Mechanical Properties and Mechanisms of Concentration-Controlled Alkali-activated Fly Ash Stabilized Saline soil in Seasonally Frozen regions

doi:10.21203/rs.3.rs-4813604/v1

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Mechanical Properties and Mechanisms of Concentration-Controlled Alkali-activated Fly Ash Stabilized Saline soil in Seasonally Frozen regions

https://doi.org/10.21203/rs.3.rs-4813604/v1

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Sulfate saline soil in the seasonally frozen zone generate soil freeze-thaw cycles, as well as saline expansion and thawing, which causes road engineering issues, thus they should be strengthened. This work investigates the effect of alkali content on the activation of fly ash (FA) stabilized sulfate saline soil by conducting unconfined compressive strength (UCS), splitting strength, and freeze-thaw cycles (FTs) tests. The results demonstrate that as NaOH concentration increases, the UCS and splitting strength tend to increase first, then decrease. N3 (stabilized soil with 0.5 M stimulated FA) had the best performance, with UCS and splitting strength of 7.18 MPa and 1.89 MPa, respectively. The residual values of the UCS and N3 splitting strength after 5 FTs were 46.35% and 39.92%, respectively. The NaOH activated FA hydration products consisted primarily of C-S-H, C-A-H gel and ettringite, with the gels linking the saline soil particles and filled the cracks between them. As a result, the mechanical characteristics and frost resistance of saline soil were much enhanced. The results of the study can provide a reference for the solidification treatment of sulfate saline soil foundations in seasonally frozen regions.

Physical sciences/Engineering

Physical sciences/Materials science

Alkali concentration

Activated cementitious materials

Hydration mechanism analysis

Stabilized saline soil

Unconfined compressive strength

Freeze-thaw cycles

Frozen soil is a common phenomenon at high latitudes that develops when temperatures fall below 0°C^1,2. When water condenses into ice, its volume increases by 9%, causing high pressure^3–5. Frozen soil is categorized into two types based on their freezing length of time: year-round frozen soil and seasonally frozen soil⁶. Additionally, high saline soil can form, generating engineering issues such saline expansion and erosion⁷. Most of the Xinjiang region is a seasonally saline frozen soil zone, with a large temperature difference between day and night, which provide a fine external environment for the freezing and thawing of saline soil⁸.

Prolonged temperature changes can produce freeze-thaw cycles (FTs), which can lead to saline expansion, frost heave⁹, and thaw-sinking¹⁰ from moisture condensation and thawing, leading to a reduction in the bearing capacity of the road base, resulting in problems such as cracks in the pavement and cracking of the road subgrade¹¹. Seasonally frozen saline soil with saline-frost coupling are the main reason for engineering diseases. Therefore, in order to solve these problems, consolidation of saline soil in the seasonally frozen zone is necessary¹².

As therefore, current research on saline soil engineering in the seasonally frozen zone focuses on developing an effective approach to consolidate saline soil in order to prevent deformation and corrosion. Fly ash (FA) as a kind of industrial solid waste produced by coal-fired power plants¹³, consisting primarily of glassy spheres, some crystalline minerals, and carbon^14,15. FA has a wide range of applications¹⁶, including concrete¹⁷, bricks¹⁸ water-stabilized sub-base for roadways, and metal extraction^19,20. Qin et al²¹ effectively enhanced the bearing capacity of saline clay foundations in the seasonally frost regions by synergistically applying activated MgO and FA. Li et al²² found a significant reduction in the temperature sensitivity of stabilized soil when curing saline soil with composite curing agents. Dang²³ added 6% lime and 18% FA to the stabilizer, resulting in a UCS of 0.75 MPa at 7 d. The strength barely met the application requirements, and there were limitations in application. Temuujin discovered²⁴ that when CaO and Ca(OH)₂ were added to the FA based geopolymer at a mass ratio of 3% and 3 wt%, respectively, the 7 d UCS increased from 11.8 MPa to 22.8 MPa and 29.2 MPa.

FA is a silica-aluminate material, which hydrates under alkaline conditions to form products such as hydrated calcium silicate (C-S-H) and hydrated calcium aluminate (C-A-H). However, the low alkalinity of FA results in a limited degree of hydration, and the addition of an exciter is required to promote the FA hydration reaction²⁵. The alkali content influences the solubility of aluminum silicate and was thus an important parameter for FA activation²⁶. Increasing the OH⁻ content in the FA/slag solution increases the solubility of the FA and slag, resulting in a higher compressive strength²⁷. Most of the previous studies on FA are mechanical property studies²⁸, and the studies on stabilized saline soil have focused on compressive strength²⁹, compaction³⁰, CBR, permeability³¹ and volume change³². The performance and mechanism of alkali-activated FA for the stabilization saline soil in the seasonally frozen zone have received little attention^33,34. As a result, it was critical to investigate the evolution of the microstructure of saline soil curing for road engineering applications.

Considering the limitations of the preceding investigations, the primary goal of this research was to investigate the mechanism and attributes of alkali concentration on FA activated controlled stabilized sulfated saline soil. The mechanism of alkali-activation in FA stabilized saline soil was explained in terms of chemical reactions and microstructural changes, and the effects of alkali concentration and FTs on the mechanical properties of stabilized saline soil were investigated. The research used the UCS, splitting test, FTs test, X-ray diffraction (XRD), scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), thermogravimetric by differential scanning calorimetry (TG-DSC), and other techniques to determine the relationship between macroscopic mechanical properties and microscopic characteristics. The mechanism of NaOH activated FA stabilized of saline soil was investigated as well to provide a theoretical foundation for alkali activated FA stabilized of saline soil. The results provided in this work may give a theoretical foundation for the solidification of saline soil in seasonally frost regions.

Materials

The saline soil was gathered from Xinjiang University Boda Campus in Urumqi, Xinjiang, China, which was a severely salinized location. To ensure uniformity of salinity in saline soil, fake saline soil was employed to make NaOH-activated FA stabilized saline soil. Prior to the manufacture of stabilized soil, the collected saline soil was desalinated and dried before being crushed and sieved through a 2 mm sieve. In addition, compaction tests showed that the maximum dry density and optimum moisture content of FA stabilized saline soil were 1.63 g/cm³ and 12.5%, respectively. FA was produced from Class II FA generated by the 102 Mission thermal power plant in the Zhundong region in Xinjiang; Table 1 shows the oxide content of the FA, and Fig. 1 shows the XRD of the FA. General-Reagent Company of China provided the sodium sulfate (powder, purity > 99.0%). Xinjiang Zhongtai Chemical Co Ltd provided the NaOH (flaky, purity ≥ 98%). The water was laboratory-grade deionized water.

Table 1

Chemical composition of FA (wt%).
Materials	SiO₂	Al₂O₃	CaO	Fe₂O₃	MgO	K₂O	Na₂O	Cl	SO₃
FA	37.31	17.38	13.14	15.07	5.95	1.28	2.60	0.27	5.23

Specimen preparation

NaOH, Na₂SO₄, FA, soil and deionized water will be weighed in calculated proportions. Dissolve NaOH, Na₂SO₄ in deionized water and spray the above solution on the mixture of FA and soil and mix well. The soil samples were packed in a bag with sealed to ensure uniform distribution of water and saline. To conduct various FA-inspired microscopic investigations, soil-free materials were weighed in the same quantities. Part of the microscopic experiment of FA activation was studied by weighing the soil-free materials in the same proportions, mixing them thoroughly, and sealing them in a beaker.

The saline soil had a moisture content of 12.5% and a sodium sulfate concentration of 1%. The stabilized saline soil with NaOH concentrations of 0.1, 0.3, 0.5, 0.7 and 0.9 M have been designated N1, N2, N3, N4 and N5, respectively. FA was 13% by volume in stabilized saline soil. Tested cured soil was divided into three layers, compressed, demolded, and made into a specimen of Φ100 mm×100 mm high, sealed with cling film, and marked based on its maximum dry density. The specimens were then placed in sealed plastic boxes and tested after 28 d of curing at a temperature of 20°C.

Unconfined compressive strength and splitting strength tests

The tests were performed using a cement flexural and compressive integrated machine (YAW-SERIES) for UCS and splitting strength tests on stabilized saline soil after 28 d of curing and FTs, with a loading rate of 1.0 mm/min, and the peak load at the time of specimen damage was recorded, as shown in Fig. 2.

Freeze-thaw cycles test

Because the saline soil sampling site was located in a region with seasonally frozen soil, the sampling layer usually has changes in temperature over time. In order to investigate the effect of FTs on the mechanical strength of alkali activated stabilized saline soils, indoor FTs tests were conducted. According to the Specification for "Hydraulic Concrete Test Specification (SL352-2006)", FTs experiments without water replenishment were carried out using a constant temperature box. Stabilized soil specimens cured for 28 d were frozen at -20°C for 12 h and then thawed at 20°C for 12 h to obtain FTs = N (where N = 1, 3, 5, 10 and 20). The residual strength ratio (BDR) was used to evaluate the frost resistance of stabilized soil as shown in Eq. (1):

$$\:\text{D}\text{B}\text{R}=\frac{{q}_{r}}{{q}_{p}}\times\:100\%$$

where q_p is the peak strength; q_r is the residual strength.

Microstructural test

After breaking the specimens, internal soil samples or alkaline-activated FA products were extracted and dried at 60℃ before being examined for mineralogical compositions and microstructures. The raw materials and FA hydration products were examined using a (D8 Advance) X-ray diffractometer with an X-ray scanning step width of 0.02° and a scanning range of 10°-80°. A scanning electron microscope (Hitachi S 4800) was used to examine the microstructure and morphology of FA hydration products, specimens that had been stored for 28 d, and representative samples with FTs = N. The FA hydration products were assessed using a Fourier transform infrared spectrometer (Great 10) with an infrared spectral scanning range of 400 cm^− 1 ~ 4000 cm^− 1. A thermogravimetric analyzer model (DSC200F3) was chosen to test the TG-DSC of FA hydration products, and the temperature was ramped up from room temperature to 800℃ at a rate of 10 ℃/min ³⁵. The binding energies of representative samples of FA hydration products were determined using an X-ray photoelectron spectrometer (Thermo Fisher Scientific ESCALAB 250 Xi) in 0.05 eV increments.

Compressive strength

The influence of alkali content on unconfined compressive strength

The UCS of various amounts of NaOH-activated FA stabilized saline soil after 28 d of cured is shown in Fig. 3. Because the specimens were stabilized with varied concentrations of NaOH-activated FA at room temperature to stabilize the same saline soil with sulfate, the strength variation was proportional to the NaOH concentration.

Figure 3 shows that there is a relationship between stabilized saline soil UCS and NaOH content. The UCS of N1, N2, N3, N4 and N5 stabilized saline soil after 28 d of cured were 4.85 MPa, 6.11 MPa, 7.18 MPa, 5.98 MPa and 3.92 MPa, respectively. The UCS of stabilized saline soil increases and then decreases with increasing alkali concentration. The best UCS of N3 stabilized saline soil was obtained at 0.5 M NaOH content. The results showed that 0.5 M alkali concentration had the best growth promotion effect on FA-stabilized saline soil. The reduced strengths of N1 and N5 can be attributed to two factors: the increased porosity of the stabilized soil and the decreased degree of polymerization of the alkali-activated FA gel. The strength of FA gel polymers mostly relies on the degree of polymerization³⁶ which is influenced by the solubility of the silicate and aluminate monomers in the alkaline medium and their subsequent nucleation.

The influence of alkali content on splitting strength

Figure 4 shows the splitting strength of FA-stabilized saline soil after 28 d of cured at different NaOH-activation concentrations.

Figure 4 shows that the relationship between splitting strength and NaOH concentration was similar to that between UCS and NaOH concentration, both of which showed a tendency to increase and then decrease, and N3 stabilized saline soil had the best splitting strength; The effect of alkali-activated FA on the splitting strength of stabilized soil was more pronounced than in the UCS test. The difference between the UCS and splitting strength of stabilized saline soil is mainly due to the formation of gelling substances by sodium hydroxide-activated FA as a cementing agent, which better connects the soil particles and improves the splitting properties. The remaining gelling material fills the voids in the saline soil and improves the performance of the UCS. In order to increase the UCS of stabilized saline soils, not only is it necessary to use a cementitious material to join the soil particles, but there must also be enough material to fill the pores. The results showed that the effect of NaOH-activated FA on stabilized soil splitting strength was greater than that of UCS.

Freeze resistance experiments

Figure 5 shows the relationship between DBR and the number of FTs for UCS, as well as the splitting strength of FA-stabilized saline soil with varying NaOH-activation degrees.

Figure 5 shows that the DBR of stabilized saline soil steadily falls as FTs = N increases. After FTs = 5, the DBR of stabilized soil UCS and splitting strength declined dramatically, indicating a rapid deterioration stage. With an increase in FTs = N, the DBR of stabilized soil UCS and splitting strength continued to decline, but at a slower rate, and it was in the stable stage. In addition, the relative magnitude connections of UCS and DBR of splitting strength remained mostly unchanged between groups, with the only difference being the degree of reduction in UCS and splitting strength in each group.

The DBR of N1, N2, N3, N4 and N5 stabilized saline soil UCS at FTs = 5 were 39.37%, 42.76%, 46.35%, 48.29% and 42.82%, respectively. The DBR of stabilized soil UCS at FTs = 20 was approximately 25%. The splitting strength DBR at FTs = 5 were 51.26%, 48.15%, 39.9%, 52.30% and 57.88%, respectively, whereas FTs = 20 stabilized saline soil splitting strength DBR by approximately 15%. The splitting strength decreased more in DBR than in UCS.

The decline in DBR of UCS and splitting strength of stabilized saline soil specimens at FTs = 20 was comparable, given that N3 stabilized saline soil had higher UCS and splitting strength in the starting state than the other groups. This suggests that the N3 stabilized saline soil not only had the best mechanical qualities in the group, but also had outstanding frost resistance.

Microanalysis

XRD analysis

Figure 6 shows the XRD of cured sulfate materials with different concentrations of NaOH activated FA for different times.

Figure 6 shows that the diffraction peak intensities vary only slightly, and that the phase compositions at different alkali-activated ages are virtually identical. Different alkaline concentrations of NaOH activation of FA showed distinct diffuse peaks between 2θ of 20–40° at different ages, corresponding to amorphous C-A-H and C-S-H gels³⁷. However, the intensity of SiO₂ diffraction peaks decreased significantly with time. Figure 6 (a, b and c) diffraction peaks of C-S-H, C-A-H gels, sodium sulfate, gypsum, unnamed zeolite, and quartz were observed for all alkali-activation of FA 1 d. With the prolongation of the activation time, the C-A-H and C-S-H diffraction peaks first increased and then decreased, and the sodium sulfate diffraction peak disappeared. This indicates that NaOH activation of FA generates C-S-H and C-A-H gels, in which C-A-H reacts with sodium sulfate to be converted into ettringite(AFt). C-A-H and C-S-H gels substances produced by NaOH-activated FA may greatly improve the UCS and splitting strength of stabilized soil ²⁷.

At 28 d of alkali activation of FA, the SiO₂ diffraction peaks decreased as the alkali concentration increased, however the amorphous gelation did not change much. It is possible that the higher alkali concentration resulted in the dissolution of SiO₂ and Al₂O₃, but the polymerization process of [SiO₄]⁴⁻ and [AlO₄]⁴⁻ was blocked, which was manifested by a decrease in the SiO₂ diffraction peaks, and the gel peaks did not change significantly. The 0.5 M alkali concentration acilitates the breaking and reorganization of the covalent bonds of silica-aluminum oxides, as well as the polycondensation reaction, which produces gel substances on a continual basis. When the alkali concentration reaches 0.9 M, it slows down the stimulated FA process and inhibits [SiO₄]⁴⁻ and [AlO₄]⁴⁻ polymerization.

SEM analysis of FA hydration with time

Figure 7 shows SEM images of N1, N3 and N5 at 1, 3, 7 and 28 d after FA activation. Figure 7 (a, b and c) shows a considerable number of needle-like AFt and scale-like C-(A)-S-H gel, as well as a tiny amount of flake-like Ca(OH)₂. N1 shows more needle-like AFt, whereas N3 and N5 show more flocculent gelling material, flake-like Ca(OH)₂.

Figure 7 (d, e and f) shows the SEM of N1, N3 and N5 3 d after alkali activation of FA. It can be seen that N1 has more needle-like AFt, while N3 and N5 have more Ca(OH)₂ and flocculent materials. SEM of N1, N3 and N5 after 7 d of FA activation shows an increase in short rod-like AFt for N1, but N3 and N5 show an increase in Ca(OH)₂ and flocculent materials. SEM of FA activation of N1, N3 and N5 over 28 d reveals that the gel layer thins and a substantial amount of needle-like AFt was appeared.

Figure 7 (a, d, g and j) can be seen that N1 has the greatest number of heterogeneous gels on the first day, which gradually declines over time, but N3 and N5 have similar conditions. The key explanation is the conversion of C-S-H and C-A-H into tiny crystals, as well as the formation of AFt from C-A-H and sulfate; these results were consistent with the XRD.

SEM of specimens with FTs

Figure 8 shows representative SEM images of N1, N3 and N5 after the stabilized soil FTs tests. As shown in Fig. 8 (a, b and c), the soil particles are tightly contained by the flocculated gel of alkali-activated FA, resulting in full agglomerates. Filling the voids between stabilized soil with FA and flocculated ground polymer gel products, the higher the concentration, the fewer voids and soil particles are exposed. When FTs = 1, no cracks were seen in the SEM of all stabilized soil (Fig. 8 (d, e and f)). When FTs = 3, all stabilized soil began to show microcracks, with N1 had the most fractures (Fig. 5 (g)), while N3 (Fig. 8 (h)) and N5 (Fig. 8 (i)) had less cracks internally and were less damaged than N1. In the FTs, stabilized soil experiences volume expansion as free water condenses into ice crystals, generating enormous pressure; the pressure disappears when the ice crystals melt, resulting in the fragmentation of some large soil aggregates, changes in the ordering of soil particles, and cracks in the gel film and bonding material. N1 has the least quantity of gel material, the least amount of coagulation, and the greatest gaps, making it most vulnerable to freeze-thawing damage.

Cracks in the stabilized soil continued to form as a result of FTs, and the soil sample particles adjusted to their equilibrium state. At FTs = 20, extensive gaps and cracks occurred in the region shown by the red circles in Fig. 5 (j, k and l). The hydrated cementitious material continued to degrade, and cementitious material was shed from the surface of the particles in the area depicted by the red circle in Fig. 5 (J).

FTIR analyses

Figure 9 shows the FTIR of FA with time for different concentrations of NaOH activation. The alkali-activated FA 28 d increased with increasing NaOH concentration, and the absorption peak at 3440 cm^− 1 corresponded to an increase in -OH absorption peaks in C-S-H and C-(A)-S-H. It indicated that C-A-H, C-S-H increased with increasing NaOH concentration. The absorption peaks from 1390 cm^− 1 to 1420 cm^− 1 were related to the C-O expansion vibrations in carbonates³⁸. The absorption peak at 1640 cm^− 1 corresponds to the molecular water O-H bending vibration, as well as the reaction products C-A-H and C-S-H bound water³⁹. The absorption peaks around 1000 cm^− 1 are asymmetric stretching vibrations of Si-O-T, which were mainly found in C-S-H and C-A-S-H gel⁴⁰. The absorption peak at 1005 cm^− 1 shifted to higher wavelengths with increasing activation time, indicating a higher Si/Al ratio in the C-A-S-H gel⁴¹. The peaks at 528 cm^− 1 and 469 cm^− 1 correspond to the bending vibration of the Si-O-Al (Mg) bond. SiO₂ has significant absorption peaks at 755 cm^− 1 ~ 800 cm^− 1. The absorption peaks at 660 cm^− 1 ~ 670 cm^− 1 correspond to the bending vibration of the Si-O-Si. With the increase of NaOH concentration, the absorption peaks of OH, H-O-H, Si-O-T and Si-O were enhanced, the Si-O-T band became narrower and wider, and the intensity of the peaks decreased, which was mainly due to the formation of the aluminum gel phase.

TG analyses

Figure 10 (a, b and c) shows the TG-DSC curves for N1, N3 and N5 activation for 28 d. The mass loss was about 10%, and the mass loss increased with increasing NaOH concentration. As shown in Fig. 10, the mass percentages of N1, N3 and N5 decreased by 1.34%, 1.08% and 1.55%, respectively, in the temperature range of 20°C ~ 100°C, due mostly to water loss due to FA adsorption and hydration products⁴². The mass losses for the water-bound dehydration processes of the hydration products C-S-H, C-A-H and AFt were 1.20%, 1.10% and 1.53%, respectively, at temperatures ranging from 100 to 380°C. It was also discovered that the first two mass losses of N1, N3 and N5 increased with increasing NaOH concentration^43,44. At 380 ~ 550°C, exothermic oxidation of organic materials in FA resulted in a mass loss of around 5.50%. At 550°C ~ 620°C, a mass loss of 0.42%, 0.72% and 0.66% was detected, primarily due to the carbonate decomposition reaction^45,46.

The main components of alkali-activated FA hydration products were C-S-H, C-A-H and AFt, and the more gel and the higher degree of polymerization, the higher the strength of stabilized soil. The first and second mass losses of N1 were lower than those of N3 and N5, indicating that the amount of C-S-H and C-A-H in the hydration products of 0.1 M alkali-activated FA was less than that of N3 and N5. It was also discovered that N5 had the fastest loss of the first two masses, indicating that the overall polymerization of C-S-H and C-A-H formed by FA stimulated by high NaOH concentration was modest, which is compatible with the results of XRD and FTIR examinations. These results of low gel volume for N1 and poor gel polymerization for N5 were compatible with the results of low UCS and splitting strength of N1 and N5 stabilized soil.

XPS analyses

The XPS spectra of the alkali-activated FA gel was shown in Fig. 11. As can be observed from the spectra, the primary chemical element compositions were consistent with XRF. Following alkali-activation of FA, the binding energies of O 1s, Ca 2p, S 2p, Si 2p and Al 2p decreased before increasing with alkali concentration. It suggests that the activity of FA hydration products initially increases and subsequently diminishes as concentration increases.

The Ca 2p orbital activation energy has two distinct peaks that correspond to the Ca 2p _3/2 orbitals (left) and Ca 2p _5/2 orbitals (right). The breakdown of the calcium-rich phase in FA results in a huge amount of free Ca²⁺, which interacts with groups like [SiO₄], [AlO₄] and SO₄²⁻ to form a new calcium-containing phase. The Ca 2p orbital activation energy has two distinct peaks that correspond to the Ca 2p _3/2 orbital (left) and the Ca 2p _5/2 orbital (right).The dissolution of the calcium-rich phase in the FA produces a considerable quantity of free Ca²⁺, which reacts with [SiO₄], [AlO₄] and SO₄²⁻, and other groups to form a new calcium-containing phase.

The Si 2p and Al 2p orbital activation energy increase and then fall for comparable reasons. There are two main reasons: first, the silicon in FA is activated, and the low concentration causes insufficient hydrolysis of Si-O and Al-O bond breaking, resulting in a limited decrease in binding energy; second, increasing its polymerization degree of silicon/aluminates increases orbital activation energies as well. This indicates that low and high concentration activation of hydrated silicon/aluminates is likely to result in higher polymerization chains or even shelf silicates. As a consequence, the binding energies of Si 2p and Al 2p can likewise be used to characterize the degree of polymerization of Si-O and Al-O. As a result, the chemical bonding information on the FA surface examined by XPS partially reflects the surface's structural information, but it was not entirely reactive⁴⁷.

When NaOH come into touch with FA, the reliant SiO₂ and Al₂O₃ on its surface dissolve under alkaline circumstances, and the Si-O-Si, Si-O-Al, and Al-O-Al bonds break⁴⁸, breaking down into monomers like Al(OH)₄⁻, Si(OH)₃⁻. The monomers are linked by -OH, then dehydrated and condensed to create oligomeric gel that are linked by cations to form a hydrated layer of FA. The formation of a hydrated layer on the FA surface reduces the contact between FA and the alkali solution; OH⁻ diffuses through the pores of the hydrated layer to continue the reaction with FA; and the generated monomers then polymerize on the surface of the hydrated layer via the pores of the hydrated layer ⁴⁷. Some of the hydration products reacted with SO₄²⁻ to form needle-like AFt, and the floating gel gradually dissipated during the polymerization reaction. It was discovered that in the early stages of the reaction, dissolution becomes the rate-controlling step, while diffusion becomes the rate-controlling phase after the FA surface was totally covered by hydration products⁴³.

The NaOH-activated FA curing process of saline soil was shown in Fig. 12. The macroscopic manifestation of the FA surface structure was damaged by the action of alkali, the produced monomer polymerization into C-S-H, C-A-H, and other gels, FA was gradually wrapped in C-S-H, C-A-H gel material, FA was covered by hydration products, and SEM detected the phenomena of "hairy". As equilibrium was established, the hydration cycle slowed, and C-S-H and C-A-H reacted with sulfate to produce needle-like AFt.

After the stabilized soil had been cured for 28 d, alkali-activated FA to produce C-S-H and C-A-H gels, which connected the saline soil from point to point and filled the gaps. Free water conversion ice crystals when it frozen and thawed, enlarging the gaps between soil particles and destroying the gel substance. These pores persist after the ice crystals have melted, resulting in a continuing decline in the strength of the stabilized soil. As the number of FTs grows, the arrangement of soil particles gradually approaches equilibrium, and the inter-particle bonding mode tends to stabilize, lowering the rate of strength loss. As a consequence, the gel produced by alkali-activated FA improves the UCS, splitting strength, and frost resistance of stabilized saline soil by filling the saline soil voids and reacting with sodium sulfate.

In this paper, the macroscopic and microscopic characteristics of alkali-activated FA stabilized saline soil in the seasonally frost zone are researched. By alkali-activated FA materials were used to stabilize saline soil, the stabilized soil UCS, splitting strength, FTs tests were researched and the mechanical and frost resistance properties were analyzed. The microstructure of alkali-activated FA materials and stabilized soil were analyzed by XRD, SEM, FTIR, TG and XPS experiments and the stabilization mechanism was discussed. The key findings were as follows:

(1) Alkali-activated FA had a considerable effect on stabilized saline soil UCS and splitting strength, with a stronger effect on the latter. Low alkali concentrations of activated FA produced less gelling material, whereas high alkali concentrations of activated FA resulted in low polymerization of gelling material and reduced strength. The optimum mechanical properties of stabilized soil were achieved when the NaOH concentration was 0.5 M for stimulated FA.

(2) The DBR of strength of stabilized soil for mechanical strength decreased gradually with increasing FTs, and the strength DBR of stabilized soil tended to stabilize at FTs = 5. The DBR of UCS and splitting strength at the same number of FTs first increased and then gradually decreased with increasing concentration, with N3 cured soil having the best frost resistance.

(3) XRD studies showed that alkali stimulated FA generated C-S-H, C-A-H, and hydrated gel products, which developed with time and reacted with sulfate to form AFt. SEM revealed that C-S-H and C-A-H gels increased splitting strength by enclosing and connecting saline soil particles. The gel fills the soil-stabilized cavities, resulting in a denser structure and higher UCS.

(4) The XRD, SEM, and FTIR data revealed that NaOH stimulated FA to create amorphous C-A-H and C-S-H gels, which increased the strength of the stabilized soil. XRD and TG-DSC revealed that N5 gel was the most prevalent and had a lower degree of polymerization. The XPS results showed that N3 gel had the highest reactivity and performance in cured sulfate. NaOH-activated FA materials can effectively increase the capacity of saline soil in the seasonally frost zone.

According to research, alkali concentration and FTs have a significant impact on the microstructure of alkali-activated FA-stabilized saline soil, altering the mechanical properties of stabilized soil specimens. However, in the field of chemically stabilized saline soil in the seasonally frost zone, stabilization time, saline content, freezing and thawing temperatures, and water content are all important factors influencing the stabilization effect. These factors have not yet been investigated in this paper, but we hope to investigate them in future work.

The datasets generated and/or analyzed during the current study are not publicly available due to the research is ongoing, but are available from the corresponding author on reasonable request.

Author Contribution

Sining Li: Writing – review & editing, Writing – original draft. Yong Huang: Resources, Funding acquisition, Writing – review. Jian Sun: Supervision, Resources, Writing – review & editing. Qiushuang Cui: Visualization, Data curation. Rui Yu: Formal analysis, Data curation. Yubin Liu: Investigation, Formal analysis, Data curation.

Data Availability

The datasets generated and/or analysed during the current study are not publicly available due to the research is ongoing, but are available from the corresponding author on reasonable request.

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No competing interests reported.

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Mechanical Properties and Mechanisms of Concentration-Controlled Alkali-activated Fly Ash Stabilized Saline soil in Seasonally Frozen regions

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Abstract

Figures

Introduction

Materials and Methods

Materials

Specimen preparation

Unconfined compressive strength and splitting strength tests

Freeze-thaw cycles test

Microstructural test

Results

Compressive strength

The influence of alkali content on unconfined compressive strength

The influence of alkali content on splitting strength

Freeze resistance experiments

Microanalysis

XRD analysis

SEM analysis of FA hydration with time

SEM of specimens with FTs

FTIR analyses

TG analyses

XPS analyses

Discussion

Conclusions

Declarations

Author Contribution

Data Availability

References

Additional Declarations

Status:

Version 1