Experimental Study on the Curing Mechanism of Red Mud-Based Stabilized Soil Co-Modified by Nano-SiO2 and Gypsum

1. Introduction

Red mud, which is of high iron oxide content and red color, is a solid waste generated during the process of alkali alumina production. According to relevant statistics, for every 1 ton of alumina produced, 1 to 2 tons of red mud is generated (Wang et al., 2019; Zhu et al., 2016a). Currently, red mud is disposed of by open damming and stockpiling. The utilization of red mud is strictly controlled (Bombik et al., 2020; Xue et al., 2016a), making the utilization rate of red mud extremely low (less than 10% utilized so far, Xie et al., 2020; Mukiza et al., 2019), and red mud has become one of the bulk industrial solid wastes . Therefore, it is of practical significance to change the treatment of red mud from simple stockpiling to resource-based integrated utilization.

Red mud can be used to make ceramics and bricks, used as raw material for cement production, applied to road base materials, etc. By considering that road base materials can consume large amounts of red mud (Liang et al., 2021; Liu et al., 2018), currently, the research on the application of red mud in engineering is hot and has shown great potential (Ou et al., 2022; Suo et al., 2021; Atan et al., 2021). Mukiza et al., (2019) investigated the possibility of using red mud as a road base material, showing that the synergistic use of red mud and other wastes can improved the mechanical properties and durability performance of the material compared to red mud alone. Zhang et al., (2018) conducted experiments to determine the optimal ratio of red mud-based blends as road base materials, and proved that they can be used in road base through freeze-thaw tests, dry shrinkage tests, temperature shrinkage tests, and erosion resistance Li et al., (2020) studied the effect of gypsum on red mud-slag roadbed grouting material and found that gypsum can reduce the fluidity of red mud-slag grouting material and can improve the compressive strength of grouting material. Zhang et al., (2019) prepared road base material using electrolytic manganese slag-red mud-electrolytic slag as the main raw material, and the results showed that the road base material hydrated to produce C-A-S-H gel and calcium alumina, and the road base material had high unconfined compressive strength strength and good durability. Li et al., (2022) studied the effect of calcium bentonite on the working performance of red mud-based grouting material, and further demonstrated that the road base material had high strength and durability because the red mud-based grouting material has a dense pore structure and high material strength. Through the above literature, it can be seen that scholars have conducted in-depth research on the feasibility of the red mud road base material and also analyzed its causal mechanism in depth.

Nanomaterials are extremely small particles with a particle size of 1 to 100 nm, with high specific surface area and good volcanic ash activity (Li et al., 2020). Scholars have introduced nano-SiO₂ materials into the field of civil engineering, and their main applications are improvement of concrete materials and soils (Kulkarni et al., 2022; Farajzadehha et al., 2021; Almurshedi et al., 2020; Adamu et al., 2018). The performance advantages of nano-SiO₂ are obvious, but how to utilize the advantages of nanomaterials and exploit them in red mud modification in strength aspects are rarely studied. Therefore, in this paper, nano-SiO₂ is synergistically involved in the modification of red mud-based stabilized soil, mechanical property and microstructure of the stabilized soil are experimentally studied. Based on the obtained results, the curing mechanism of nano-SiO₂ synergistically modified red mud-based stabilized soil is analyzed.

2. Materials and methods

2.1. Raw materials

2.1.1. Red mud

The red mud used in the test was taken from an aluminum company in Guangxi Province, which is a Bayer red mud. The chemical composition of the red mud is shown in Table 1. It shows that the red mud contains 5.23% of Na₂O, which can provide hydroxide for the hydration process.

2.1.2. Nano-SiO₂

The nano-SiO₂ used was produced by the mechanical crushing process. The nano-SiO₂ has a large specific surface area and a high surface activity. Its technical parameters are listed in Table 2.

2.1.3. Cement

The cement used in the test is 42.5 standard ordinary silicate cement. It has a specific surface area of 340 m² /kg and a density of 3.10g /cm³. The chemical composition is shown in Table 3.

It shows that the main mineral composition of the cement is tricalcium silicate 3CaO-SiO₂ (C3S), accounting for 50-60%; dicalcium silicate 2CaO-SiO₂ (C2S), accounting for 20-25%, called Belite or B ore; tricalcium aluminate 3CaO-AlO₂₃ (C3A), accounting for 5-10%; tetra calcium iron aluminate, 4CaO-AlO₂₃-FeO₂₃ (C4AF ), accounting for 10-15%.

2.1.4. Gypsum

The selected gypsum is a kind of construction gypsum produced by Jinan Desheng Chemical Technology Co. Ltd., Shandong Province. It is white powder with a density of 2.32 g/cm³. Its chemical formula is CaSO₄-0.5H₂O.

2.2. Specimen preparation

When stabilized soil is applied to the subgrade material, the strength requirements need to be met first. Combining the studies of Xue et al., (2017), Xue et al., (2016a), the designed mass ratios added in the red mud-based stabilized soil of nano-SiO₂ are 0.5%, 1%, 1.5%, 2%, 2.5% and 3%; gypsum is 6%; cement are at 1%, 3%, 5%, 7%, and 9%. Referring to Table 4.2.4 in Technical Guidelines for Construction of Highway Roadbases (JTG/T F20-2015), the 7-d unconfined compressive strength of red mud-based stabilized soil should be greater than or equal to 2 MPa, so as to serve as the base material used for medium and light traffic secondary and secondary roads.

The specimen preparation are as follows:

①

Drying and grinding of red mud specimens into powder form;

②

Add cement and powdered gypsum to the red mud specimen in accordance with the designed dosage and mix well;

③

Add deionized water to the specimen in accordance with the maximum dry density and optimum water content determined by the compaction test, and mix well;

④

The test material will be made into 5mm×5mm cylindrical compressive specimens, and the specimen production time should be controlled within 1 hour after adding cement；

⑤

After the specimens are made, they are left to standing curing for different periods (i.e., 1, 7, 14 and 28 days) before the mechanical and microstructure experiments.

Following the above preparation procedures, stabilized soil specimen with different contents of red mud, cement and nano-SiO₂ were prepared. The synergistic combination scheme of the added stabilizers is shown in Table 4, where NS1CS6PC3 denotes indicates that the doping mass ratio of nano-SiO₂ is 1%, the doping mass ratio of gypsum is 6%, and the doping mass ratio of cement is 3%.

2.3. Testing methods

2.3.1. Unconfined compression test

The prepared specimens, cured for 1, 7, 14 and 28 days, respectively, were subjected to water immersion for 24 h before the unconfined compression test. The compression was conducted under the condition of the unconfined compression test is carried out using the TSZ series automatic triaxial instrument (Figure 1) produced by Nanjing Soil Instrument Factory Co. The instrument can directly obtain the experimental maximum compression strength and the relationship curve between the main stress difference and axial strain.

2.4.2. Scanning electron microscope test

A scanning electron microscope (SEM) of type S-3400 produced by Hitachi, Tokyo, Japan, was used for microstructure observation, which has a magnification of 20 ~ 30,000 times. Before observation, the dried specimen is cut into observation cubes with dimensions of 4mm (length) × 4mm (width) × 2mm (thickness) using a geotechnical knife. The natural cross-section is used as the observation surface, then the observation cube is connected to the metal panel for observation through the conductive strip, and after the specimen surface and sides are sprayed with gold, it can be tested and observed.

2.4.3. Energy spectrum analysis test

Energy spectrometry (EDX) is the analysis of all elemental species and contents between Be-U within the micro-zone of the material using an energy dispersive spectrometer, which is used in conjunction with a scanning electron microscope.

2.4.4. X-ray diffraction test

X-ray diffraction (XRD) tests are used to measure the mineral composition of test materials. Before testing, the red mud-based stabilized soil material was oven-dried, after which the samples were ground into powder form and set aside. The mineral composition was qualitatively analyzed using an X'Pert PRO MRD/XL high-resolution diffractometer. During the experiments, the ray wavelength λκα was 1.54060 Å, the tube pressure was 40 KV, the tube electric current was 40 mA, the scanning range was 5° to 75°, the step size was 0.02°, and the scanning speed was 5°/min.

2.4.5. X-ray photoelectron spectroscopy test

X-ray photoelectron spectroscopy (XPS) is an advanced analytical technique in the microscopic analysis of electronic materials and components. Before the test, the red mud-based stabilized soil material was oven-dried, after which the sample was ground into powder form and set aside. A PHI-5300ESCA X-ray photoelectron spectrometer was used for XPS analysis of the red mud-based stabilized soil specimens with an Mg / Al anode target with 400 W power and the analyzer charge set to 17.5 eV, which was detected by a position-sensitive detector.

3. Results and analysis

3.1. The unconfined compressive strength

3.1.1. Effect of cement content

The unconfined compression tests were conducted on the cylindrical specimens of red mud-based stabilized soil mixed with cement alone at 1%, 3%, 5%, 7%, and 9%, respectively. The test results are shown in Figure 2.

Cement-modified red mud-based stabilized soil has higher compressive strength with increasing cement admixture, the unconfined compressive strength of the cement-modified red mud-based stabilized soil was 491 kPa, 928 kPa, 1262 kPa, 1639 kPa, and 1797 kPa for the specimens with 1%, 3%, 5%, 7%, and 9% cement dosing, respectively.

The unconfined compressive strength of cement-modified red clay-based stabilized soil increased with the increase of curing time, but the increase of unconfined compressive strength from 1 d to 7 d was greater than the increase of curing time from 7 d to 14 d and that from 14 d to 28 d. The increase of unconfined compressive strength of cement-modified red clay-based stabilized soil was calculated as the increase of curing time from 1d to 7d and from 14 d to 28 d only. It is assumed that the presence of soluble alkali in the red mud can promote the hydration reaction, and the strength increase of the stabilized soil by the modification of cement is also mainly concentrated in the pre-curing period.

3.1.2. Effect of synergistic modification of nano-SiO₂, gypsum and cement

To verify the optimum dosing of nano-SiO₂ in the modified material, the dosing of gypsum was chosen to be 6% in this section, the dosing of cement was chosen to be 3% considering the economy, and the dosing of nano-SiO₂ was chosen to be 0.5%, 1%, 2% and 3% for the tests. Figure 3 shows the relationship between the unconfined compressive strength of the modified material and the variation of nano-SiO₂ dosing. It can be found that the unconfined compressive strength did not increase with the increase of nano-SiO₂ dosing, and the unconfined compressive strength was maximum at 1% dosing. For example, the unconfined compressive strength at 7 d curing time is 2421, 2748, 2467 and 2156 kPa for 0.5%, 1%, 2%, and 3% of nano-SiO₂, respectively. They all satisfy the condition of 7 d unconfined compressive strength of red mud-based stabilized soil as proposed in Section 2.2.

Figure 3 show that the unconfined compressive strength of the modified red mud-based stabilized soil increased with the increase of the maintenance time, and the growth rate of the unconfined compressive strength was the largest from 1 to 7 d in the early stage. In the case of 1% dosing of nano-SiO₂, The growth value of the first 7d compressive strength accounted for 69.2% of the growth value of the 28d compressive strength. The highest unconfined compressive strength of the modified combination can be obtained at 1% of nano-SiO₂. Wang et al. (2020) concluded that when the nano-SiO₂ doping is too much, the nano-SiO₂ tends to form agglomerates and adsorb water, encroaching on the water for hydration reaction, affecting the degree of hydration and causing the strength of the red mud-based stabilized soil to decrease, so the NS2CS6PC3 and NS3CS6PC3 combinations have lower unconfined compressive strengths than the NS1CS6PC3 combination.

Figure 4 shows that the age of 7d was the inflection point of unconfined compressive strength growth rate, and after 7d the growth rate of unconfined compressive strength gradually decreases after the age of 7d, and the average growth rate of unconfined compressive strength is 7.4 kPa/d during 60d~120d.

3.2. Micro-morphology and curing characteristics of red mud-based stabilized soil

3.2.1. Micromorphology

The gypsum dose of 6%, the cement dose of 3%, and the nano-SiO₂ dose of 1%, 2%, and 3%, respectively, were selected for electron microscope scanning at 28 d. The observation magnification of the electron microscope used in the test was 500 times. The SEM images after the test are shown in Figure 5. It shows that the structural compactness of the soil did not increase with the increase of nano-SiO₂ under the condition of a certain amount of gypsum and cement admixture. On the contrary, it showed a decreasing trend, which indicates that in the synergistic modification combination of nano-SiO₂, gypsum and cement, there exists a limit for the amount of nano-SiO₂. When the admixture of nano-SiO₂ is 1%, the surface of the soil is relatively flat, the pore development is less, the fracture rate is low, and the soil structural compactness is relatively good.

3.3.2. Curing characteristics

(1) SEM-EDX

The gypsum dose of 6%, the cement dose of 3%, and the nano-SiO₂ dose of 1%, 2%, and 3%, respectively, were selected for electron microscope scanning at 28 d. The observation magnification of the electron microscope used in the test was 10,000 times. The SEM images after the test are shown in Figure 6.

The content and morphology of some gelling products in the 10,000x electron microscope scan images differ. Figure 6(a), the gelling products are mainly needle-like and columnar calcium alumina (AFt), and the AFt is longitudinally and horizontally distributed between particles or agglomerates, interconnected and interwoven, forming a huge spatial mesh structure to support the material skeleton system, while Ca(OH)₂ is occasionally seen to be distributed; Figure 6(b), the gelling products are mainly columnar AFt, which is in clusters Figure 6(c), the gelling products are not obvious, and the distribution of needle-like AFt is occasionally seen, and the red mud agglomerates are also found. It is assumed that the excessive nano-SiO₂ fills and blocks the pores and channels of the stabilized soil, which reduces the migration of ions such as OH^-, Ca²⁺, and Al³⁺, and hinders the generation of AFt and other products.

The synergistic participation of gypsum promotes more AFt generation, which is more obvious at nano-SiO₂, doping of 1% and 2%. AFt is generated by the reaction between gypsum and tricalcium aluminate in cement clinker with the reaction equation: 3C3A+3(CaSO₄ -2H₂ O)+26H₂ O→3CaO-Al O₂₃ -3CaSO₄ -32H₂ O. Usually The percentage of gypsum in silicate cement is less than 3%, while 6% of gypsum is added to this group of modified materials, which makes the red mud-based stabilized soil materials have the conditions to generate more AFt.

Figure 7 shows the EDX energy spectrum of the NS1CS6PC3 modified combined agglomerates. Compared with the EDX energy spectrum of pure red mud, the presence of S elements within the stabilized soil material indicates that gypsum intervenes in the reaction and the material has a higher content of O and Ca elements, presumably more AFt is generated within the modified material.

(2) XRD

Figure 8 shows that the XRD pattern of modified red mud-based stabilized soil is similar to that of red mud in terms of peak area, peak size, and peak trend, mainly because the amount of modified materials in red mud-based stabilized soil is very small, and the new material generation should be relatively limited. However, due to the addition of the modified materials, new products were generated, so its XRD pattern had peaks that were not present in the red mud XRD pattern.

The XRD pattern of the modified red mud-based stabilized soil contains physical phases of hematite, calcium-iron garnet, sodium calcite, calcium chalcocite, and calcite in red mud, and also contains physical phases of AFt, silica, gypsum, and tricalcium silicate, which are not present in red mud, where AFt is a hydration product, tricalcium, gypsum and silica are incompletely reacted modified materials. The peak of Ca(OH)₂ in the modified red mud spectrum overlaps with the red mud, and the analysis of this peak may be the physical phase of portlandite in the red mud (Rao, 2008), and the main peak does not see the hydrated Ca(OH)2 physical phase. According to the analysis, after the hydration reaction between the cement with little admixture and the water contact in the modified material, the hydrated Ca(OH)₂ is generated, but the alumina and silicon oxide, which exist in large quantities in the red mud, react with the hydrated Ca(OH)2 of cement under the alkaline environment conditions in a pozzolanic reaction, and the Ca(OH)₂ generated by its hydration is also small due to the little admixture of the cement, then the alumina and silicon oxide, which exist in large quantities in the red mud, gradually consume the hydrated Ca(OH)₂. The cement hydration products, C-S-H gels, and C-A-H gels, have no peaks in the XRD patterns because of their non-crystalline structure.

(3) XPS

The binding energy refers to the mutual attraction between the components within an object that binds them together, and if one wants to separate these components, a certain amount of energy is required to overcome the attraction between them, which is the binding energy of the object, and the amount of work required indicates the tightness of the combination of these components. The greater the binding energy, the greater the attraction or cohesion between the components of the material (Zhang et al., 2009).

Based on the XPS energy spectra of the red mud-based stabilized soil materials with different modification combinations and different hydration ages, the binding energies of the main elements such as Ca, Si, Al, Na, O, and S during hydration can be derived, as shown in Table 5.

As shown in Table 4, in the same set of modified materials, the binding energy of the main elements such as Ca, Si, Al, Na, O, and S in the red mud-based stabilized soil materials are increasing with the increase of the maintenance age, which indicates that the free Ca, Si, Al, Na, O and S elements in the liquid phase of the red mud-based stabilized soil materials continuously participate in the hydration reaction with the increase of the maintenance age and generate Ca(OH)₂, C-S-H gels, AFt and other hydration products, while new covalent and ionic bonds are formed in this process, which makes the binding energy of each element enhanced (Zhang et al., 2014).

However, the increase or decrease of the binding energy of each major element with different doping amounts of nano-SiO₂ did not exactly show a positive or negative correlation with the amount of nano-SiO₂. It is speculated that this phenomenon is related to the elemental properties and the calcium-silicon ratio.

4. Curing Mechanics of red mud-based stabilized soil

We analyze the properties and change mechanisms of red mud-based stabilized soils by SEM-EDX, MIP, XRD, and XPS, and conclude that the curing mechanisms of red mud-based stabilized soils include hydration reactions, pozzolanic reactions (secondary hydration reactions), the promotion effect of nano-SiO₂ and the enhancement effect of gypsum, which are chemical reactions. In the case of stabilized soil bulk materials, the premise for these reactions is that the materials first undergo a physical process - mechanical compaction - and when mechanically compacted, the bulk materials are tightly joined together so that they have the contact conditions for chemical reactions to occur between them. The curing mechanisms are independent of each other but are interconnected and the common interconnection formed a synergistic curing mechanism that promotes the formation of the strength of the red mud-based stabilized soil.

4.1. Mechanical compaction

Through mechanical compaction, the friction and embedded force between the particles of the red mud-based stabilized soil material formed, and the friction and embedded force between the particles contributed to the macroscopic mechanical strength, which is the mechanism of mechanical compaction to obtain the strength of the material. Once the particles of the red mud-based stabilized soil material are in close contact with each other through mechanical compaction and have gained initial strength, the chemical reaction between the mixes continues to occur as the maintenance time increases, generating cementing substances that result in tighter particle bonding and higher structural strength.

4.2. Hydration reaction and volcanic ash reaction

The modified material in the red mud-based stabilized soil contains cement clinker, which is highly reactive. When it meets water, each component of the cement dissolves rapidly and undergoes a hydration reaction, which is the main reason for the strength of red mud-based stabilized soil. The hydration reaction of cement is a series of reactions of tricalcium silicate (C3S), dicalcium silicate (C2S), tricalcium aluminate (C3A), and tetra calcium iron aluminate (C4AF) in cement with the participation of water, and the process of hardening strength of the material is improved by the generation of cementitious products.

Pozzolanic reaction, also known as secondary hydration reaction, refers to the process that active components such as SiO₂, and Al₂O₃ in minerals react with Ca(OH)₂ in an alkaline environment to produce gelling products such as C-S-H, C-A-H, AFt, etc. (Peng, 2020).The proportion of SiO₂, and Al₂O₃ in red mud is 12.35% and 18.89% respectively, and the active component accounts for a relatively large proportion; nano SiO₂ soluble in alkali, in the alkaline environment belongs to the active material, can provide Si⁴⁺; gypsum can provide additional Ca²⁺; red mud soluble alkali such as NaOH, KOH and can provide OH^- in addition to cement hydration and then supplement OH^-. Therefore, the modified red mud-based stabilized soil itself already has good conditions for the pozzolanic reaction. The pozzolanic reaction is an important reason for the late strength growth of red mud-based stabilized soil.

4.3. Facilitating effect of nano-SiO₂

The synergistic participation of nano-SiO₂ improves the strength of the red mud-based stabilized soil. The contribution of nano-SiO₂ is reflected in the promotion of the early hydration process and hydration degree through the "nucleation effect" on the one hand, the provision of more active silicon sources on the other hand, and the generation of more cementation products under alkaline environment on the other hand, and the reduction of material porosity and improvement of material compactness through the filling effect.

4.3.1. Promotion of early hydration reaction

According to Jianrong Wang (2019), G. Land et al. (2012), the high specific surface area of nano-silica and its high activity in the alkaline environment make it have a "crystalline nucleation effect", and the nano-silica uniformly distributed in the red mud-based stabilized soil acts as The nano-silica uniformly distributed in the red mud-based stabilized soil acts as a "nucleation crystal species", providing nucleation sites, allowing more hydration ions to gather and react in the periphery, accelerating the early hydration process of the red mud-based stabilized soil, promoting the early production of more hydration products, and improving the strength of the red mud-based stabilized soil.

For the characteristics of the granular material of the red mud-based stabilized soil, the model of nano-SiO₂ promoting the early hydration process of the red mud-based stabilized soil was established in this study with individual red mud particles, see Figure 3-9.

In the first step, as shown in Figure 3-9(a), the ground red mud in powder form is mixed with the modified material in a dry state so that the material particles are uniformly distributed. The red mud particles are surrounded by the cement, while the smaller particle size nano-SiO₂ is filled between the red mud and cement particles.

In the second step, as shown in Figure 3-9(b), water is added to the mixed particles in the designed proportion, and the hydration reaction occurs rapidly after the water is added, and the hydration products are formed around the cement particles, while the "nucleation effect" formed by the nano-SiO₂ due to its high surface energy makes the hydration ions of cement also gather around it rapidly, which in turn makes the hydration products are also formed around it and gathered around it.

In the third step, as shown in Figure 3-9(c), with the hydration reaction, the hydration products are continuously generated. The original state of large pores between cement particles due to the small amount of cement and the hydration products are not easily connected in series, due to the dispersed presence of nano-SiO₂ and the continuous gathering of hydration products around themselves, the cement particles can be continuously connected and bonded by nano-SiO₂ and hydration products.

In the fourth step, as shown in Figure 3-9(d), the hydration reaction continues and the hydration products keep connecting to fill the particle voids, and finally a network body is formed around the red mud particles, which wraps the red mud particles and also serves to bond the different red mud particles, thus improving the overall compactness and structural strength of the red mud-based stabilized soil. The variation of this step can be better explained by the staggered distribution of Aft and wrapping of red mud particles shown in Figure 3-6(a).

The content of C3S is about 50-60% in cement, which is the main contributor to the early strength of cement, then the promotion effect of nano-SiO₂ is mainly concentrated in the early stage during the synergistic process of nano-SiO₂, which is the mechanism of nano-SiO₂ to promote the early hydration reaction.

4.3.2. Provide silicon source

Nano-silica is stable but soluble and reactive under alkaline environmental conditions, so the synergistic participation of nano-SiO₂ continuously replenishes the liquid-phase system with Si⁴⁺ ions, providing more silicon sources for the red mud-based stabilized soil. Si⁴⁺ provided by nano-SiO₂ and Ca²⁺ together with OH^- ions (generated by cement hydration and by soluble alkali in red mud) in turn undergo secondary hydration reactions to generate more C-S-H, C-A-H, AFt and other gels, further enhancing strength. The red mud acts as a "hotbed", and the alkali environment provided by it allows the relatively stable nano-SiO₂ and gypsum to react continuously, but the reaction is more moderate and can last for a longer period.

4.4. Enhancement effect of gypsum

The synergistic involvement of CS in the modified material likewise significantly improved the mechanical strength of the red mud-based stabilized soil. The gypsum was blended at 6%, and in an alkaline environment, gypsum was gradually dissolved in the liquid phase, which could continuously provide Ca²⁺ ions to the liquid phase system. During the hydration of cement, tricalcium aluminate C3A reacts with Ca(OH)₂ and gypsum to form trisulfate hydrated calcium sulfate (AFt), and simply in the cement system, if gypsum is insufficient and consumed before C3A is fully hydrated, the trisulfate hydrated calcium sulfate will again react with the unconsumed C3A to convert into monosulfate hydrated calcium sulfate (AFm). According to Wenwen Tian (2020), Shuhua L (2010), and others, the strength of AFt or its contribution to material strength is greater than that of AFm, so once AFt is converted to AFm, it will have a negative impact on material strength. Since the red mud-based stabilized soil modification material has an additional 6% gypsum co-added in addition to 3% cement, the amount of gypsum is sufficient for the hydration reaction, and it is presumed that all or the vast majority of the end product of C3A in cement is AFt, which enhances the strength of the modified stabilized soil. Figure 3-6 shows the characteristics.

4.5. Effect of calcium to silicon ratio

In the alkaline environment of red mud, the modified material and the active ingredients in red mud are excited and chemically reacted to produce more cementation products and promote strength. Different materials have different calcium-silica ratios, and different calcium-silica ratios have different effects on material strength.

Calculation of calcium to silicon ratio based on Equation 1:

$$\frac{C}{S}=\frac{CaOtotalcontent÷56}{Si{O}_{2}totalcontent÷60}$$

(1)

Where, the total calcium oxide content = ∑ material ratio × calcium oxide content, the total silicon oxide content = ∑ material ratio × silicon oxide content.

The calculated calcium-silica ratios of NS1CS6PC3, NS2CS6PC3, and NS3CS6PC3 were 2.83, 2.55, and 2.32, respectively, which were higher than 0.92 for red mud and lower than 3.23 for cement, and the addition of modified materials improved the calcium-silica ratio of red mud and the calcium-silica ratio was close to that of cement. section 3.1.2 revealed that the unconfined compressive strengths of NS1CS6PC3, NS2CS6PC3, and NS3CS6PC3 at the age of 7 d were 2748, 2467 and 2156 kPa, respectively, indicating that the unconfined compressive strength increased with the increase in calcium-silica ratio in the three groups of modified materials. It indicates that the increase of calcium-silica ratio in the tested red mud-based stabilized soil contributes to the improvement of the strength of the red mud-based stabilized soil.

5. Conclusions

In this experiment, the stabilized soil material was prepared by modifying the red mud of aluminum industrial waste, and the strength and microscopic characteristics of the red mud-based stabilized soil with different modified materials and different amounts of modified materials were tested, and the curing mechanism was analyzed, and the main findings were as follows:

(1) Cement alone can improve the unconfined compressive strength of red mud-based stabilized soil; with the synergistic modification of nano-SiO2, gypsum and cement, the 7-d unconfined compressive strength of red mud-based stabilized soil is greater than 2 MPa under the synergistic effect of nano-SiO2 (1%, 2% and 3%, respectively), which meets the compressive strength requirement of road subgrade material, and the highest unconfined compressive strength of nano-SiO2 combination is 2748 kPa.

(2) In the microstructure study, the SEM test results showed that the soil structural compactness did not increase with the increase of nano-SiO₂ when nano-SiO₂, gypsum, and cement were co-modified, and the soil structural crack rate was the lowest and the structural compactness was the best when nano-SiO₂ was used at 1%.

(3) High magnification SEM tests reveal that when nano-SiO2, gypsum and cement are synergistically modified, it is found that the increase of needle-like and columnar AFt in the cementitious products is due to the 6% gypsum added to the modified material, which creates conditions for the formation of more AFt; The XRD results showed that the gypsum diffraction peaks of the NS1CS6PC3 modified combination of red clay-based stabilized soil tended to disappear with the growth of the maintenance age, indicating that it was continuously transformed into AFt. The increase of binding energy of hydration product-related ions in the modified material also indicates the strength of the modified material is improved.

(4) Mechanical compaction is a prerequisite for chemical curing, and the chemical curing mechanism contains the hydration reaction, pozzolanic reaction, the promotion effect of nano-SiO2, and the enhancement effect of gypsum. The amount of nano-SiO₂ is small, but it can promote the early hydration reaction process and hydration degree, provide more silica source for the stabilized soil material; The paper established a model of nano-SiO₂ to promote the early hydration process of red clay-based stabilized soil, and revealed the mechanism of nano-SiO₂ to promote the hydration process of red clay-based stabilized soil; the modification effect of gypsum is key in providing a calcium source to the red clay-based stabilized soil system The key role of gypsum in the modification is to provide a calcium source, which contributes to the conversion of all or most of the C3A in cement into AFt, and at the same time, with Si⁴⁺ ions in the material, to generate C-S-H hydration gel under alkaline environment, which further enhances the strength of the modified stabilized soil.