with a 200-mesh standard sieve. The chemical compositions of RM and RHA are presented in Table 1; these were determined using an X-ray fluorescence spectrometer (XRF-1800, Shimadzu, Japan). In addition, an X-ray diffractometer (XRD-6000, Shimadzu, Japan) was used to analyze the mineral component of the raw material (Figure 1).

TABLE 1. Main chemical components of red mud and rice husk ash (wt%).

Table 1 shows that the content of Al₂O₃ and Fe₂O₃ in RM is very high, with only 13.82% SiO₂ and an Si/Al value of 0.61, characteristic of low Si and high Al Fe solid waste. RHA, however, has much higher content of SiO₂ at 92.15%, while Al₂O₃ is only 2.08%. This indicates high-Si–low-Al solid waste. The combination of the two can adjust the Si/Al ratio of the geopolymer system more effectively and provide enough Si and Al raw material for geological polymerization reaction. The content of Na₂O in RM is 9.52%, and its alkalinity can reduce the amount of alkali activator and reduce preparation cost.

It can be observed from Figure 1 that the characteristic diffraction peaks of hematite (Fe₂O₃), lepidocrocite (γ-FeOOH), albite (NaAlSi₃O₃), and quartz (SiO₂) appear in the XRD pattern of RM, and the mineral phase is mostly crystalline and reactivity is low. The XRD pattern of RHA shows a wide and slow diffraction peak in the range of 2θ = 20°–30°. There is no obvious characteristic peak of crystalline minerals, indicating that SiO₂ mainly exists in an amorphous state, which is easy to dissociate in an alkaline environment and has high reactivity.

The particle size distribution of RM and RHA was determined by a laser particle size analyzer (Mastersizer 2000; Malvern, United Kingdom). The results are summarized in Table 2, in which the median particle sizes (D50) of RM and RHA are 18.73 μm and 3.85 μm, respectively, and the specific surface areas are 12.16 m²/g and 25.83 m²/g, respectively. Compared with RM, RHA has a significantly finer particle size and a larger specific surface area, which confirms that RHA particles can effectively fill the pores between RM particles and provide more reaction sites for geopolymerization.

TABLE 2. Particle size distribution characteristics of red mud and rice husk ash.

3.1.2 Alkali activator

In this experiment, the composite alkali activator was NaOH and water glass. NaOH is an analytical pure particle (purity >96%), with an aqueous solution of prepared substance concentration (Rihan and Alahmari, 2025). Water glass was industrial grade liquid water glass (modulus 3.3, SiO₂ content 26.5%, and Na₂O content 8.2%) (Abdullah et al., 2024). The mass ratio of NaOH solution to water glass in the composite alkali activator was 1.0 : 2.5, and the concentration of NaOH solution was 10 mol/L. The ratio was determined by preliminary tests: a fixed liquid–solid ratio of 0.65 and RHA content of 30%. The effects of NaOH concentration (8, 10, and 12 mol/L) and mass ratio of NaOH to water glass (1.0:1.5, 1.0:2.0, 1.0:2.5, and 1.0:3.0) on the compressive strength of geopolymer at 28 days were investigated. The results show that when the concentration of NaOH is 10 mol/L and the mass ratio of NaOH to water glass is 1.0:2.5, the compressive strength at 28 days reaches the optimal value, so this ratio is adopted in this study.

3.1.3 Mixing water

The mixed water used in the test is laboratory distilled water, which meets the requirements of the “Concrete Water Standard” (JGJ 63–2006) (Ministry of Construction of the PRC, 2006).

3.2 Experimental scheme

3.2.1 Mix proportion design

The reference raw material is red mud (RM), while the variable is rice husk ash (RHA), varying 0%, 10%, 20%, 30%, and 40%. After mixing both materials by their total mass, we labeled our samples “RHA-0,” “RHA-10,” “RHA-20,” “RHA-30,” and “RHA-40”. The proportion of liquid/solid (total mass of alkali activator/total mass of solid raw material) in the preparation of geopolymer is 0.65, which was calculated based on preliminary testing and referring to the conventional method of producing RM-based geopolymer (Zhao et al., 2026). The detailed mixture proportion of each sample is shown in Table 3.

TABLE 3. Specific mixing ratio of each sample.

3.2.2 Specimen preparation

The preparation process of the geopolymer specimen was optimized by referring to the “Cement Mortar Strength Test Method (ISO Method)” (GB/T 17671-2021) (State Administration for Market Regulation, 2025 Standardization Administration of the People’s Republic of China, 2021) combined with the preparation characteristics of solid waste geopolymer.

• RM and RHA powder weighed in proportion were put in a cement paste mixer and dry mixed for 30 s, mixing the solid feedstocks thoroughly.
• The NaOH solution prepared in advance was mixed with water glass in proportion and stirred evenly to prepare a composite alkali activator, which was allowed to stand for 10 min to eliminate bubbles in the solution.
• After dry mixing, the composite alkali activator was added steadily into the solid powder and then stirred by using the cement paste mixer at slow speed for 2 min and fast speed for 1 min, resulting in a homogeneous geopolymer slurry.
• The slurry was poured the into a 70.7 mm × 70.7 mm × 70.7 mm triple steel mold and placed on a vibrating table for 60 s to discharge bubbles and scrape off the surface of the mold.
• The specimens, which were molded and placed into a standard curing box at 20 °C ± 2 °C and an RH above 95%, were cured for 24 h, demolded, and then maintained under the same conditions until reaching their required age (7, 14, 28, 45, and 60 days, where “d” represents the curing age in days).

The main equipment used for specimen preparation were a JJ-5 cement paste mixer (Wuxi Jianyi Instrument Co., Ltd.) for slurry mixing, a ZS-15 shaking table (Beijing Zhongke Luda Test Instrument Co., Ltd.) used for the vibration molding of the test piece, and a YH-40B standard constant temperature and humidity curing box (Shanghai Luda Experimental Instrument Co., Ltd., temperature 20 °C ± 2 °C, relative humidity >95%) for specimen maintenance. A YES-300 electro-hydraulic servo pressure testing machine (Shanghai Sansi Vertical and Horizontal Machinery Manufacturing Co., Ltd., loading rate 2.0 kN/s) was used for compressive strength testing.

3.2.3 Test method

3.2.3.1 X-ray diffraction analysis

XRD pattern retrieval and analysis were performed by Jade 6.0 software, and the phase of raw materials and geopolymers with different ages and ratios was evaluated using an XRD-6000 type X-ray diffractometer. Test condition: the Kα radiation of Cu, with a tube voltage of 40 kV and a current of 30 mA, has a scanning range of 2θ from 5° to 90° and a rate of 10/min, with a step of 0.02 (Wu et al., 2025a).

3.2.3.2 Fourier transform infrared spectroscopy analysis

The chemical bonds and functional groups of the sample were studied by a Nicolet iS50 Fourier transform infrared spectrometer. Test condition: KBr tablet method, sample mass ratio 1:100, 15 MPa tableting pressure, scanning range 4000–400 cm^-1, 32 scan times, resolution 4 cm^-1. FT-IR spectra were analyzed by OMNIC software (Teerawattanasuk et al., 2024).

3.2.3.3 Compressive strength test

The compressive strengths of the geopolymer specimens were measured using a YES-300 computer-controlled electrohydraulic servo pressure testing machine, with a loading rate of 2.0 kN/s with three parallel samples per set. It took the arithmetic mean of the test results as its final compressive strength value, precise to 0.1 MPa. The core pieces of specimens after testing were immersed in anhydrous ethanol for 48 h to complete the water absorption treatment. After drying at 60 °C for 24 h and ground through a 200-mesh screen, they were used for scopic examination (Tesanasin et al., 2023; Xu et al., 2025).

In this study, compressive strength was selected as the core evaluation index of the mechanical properties. The main reason for this is that compressive strength is the most basic and critical mechanical parameter of geopolymer materials, which directly reflects the degree of gelation and structural integrity of materials. It is also the core design reference index in engineering applications. The core goal of this study is to reveal the reaction mechanism of RM–RHA co-excitation and its influence on compressive strength. Therefore, compressive strength can most effectively support the mechanism analysis of this study.

4 Test results and analysis

4.1 Phase composition analysis

4.1.1 XRD analysis of geopolymer with different rice husk ash content

The XRD pattern of geopolymer samples with different amounts of RHA at 28 days are shown in Figure 2 and can be observed from Figure 2. There are no peaks belonging to new crystal minerals found on XRD patterns of all geopolymers that consisted of wide slow amorphous humps. This indicates that the reaction product of RM–RHA geopolymer is mainly amorphous and/or low crystalline gel materials, which represents the general characteristic phase for geopolymers (Xu, et al., 2025).

The spectra of the RM raw material show the characteristic diffraction peaks of the inert mineral phase: hematite (Fe₂O₃, 2θ = 33.2°, 35.7°) and quartz (SiO₂, 2θ = 26.6^o). With an increase in RHA proportion, the relative intensity of the characteristic diffraction peak of quartz increases progressively. Although amorphous SiO₂ dominates in the RHA, there is still a small quantity of crystalline quartz left. The diffraction peak of hematite still exists, and the intensity change little. However, it should be noted that XRD mainly reflects the crystalline phase, and Fe may be incorporated into low crystallinity or amorphous gel products in the form of no obvious diffraction peaks. Therefore, it is difficult to draw a clear conclusion on whether Fe₂O₃ participates in the polymerization reaction just by XRD analysis. Recent studies have shown that Fe³⁺ can partially replace Al³⁺ in the aluminosilicate network of geopolymers, and Fe³⁺ may exist in C-(F)-A-S-H gels in different coordination environments (Fang, et al., 2023). In the future, characterization techniques such as Mössbauer spectroscopy or X-ray photoelectron spectroscopy are needed to further verify the actual role of Fe species in this system.

Compared with the pure RM geopolymer (RHA-0), a significant increase in the intensity of the amorphous bulge strength of samples mixed with RHA has been observed at an angle of 2θ = 20–35, and the bulge peak position gradually shifted to a high angle; among the samples, RHA-30 had the highest bulge strength. This phenomenon indicates that the incorporation of RHA promotes the dissociation and polymerization of active Si and Al components and generates a large number of amorphous gel products such as C-S-H, C-A-H, and C-A-S-H, with a higher degree of polymerization of these gel products. The peak shift is due to the formation of the Si-O-Al bond in the gel product, which changes the bond length and angle of the chemical bond, resulting in a change in the diffraction peak position (González, et al., 2017). The RHA-30 sample has the highest amorphous diffuse scattering peak intensity at 28 days, which is attributed to the optimal Si/Al ratio of the system (approximately 2.8), which promotes the formation of the largest amount of N-A-S-H gel. The silicon source is insufficient when the RHA content is too low (≤20%), while the excess Si cannot effectively enter the gel network due to the relative lack of Al component when the RHA content is too high (40%).

4.1.2 XRD analysis of geopolymers at different ages

The XRD patterns for 7, 14, 28, and 60 days of age were studied, and then an appropriate ratio of RHA-30 sample was chosen. The result is shown in Figure 3. According to the XRD diagram of the RHA-30 specimen, there are clear development rules for the increased curing time: the strength of the amorphous protrusion at 2θ = 20°–35° has not changed, and 60 days bulging strength is much higher than 7 days, indicating that the formation of gel products keeps increasing with time and that the geological polymerization reaction continues. There is no new characteristic peak of crystalline minerals, indicating that the geological polymerization reaction is always dominated by the formation of amorphous gel products. The later strength increase is mainly due to the polycondensation of gel products. A low-intensity diffraction peak of polyaluminosilicate siloxane appears in the range 40°–50° and becomes stronger with an increase in curing age; therefore, it can be concluded that with increasing curing time, the gel product undergoes another polycondensation process to form a long-chain polymer and serve as the skeleton of the strength improvement of the geopolymer. For RHA-30 sample, the peak strength of 28 days was between 7 days and 60 days, which reflected the time dependence of geological polymerization reaction. With the extension of age, the active components continued to depolymerize and polymerize, and the gel products continued to accumulate. The peak strength reached the highest at 60 days, which was consistent with the law of strength growth.

4.2 Analysis of chemical bonds and functional groups

4.2.1 FT-IR analysis of geopolymer with different rice husk ash content

The FT-IR spectrum of the geopolymer samples having different proportions of RHA at 28 days age is shown in Figure 4. All samples showed similar absorption bands in the FT-IR spectrum, mainly including four characteristic regions of 4,000–3,000 cm^-1, 1,700–1,500 cm^-1, 1,200–800 cm^-1, and 800–400 cm^-1, indicating that geopolymers with different RHA content had similar chemical bond composition. Importantly, however, both the location and magnitude of the absorption maxima differed, reflecting the different reaction degree and product structure (Wu et al., 2025b).

The attribution and analysis of the characteristic absorption peaks are as follows. (1) 3,420–3,430 cm^-1: the maximum of the absorption band corresponding to the stretching vibration of O-H bonds comes primarily from the absorbed water, bonded water in geopolymer, and the hydroxyl (-OH) group in the gel product. The wide and weak absorption peak for pure RM geopolymer (RHA-0) can be observed. After the incorporation of RHA, the absorption peak intensity is significantly enhanced, and the strength of the RHA-30 sample is the highest, indicating that the incorporation of RHA promotes the formation of gel products, and that the combined water content increases—consistent with the XRD analysis results. The maximum of the RHA-40 sample’s intensity slightly decreased in absorbance, which indicates a reduction in reaction amount. (2) 1,630–1,640 cm^-1: this is the bending vibration absorption peak of the H-O-H bond, corresponding to the free water and adsorbed water in the geopolymer. The change trend of the intensity of the absorption peak is consistent with that of the O-H bond. The strength of the RHA-30 sample is the highest, indicating that the content of the gel product is high, so the content of bound water and adsorbed water is correspondingly high, consistent with the results of the highest strength of the RHA-30 amorphous bulge in the XRD analysis. (3) 1,450–1,460 cm^-1: the peak corresponding to the asymmetric stretching vibration of O-C-O bonds of CO₂ is mainly attributed to carbonate minerals contained by RM as well as CaCO₃ that were produced due to the reaction between Ca²⁺ and CO₂ in the air when it was cured. No change in the absorption peak strength was observed with increasing RHA content, indicating that the involvement of carbonate minerals has played no role in the process of geopolymerization. (4) 980–1,000 cm^-1: the asymmetrical stretching vibration peak of the Si-O-Si and Si-O-Al bonds was typical for the geopolymer’s absorption peak. The strength and location reflect how much the Si-Al-O net was formed. Pure RM geopolymer (RHA-0) has a very weak absorption peak strength, and the peak position is located at 982 cm^-1. The absorption maximum strength was greatly increased after adding RHA and gradually shifted towards higher wave numbers in the position of the maximum. Sample RHA-30 has an absorption maximum of 996 cm^-1. A positive shift of the maximum towards a larger wave number indicates the formation of the Si-O-Al bond and an increase in the degree of polymerization of the Si-Al-O skeleton, which is the main reason for enhanced geopolymer strength. The absorbance peak strength of RHA-40 is lower than that of RHA-30, and the peak location returns to 990 cm^-1, indicating that excess RHA resulted in reduced Si-O-Al bonding and incomplete network structures. (5) 460–470 cm^-1: the formation and evolution of the Si-Al-O structure are also supported by the bending vibration absorption peak of the Si-O bond, which shows the same changing tendency of strength as that of the adsorption peak at 980–1000 cm^-1.

4.2.2 FT-IR analysis of geopolymers at different ages

The FT-IR spectra of RHA-30 samples at 7, 14, 28, and 60 days were analyzed (Figure 5). It can be observed from Figure 5 that with the increase of curing age, the FT-IR spectrum of geopolymer shows obvious evolution characteristics, reflecting the gradual process of geological polymerization reaction. (1) O-H bond absorption peak at 3,425 cm^-1: the absorbance peak’s strength continued to increase with age and began to narrow in form, indicating that the bound water content increased with age and that the gel product’s consistency was improved. (2) H-O-H bond absorption peak at 1,635 cm^-1: the intensity of the absorption peak gradually increases with the increase in curing age, indicating that the polymerization reaction continues and that the generated gel products continue to increase. The gel products have a large number of surface hydroxyl groups and nano-scale pore structures which can adsorb and accommodate more water molecules. Therefore, the intensity of the H-O-H absorption peak related to adsorbed water and bound water gradually increases. This is consistent with the trend of the strength of the O-H bond stretching vibration peak at 3425 cm-1 with the increase of age, and it is also consistent with the results of the growth of amorphous bulge strength with age in XRD analysis. (3) Si-O-Si/Si-O-Al bond absorption peak at 996 cm^-1: absorption maxima shift to higher values with increasing age as well as becoming sharper, indicating an increase in the number of Si-O-Al bonds. The degree of polymerization and regularity of the Si-Al-O network structure are also increasing, which is critical to the strengthening of geopolymer strength with age. (4) Si-O bond absorption peak at 465 cm^-1: the height of the absorption peak increases along with the ageing time and agrees with the evolution of the Si-Al-O structure.

The above characteristics indicate that the geological polymerization reaction is a continuous dissociation–polymerization–condensation process. With the increase in curing age, the active Si and Al components are continuously dissociated, and the gel products are continuously generated and condensed to form a more stable and higher polymerization degree of Si-Al-O network structure.

4.3 Development law of compressive strength

Figure 6 shows the compressive strength test results of RM–RHA geopolymer with various concentrations of RHA at 7, 14, 28, 45, and 60 days (see also Figure 6). The compressive strength of RM–RHA geopolymer is greatly influenced by the quantity of RHA as well as the curing age, thus establishing a rule of “age-related increase in the compressive strength, then decreasing when the RHA content increases”. This law shows that the proper incorporation of RHA can effectively stimulate the cementitious activity of RM, with the two forming a synergistic effect in an alkaline environment to jointly promote the development of geopolymer mechanical properties. However, there is an optimal range of RHA content. Too high or too low is not conducive to the full play of strength. In addition, the extension of curing age provides sufficient time for a geological polymerization reaction so that the polycondensation and structural rearrangement of gel products can continue, thus promoting the steady improvement of strength.

4.3.1 Effect of curing age

With increasing curing age, the compressive strengths of geopolymers at all ratios increased continuously; the increase rate was faster in the early stage (7–28 days) but then slowed at the late phase (28–60 days). Taking the best ratio RHA-30 as an example, the compressive strength was 6.8 MPa at 7 days, 9.5 MPa at 14 days, 12.3 MPa at 28 days, 14.2 MPa at 45 days, and 15.8 MPa at 60 days. The intensity growth rate of 7–28 days was 80.9%, and the growth rate of 28–60 days was 28.5%. This is because the geological polymerization reaction is a gradual dissociation–polymerization–condensation process. In the early stage, the alkaline solution rapidly eroded the surface of the solid raw material, and a considerable number of gels were prepared by largely dissolving and polymerizing the active Si and Al, leading to rapid growth in strength. In the later stage, the active components on the surface of the raw material gradually decreased, the reaction rate slowed, the formation of the gel product tended to be stable, and strength growth also slowed. However, the polycondensation reaction of the gel product continued, the structure was denser, and the strength was still slightly improved.

The strength of pure RM geopolymer (RHA-0) was at a very low level at each age, the strength at 7 days was only 1.2 MPa, the strength at 60 days was only 3.7 MPa, and the growth rate was always slow, indicating that the degree of geological polymerization of just RM under this test condition was extremely low, making it difficult to form a cementitious structure with certain strength.

4.3.2 Effect of rice husk ash content

The addition of RHA could notably increase the compressive strength of RM-based geopolymer, as its content increases from 0% to 30%, the 60 days compressive strength of geopolymer increased by 3.7–15.8 MPa (increasing 327%), but it decreased sharply after exceeding 30%. The 60-day compressive strength of the RHA-40 sample was 12.5 MPa, which is 20.9% lower than that of RHA-30.

When the RHA content was 10%–30%, strength increased with the increase in dosage because the amorphous SiO₂ in RHA is rapidly dissociated in an alkaline environment, which supplements the deficiency of Si content in the RM system, optimizes the Si/Al ratio of the system, promotes the formation of gel products such as C-S-H and C-A-S-H, and the porous structure of RHA can be used as reactive sites to accelerate the erosion and reaction of the alkaline solution. In addition, the results of particle size analysis show that the median particle size (D50 = 3.85 μm) of RHA is significantly thinner than that of RM (D50 = 18.73 μm), and the specific surface area (25.83 m²/g) is also significantly larger than that of RM (12.16 m²/g), so that it can fill the pores between RM particles, play a physical filling role, and improve the density of geopolymer.

The reasons for the decrease in strength when the RHA content is too high may include the following aspects. On the one hand, excessive RHA content leads to the relative lack of Al₂O₃ in the system, and the Si/Al ratio deviates from the optimal range (the Si/Al ratio of the RHA-40 sample is approximately 3.5). Too much SiO₂ may not fully combine with the Al component to form a three-dimensional Si-Al-O network structure but tend to generate a large number of independent C-S-H gels, with which is difficult to form a stable cementitious skeleton. On the other hand, the porous structure of RHA has strong water absorption. Too much content will lead to a large amount of free water in the slurry being adsorbed, making the effective reaction medium (free water) involved in the geological polymerization reaction insufficient and thus inhibiting the reaction. At the same time, the workability of the slurry worsens and more pores are generated during molding, thus reducing the compactness of the structure (Raj et al., 2025).

4.4 Analysis of reaction mechanism

The three-stage reaction model proposed in this study is consistent with the polymerization reaction framework established in the literature. Based on this theoretical basis and XRD/FT-IR characterization results, the reaction mechanism can be explained as follows. The XRD amorphous bulge strength increases with age corresponding to the continuous formation of the gel, and the Si-O-Al peak in FT-IR shifts to a high wave number corresponding to the continuous improvement of the degree of polymerization.

4.4.1 Stage 1: Alkaline erosion and component dissociation (0–7 days, early stage)

The NaOH and water glass in the composite alkali activator provide a strong alkaline environment (pH > 13). The strong alkaline solution first erodes the particle surface of RM and RHA and destroys its original mineral structure.

• The amorphous SiO₂ in RHA was dissociated under the action of OH⁻, and the soluble silicate ion [SiO₄]^4- was generated, as shown in Formula 1.
• The active Al₂O₃ in RM is dissociated under the action of OH⁻, and the soluble aluminate ion [AlO₄]^5- is generated, as shown in Formula 2.
• A small amount of crystalline SiO in RM is slowly dissociated in a strong alkaline environment to supplement the content of silicate ions. The core is shown in Formula 3.

There are then a large number of [SiO₄]^4-, [AlO₄]^5-, Na⁺, and Ca²⁺ ions in the slurry, which provide sufficient ion sources for the subsequent polymerization reaction.

S i O 2 amorphism + 4 O H − → S i O 4 4 − + 2 H 2 O , (1)

A l 2 O 3 + 10 O H − → 2 A l O 4 5 − + 5 H 2 O , (2)

S i O 2 crystalline + 4 O H − → S i O 4 4 − + 2 H 2 O . (3)

4.4.2 Stage 2: oligomer gel formation (7–28 days, middle stage)

The [SiO₄]^4- and [AlO₄]^5- in the slurry are polymerized in an alkaline environment to form an oligomer gel, which is combined with the Na⁺ in the system to form N-A-S-H gel. This is the main gel product in this low Ca and Na-rich system and the core skeleton for the development of geopolymer strength. In addition, a very small amount of Ca²⁺ (total CaO <2%) in the system can be combined with [SiO₄]^4- to form C-S-H gel. Under the condition that the N-A-S-H three-dimensional network is fully developed, an appropriate amount of C-S-H can be filled in the pores of the N-A-S-H skeleton to play an auxiliary enhancement role, and the two synergistically improve the compactness and mechanical properties of the geopolymer.

In a basic condition, [SiO₄]^4- and [AlO₄]^5- in the slurry polymerize into the formation of oligomeric gel and combine with Ca²⁺ and Na⁺ in the system to produce gel products such as C-S-H, C-AH, and C-ASH. This the most important process for rapidly increasing the geopolymer strength.

• [SiO₄]^4- combines with Ca²⁺ to form C-S-H gel, as shown in Formula 4.
• [AlO₄]^5- combines with Ca²⁺ to form C-A-H gel, as shown in Formula 5.
• The C-A-S-H gel is formed by the synergistic combination of [SiO₄]^4- and [AlO₄]^5- with Ca²⁺, as shown in Formula 6.

The gels formed by these oligomers (flocculent and wrapped on the surface of the solid particles) fill with pores between them, progressively developing a primary gel skeleton. Compressive strength develops quickly with the increase in content of gel products for geopolymers. On the other hand, the finely ground RHA fills the voids and increases density.

x C a 2 + + S i O 4 4 − + n H 2 O → C a x S i 1 O 2 + x / 2 · n H 2 O C − S − H , (4)

y C a 2 + + A l O 4 5 − + m H 2 O → C a y A l 1 O 2.5 + y / 2 · m H 2 O C − A − H , (5)

x C a 2 + + a S i O 4 4 − + b A l O 4 5 − + n H 2 O → C a x S i a A l b O 2 a + 2.5 b + x / 2 · n H 2 O C − A − S − H . (6)

4.4.3 Stage 3: Polymer network structure formation (28–60 days, later stage)

Oligomer gel products undergo a polycondensation reaction under the alkaline environment and increase their curing ages. The Si–O bond and an Al–O bond are connected to make a Si–O–Al link, which in turn generates long-chain polymers such as polyaluminosilicate silicate. These polymers overlap and crosslink with each other to form a three-dimensional continuous Si-Al-O network structure, which is the core reason why the strength of geopolymer continues to increase and tends to be stable. At the same time, the free water in the geopolymer is continuously discharged with the reaction, the porosity is greatly reduced, and the stability and regularity of the cementitious skeleton are significantly improved. Inert mineral particles such as hematite and quartz in RM are wrapped by a three-dimensional Si-Al-O network structure, which plays a role in physical enhancement and filling, further improving the mechanical properties of geopolymers (Yuan et al., 2025; Melnyk and Sviderskyy, 2024).

The mechanisms of these processes are mainly probed by XRD and FT-IR experiments. Future research will use SEM-EDS and ²⁹Si MAS-NMR to further investigate the gel structure, element distribution, and Qⁿ distribution at all times during the three-stage reaction model. It should be noted that the conclusions of this study on the type and distribution of gel products under excessive RHA content are mainly based on XRD and FT-IR analysis. XRD has limited ability to distinguish amorphous gels, and with FT-IR it is also difficult to accurately distinguish the microscopic morphology differences between C-S-H and N-A-S-H gels. Subsequent studies will use SEM-EDS and other means to more accurately characterize the morphology, distribution, and elemental composition of the gel products in the geopolymer with high RHA content, so as to further verify the influence mechanism of excess SiO₂ on the gel network structure.

5 Discussion

5.1 Application prospects

The 28-day compressive strength of the red mud–rice husk ash (RM–RHA)-based geopolymer prepared in this study reaches 12.3 MPa (30% RHA), which is better than some reported RM single systems (usually 30 MPa). It is therefore more suitable for non-load-bearing components, road base, and other scenarios with low strength requirements while giving full play to its dual environmental advantages of solid waste absorption and carbon emission reduction.

5.2 Practical challenges for large-scale applications

The conversion from laboratory to large-scale application still faces three major challenges. (1) Due to the different sources of raw materials (the content of SiO2 in red mud is 13%–25%), the chemical composition fluctuates, so it is necessary to establish a raw material pre-homogenization and rapid detection system. (2) The cost of alkaline activator accounts for approximately 40%–50% of the total cost of the material; in addition, the concentrated release of hydration heat during large volume pouring may cause shrinkage cracks, so it is necessary to develop low-cost activator formulations and retarding technologies. (3) At present, there is a lack of unified product standards and construction specifications for geopolymers based on solid waste, so it is necessary to cooperate with industry agencies to promote the formulation of standards.

5.3 Research limitations and future development

This study has the following main limitations. First, the distinction of different gel phases in the reaction products is mainly based on the indirect information of XRD and FT-IR. The above techniques have limited recognition ability for amorphous gels, and it is difficult to accurately identify the coexistence form and relative proportion of N-A-S-H and C- (N) -A-S-H. Second, the inference of the filling effect of RHA particles is based on the particle size distribution data but lacks direct micro-morphological evidence. In view of the above limitations, the microstructure, element distribution, and Qⁿ structure of the gel products were characterized by SEM-EDS and ²⁹Si MAS-NMR to clarify the spatial distribution and polymerization degree of different gel phases. At the same time, systematic tests of multiple mechanical parameters were carried out, and the engineering applicability of the material was evaluated more comprehensively in combination with microstructural analysis.

6 Conclusion

In this study, red mud (RM) and rice husk ash (RHA) were used as main precursors for synthesizing a synergically activated geopolymer. The effects of the RHA content and the curing ages were systematically investigated in terms of the resulting compressive strengths of geopolymers. The reaction mechanism was combined with XRD, FT-IR, and other test methods. The main conclusions are as follows.

• RHA can significantly improve the compressive strength of RM-based geopolymer. With an increase in RHA content, initial strength increases and after that decreases but thence increases with curing age. The optimal dosage at 30% gave a compressive strength of 15.8 MPa for the geopolymer with 60 days, which is greater than that obtained from pure RM geopolymer.
• The reaction products of RM–RHA geopolymer are mainly amorphous or low crystallinity N-A-S-H (sodium aluminosilicate) gel and a polyaluminosilicate silicate three-dimensional network structure. A very small amount of CaO in RM makes it possible for the system to generate a trace amount of C-(N)-A-S-H doped phase. The gel products increased with increased RHA content as well as with aging time, and the degree of polymerization continued to increase for Si-Al-O network structures.
• The maximum for the Si-O-Si/Si-O-Al bonds appears at 980–1,000 cm^-1, which represents a characteristic peak for the strength of this reaction. RHA addition enhances its activity as well as displaces it toward higher wavenumber. Increased age makes the peak shape sharper, indicating that the Si-Al-O network structure is more perfect, and the degree of polymerization and regularity are continuously improved.
• Compressive strength is closely related to the structure of the geopolymer, the sample containing 30% RHA has the most compacted structure, and their gels aggregate to form a solid 3D gel skeleton. The structure is denser with increasing curing age; gel wraps the original particle completely, and porosity is strongly reduced.
• RM–RHA synergistically stimulates the geopolymer reaction, which comprises three stages: alkaline erosion dissociation, oligomer gel formation, and polymer network structure formation. The two use chemical component complementation and optimize structure to form a three-dimensional Si-Al-O framework structure.

The application of RM with RHA as a high-value synergistic mix has been achieved in this study, which provides a new raw material mixture to manufacture solid waste geopolymer and a new way to dispose of solid waste from affiliated companies.

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Footnotes

Edited by: Babak Safaei, Eastern Mediterranean University, Türkiye

Reviewed by: Sanku Konai, National Institute of Technology, Durgapur, India

Dragan Marinkovic, Technical University of Berlin, Germany