Effects of Basalt Fiber on the Mechanical and Frost Resistance of Geopolymer-Stabilized Aeolian Sand Gravel

Research by Guo and Yang (2020) indicated that in the preparation of geopolymers with 0–28% steel slag replacing FA, the compressive strength increased with an increase in the steel slag content. Based on the preliminary experimental results, steel slag micro powder contents of 20%, 30%, and 40% were selected, corresponding to mass ratios of steel slag micro powder to fly ash (hereinafter referred to as slag-powder ratio) of 2:8, 3:7, and 4:6, respectively. Based on the findings reported by He (2020) regarding the influence of SiO₂/Na₂O ratios in water glass solutions on geopolymer activation, alkali content and water glass modulus were selected as critical evaluation factors. Alkali content (Na₂O percentage relative to solid waste materials) was set at 6%, 8%, and 10%. Research indicated that when the water glass modulus was too low (less than 1), the alkali activation effect was unsatisfactory. When it was too high (greater than 2), it might lead to a decline in the mechanical properties of the geopolymer. Therefore, this study selected water glass modulus of 1.2, 1.4, and 1.6. An orthogonal design was conducted for the above three factors at three levels, with a constant water-to-binder ratio of 0.25. The design results are shown in Table 4.

The mass of water glass and NaOH during solution preparation can be calculated using formulas (1) and (2).

(1) MNa2SiO3=W1×M2(W0×M0)/M1 {{\rm{M}}_{{\rm{N}}{{\rm{a}}_2}{\rm{Si}}{{\rm{O}}_3}}} = {{{{\rm{W}}_1} \times {M_2}} \over {\left( {{{\rm{W}}_0} \times {M_0}} \right)/{M_1}}}

(2) MNaOH=(W0×M0M1−W0)×8061.99×MNa2SiO3 {{\rm{M}}_{{\rm{NaOH}}}} = {{\left( {{{{{\rm{W}}_0} \times {M_0}} \over {{M_1}}} - {{\rm{W}}_0}} \right) \times 80} \over {61.99 \times {{\rm{M}}_{{\rm{N}}{{\rm{a}}_2}{\rm{Si}}{{\rm{O}}_3}}}}}

Where:

• M_Na2SiO3 - the mass of water glass [g],

• W₁ - the content of Na₂O after preparation,

• M₂ - the mass of FA [g],

• M₀ - the modulus of water glass before preparation,

• W₀ - the content of Na₂O before preparation,

• M₁ - the modulus of water glass after preparation,

• M_NaOH - the mass of NaOH [g].

Aeolian sand was used to completely substitute aggregates smaller than 4.75 mm for the preparation of basalt fibre-reinforced steel slag micro powder-fly ash geopolymer stabilized aeolian sand gravel (FGSG) samples. Based on preliminary experimental results, a gravel content of 34% was selected (10 ~ 20 mm: 20 ~ 30 mm = 1:1). In this context, the design of external fibre incorporation was carried out with volume fractions set at B0=0 v%, B1=0.05 v%, B2=0.1 v%, B3=0.15 v%, and B4=0.2 v%. Additionally, the fibre lengths were set to L0=0 mm, L1=9 mm, L2=12 mm, L3=15 mm, and L4=18 mm. The mix design experimental results are presented in Table 5.

The compaction tests of the mixture were conducted according to the Chinese testing code JTG E51 (2009). A total of eight mix proportions were selected, wherein the content of geopolymer constituted 6%, 8%, 10%, 12%, 14%, 16%, 18%, and 20% of the total mass of the GSG mixture. Five water contents of 5%, 6%, 7%, 8%, and 9% were designed for each ratio, with a total of 40 compaction tests. The optimum moisture content and maximum dry density were obtained through fitting calculations.

According to the standards of GB/T 17671 (2021), geopolymer paste was prepared. First, steel slag micro powder and FA were proportionally weighed (total mass 500g) and placed in a mixer, where they were mixed for 10 min to ensure uniformity. Subsequently, the water glass solution prepared with NaOH was added, having cooled to room temperature, and mixing continued for 5 minutes. The resulting slurry was then slowly poured into a cubic mold with a side length of 70.7mm and compacted on a vibrating table. The mold was wrapped with a film and kept at room temperature for 24 hours for curing. After molding, the specimens were subjected to standard curing for 7 days and 28 days. Finally, unconfined compressive strength was measured to verify compliance with highway construction technical specifications.

In accordance with JTG E51 (2009), the preparation, curing, and strength test of GSG and FGSG mixture specimens were conducted. The specimens were prepared based on the optimum moisture content and maximum dry density, resulting in cylindrical specimens of Φ150mm×150mm. The GSG specimens underwent standard curing for 7 days and 28 days, while the FGSG specimens were cured for 7 days, 28 days, and 60 days. One day prior to the end of curing, the specimens were immersed in water for 24 hours, ensuring the water surface was 2.5 cm above the top of the specimen. After soaking, the specimens were removed, surface was wiped off, and unconfined compressive strength and splitting tests were conducted using an electric-hydraulic universal testing machine, applying a loading rate of 1mm·min⁻¹.

The specimen preparation is shown in Figure 3.

Figure 3:

Flow chart of specimen preparation procedure

The freeze-thaw cycle test was conducted according to JTG E51 (2009). The preparation and curing of the specimens were consistent with Section 2.3.2, with a standard curing duration of 28 days. After curing, the specimens were removed, surface moisture was wiped off, and their weights were recorded. Subsequently, the specimens were transferred to a testing chamber for the freeze-thaw cycle test. The freezing temperature was set to −18 °C and the thawing temperature was set to 20 °C. Each freeze-thaw cycle lasted 24 hours, with 16 hours of freezing and 8 hours of thawing. A total of 12 freeze-thaw cycles were performed, and unconfined compressive strength tests were conducted after the 0th, 3rd, 6th, 9th, and 12th cycles. The average of three samples was reported as the result of the strength. Notably, the test employed a gas freeze-water thaw mode, which involved using dry cold air in the environmental chamber during freezing and filling the environmental chamber with water at a temperature of 20 °C that was above the top of the specimens during thawing. Each specimen was weighed upon completion of each cycling phase.

The SEM-EDS tests were conducted by Hitachi Cold Field Emission Scanning Electron Microscope (Hitachi Regulus8100) to observe the micro-morphology of geopolymer and FGSG at the age of 7d and 28d, as well as the structural and elemental changes of the mixes before and after freeze-thaw cycles. Following mechanical property testing and freeze-thaw cycling, fractured specimens were dried in a 50°C oven until constant mass was achieved, ensuring minimal strength deviation across all sampled units. XRD D8 ADVANCE equipped with CuKα radiation was used to analyse the phase composition of geopolymer by XRD test.

Figure 4 shows the results of 7d and 28d unconfined compressive strength test of geopolymer mixture. At both ages, all mix ratios of the geopolymer demonstrated good compressive strength performance. The strength reached its maximum values at all ages when the slag-to-powder ratio was 3:7, with an alkali content of 8%, and a water glass modulus of 1.4, resulting in a compressive strength of 2.58 MPa at 7 d and 9.83 MPa at 28 d.

A range analysis was conducted on the compressive strength of the geopolymer at different factor levels, with the results shown in Table 6. The varying k values shown in the table exhibited peak changes. This peak variation indicated that, with the increase of each factor level, the compressive strength of the geopolymer experienced a process of initially increasing and then decreasing at both 7 d and 28d. The maximum k2 corresponded to each factor, suggesting that the optimal mix ratio of the geopolymer was slag-to-powder ratio = 3:7, alkali content = 8%, and water glass modulus = 1.4. The magnitude of the R value represents the degree of influence of each factor on the compressive strength, with larger R values indicating greater contributions. By comparing the R values, the influence ranking of each factor on strength was determined to be: alkali content > water glass modulus > slag-to-powder ratio.

Figure 4:

Unconfined compressive strength of geopolymer at different curing age

To further investigate the significant effects of various factors on the performance indicators, variance analysis was conducted on the compressive strength at 7 d and 28 d, as shown in Table 7. The sums of squares (SS) for each factor were relatively small, indicating minimal error influence during the experiment and ensuring reliable and accurate results. To assess the significance of different factors on the unconfined compressive strength of the geopolymer, F-tests were performed with confidence levels of α = 0.01, 0.05, and 0.1. The F-distribution table indicates that, for degrees of freedom (DF) of (2, 2), Fα0.01 (2, 2) = 99, Fα0.05 (2, 2) = 19, and Fα0.1 (2, 2) = 9 (GB 4086.4, 1983). At both 7 d and 28 d, the alkali content and water glass modulus were significant at the 0.05 level. The influence of the slag-to-powder ratio on compressive strength varied across different ages. It was significant at the 0.05 level for 7d and at the 0.1 level for 28d. The P value indicates significance, results show significant differences when p < 0.05, and smaller P values indicate greater significance. Therefore, the ranking of the significant influence of each factor on the compressive strength of the geopolymer can be obtained as: alkali content > water glass modulus > slag-to-powder ratio. This is consistent with the results of the range analysis.

Both range and variance analyses indicated that the three factors influencing the strength of the geopolymer each had an optimum value. The alkali content contributed most significantly to the strength, which is consistent with previous research findings (Singh & Sengupta, 2022; Sukmak et al., 2013). In a strongly alkaline environment, the alkali activator solution can dissolve and release SiO₂ and Al₂O₃ from the steel slag micro powder and FA, enabling both materials to participate more effectively in the condensation phase of the polymerization process (Hwang & Huynh, 2015). Based on the above research findings, subsequent experiments used a fixed slag-to-powder ratio of 3:7, alkali content of 8%, and water glass modulus of 1.4 to prepare the geopolymer.

The mass loss rate was used as an indicator to evaluate the impact of freeze-thaw cycles on the quality of FGSG, with the calculation formula shown in Equation (3). Figure 9 shows the variations in the mass of the mixture under different freeze-thaw cycle counts.

(3) KD=KD0−KDnKD0 {{\rm{K}}_{\rm{D}}} = {{{{\rm{K}}_{{\rm{D}}0}} - {{\rm{K}}_{{\rm{Dn}}}}} \over {{{\rm{K}}_{{\rm{D}}0}}}}

Where:

• K_D - the mass loss rate of specimen [%],

• K_D0 - the initial mass of specimen before freeze-thaw cycle [g],

• K_Dn - the mass of specimen after n freeze-thaw cycles [g].

The figure indicated that the variation patterns of the five mixture compositions were generally the same. After three freeze-thaw cycles, the mass loss for all groups was negative, indicating that the mass of each composition was continuously increasing. The growth rates during this period were relatively similar, ranging from 0.15 v% to 0.2 v%. After three cycles, the mass of the specimens began to decrease, with the rate of mass loss only slowing down by the 12th cycle. The incorporation of fibres slightly inhibited the mass loss of the mixture, with the suppression effect of different lengths ordered as follows: L2 > L3 > L4 > L1. When the freeze-thaw cycles reached 12, the mass loss rates of the B3L1, B3L2, B3L3, and B3L4 groups decreased by 8.5%, 34.3%, 25.7%, and 14.3%, respectively, compared to the B3L0 group (with a mass loss rate of 3.5%) that contains no fibres.

The increase in mass of the specimens during the early stages of freeze-thaw cycles could be attributed to three main reasons: First, the initial presence of numerous irregular pores in the specimens allowed moisture to gradually enter these pores, which led to an increase in overall mass (Sun et al., 2021). Second, the Ca(OH)₂ generated by the hydration products of the geopolymer could continue to undergo secondary hydration reactions, producing additional mass. Lastly, the mass loss due to freeze-thaw cycles primarily resulted from the spalling of the mixture, and the incorporation of fibres could effectively bond the matrix, reducing mass loss. The mass gained from water absorption and secondary hydration products far exceeded the mass loss through spalling. After six cycles, three mixture compositions (B3L2, B3L3, B3L4) still had a mass greater than their initial mass, indicating that basalt fibres longer than 12 mm had a significant effect in inhibiting the spalling of the mixture. However, once the specimens were nearly saturated with water, they experienced pressure from both internal and external moisture (Pilehvar et al., 2019). When the stress exceeded the tensile strength of the matrix, cracks formed in the specimens. The further development of these cracks would damage the internal structure, leading to particle redistribution and the formation of a new skeletal structure, resulting in material detachment (Mermerdaş et al., 2021; Pang et al., 2024). This was macroscopically observed as a decrease in mass, though the rate of decline eventually slowed.

Figure 9:

Mass change rate of mixture under different freeze-thaw cycles

The frost resistance performance of FGSG was further evaluated using the frost resistance coefficient (BDR), with the calculation formula presented in Equation (4) and the results shown in Table 8.

(4) BDR=RDCRC×100% {\rm{BDR}} = {{{{\rm{R}}_{{\rm{DC}}}}} \over {{{\rm{R}}_{\rm{C}}}}} \times 100\%

Where:

• BDR - the freezing resistance coefficient,

• R_DC - the compressive strength of the specimen after freeze-thaw cycle [MPa],

• R_C - the standard compressive strength of the specimen [MPa].

As the number of freeze-thaw cycles increased, the compressive strength of the FGSG mixture showed a marked decline, and this trend became more pronounced with the increasing number of cycles. For the same number of freeze-thaw cycles, the order of BDR for the mixtures was B3L2 > B3L3 > B3L4 > B3L1 > B3L0. The test groups with fiber incorporation exhibited both higher compressive strength and strength retention rates compared to the blank control group (B3L0). When the fiber length was optimal at 12 mm, the compressive strength loss rates of the mixture after 3, 6, 9, and 12 cycles were 2.98%, 6.33%, 10.61%, and 17.13%, respectively. The strength loss rates of the fiber-free B3L0 group for different cycles are 5.24%, 9.48%, 14.11%, and 19.36%. The BDR of the B3L2 group, which showed the best frost resistance, increased by 2.26%, 3.07%, 3.5%, and 2.23% compared to the B3L0 group at different cycles. After 12 cycles, the BDR for all five fibre lengths remained above 80%. Under repeated freeze-thaw conditions, the frost resistance performance of the mixture gradually diminished. The incorporation of fibres effectively enhanced the frost resistance of the mixture at lower numbers of freeze-thaw cycles; however, this enhancement gradually lessened with increasing cycles.

Geopolymers at a slag-to-powder ratio of 3:7, an alkali content of 8% and a water glass modulus of 1.4 were selected for microanalysis. Figure 10 shows the SEM and XRD tests results of geopolymer at 7d and 28d curing age.

In Figure 10.a, it could be observed that in the geopolymers composed of steel slag micro powder and FA at 7 d, there were many exposed spherical FA particles. The surfaces of the particles had developed a substantial amount of amorphous C-S-H and a small quantity of three-dimensional C-A-S-H gel, forming an incompletely continuous spatial network structure. Additionally, numerous cracks were present, characterized by generally large widths. As the curing age extended to 28 d, hydration reactions generated more C-A-S-H gel and acicular ettringite (AFt), which interweaved and filled some of the surface cracks, resulting in a denser network structure that made the overall structure more compact (Figure 10.b). In Figure 10.c, XRD analysis of the geopolymers at 7 d and 28 d showed multiple peaks in the range of 20°~40° (2θ), primarily consisting of C-S-H, ettringite, and a small amount of Ca(OH)₂. As the curing age increased from 7 d to 28 d, the diffraction peaks of the hydration products also increased, with C-S-H being the main hydration product. The results from SEM and XRD corroborated each other, indicating that the hydration reaction of the geopolymer was a continuous process. The main hydration product, C-S-H and C-A-S-H, continuously filled the gaps, making the structure denser, while its three-dimensional network structure tightly linked the particles together, which macroscopically results in an increase in strength (Ji et al., 2023).

Figure 10:

SEM and XRD images of geopolymer

SEM-EDS tests were conducted on B3L2 group FGSG specimens cured for 7 d and 28 d to observe and analyse their internal micro-morphology and hydration products. The interface between the fibres and the mixture is shown in Figure 11, while the results of the SEM-EDS tests are presented in Figure 12.

In Figure 11, it could be observed that the fibres were closely bonded to the slurry in a cylindrical shape, with the surfaces wrapped in a significant amount of gel material, indicating that the geopolymer had undergone a sufficient polymerization process around the fibres. As seen in Figure 12. a and b, the structure of the mixture at 7 d possessed a certain level of integrity; however, there were numerous micro-cracks present at the surface. The surfaces of the fly ash particles within the mixture were smooth and attached with a small amount of fluffy material, while AFt was interspersed within the pores and cracks. EDS analysis showed that the main elemental composition at this stage includes O, Ca, Si, Na, C, and Al, indicating that the predominant early hydration product was C-S-H. In Figure 12.c and d, the matrix of the mixture tightly bonded with the aggregates, and a large amount of binding material adhered to the surfaces of the steel slag micro powder and fly ash particles. A significant quantity of AFt was uniformly distributed, emerging from the pores and cracks of the mixture matrix, creating a cross-linked distribution in space. Compared to the 7 d, the structure was more compact, with a noticeable reduction in the number and size of micro-cracks. The predominant elements at this stage were O, Si, Al, C, and K, and a large amount of aluminium interacted with the C-S-H gel to form C-A-S-H (Wei et al., 2018). The early hydration products mainly consisted of AFt, C-S-H, and Ca(OH)₂. As the curing age increased, the hydration of steel slag micro powder released a larger amount of Ca²⁺, promoting the hydration reaction rate of the fly ash in alkaline conditions and accelerating the formation rates of C-S-H and AFt. The increase in Ca²⁺ concentrations promoted the continuous formation of Ca(OH)₂, which provided suitable alkaline conditions for the generation of AFt. With the ongoing acceleration of the reaction, Ca(OH)₂ reacted with Si and Al to form C-S-H and C-A-S-H gels before it crystallized, filling the internal pores.

Figure 11:

Interface between the fibres and the mixture

Figure 12:

SEM-EDS images of FGSG at different curing ages

The internal microstructure of B3L2 specimens under different cycles was observed by SEM test, and the results are shown in Figure 13. During early stages (3 cycles), abundant ettringite formation was observed within the microstructure (Figure 13.a), which was produced by secondary hydration reactions promoted by water penetration during the melting stages. Significant internal expansion stress was generated during the crystallization of ettringite, which accelerated crack propagation (Min & Mingshu, 1994). At the same time, the structure around fly ash particles was loose, and the bond with the matrix was weak (Figure 13.b). These were direct manifestations of early deterioration.

As the number of cycles increased to six, the overall structure of the mixture became progressively loose, and the bond strength decreased (Figure 13.d), reflecting the accumulation of damage. At this time, some fly ash particles began to detach, cracks around the particles continued to propagate, and a large number of small pores began to form (Figure 13.c). In the later stages (9 and 12 cycles), the pores expanded into voids (Figure 13.e), and cracks interconnected with each other (Figure 13.g), leading to a continuous network. Internal damage transitions from local to overall degradation. A large amount of dispersed cementitious material indicated that hydration products (such as C-S-H) had fractured or been destroyed under stress. The widening and increasing of cracks (Figure 13.h) were the results of the continuous reduction in bond strength of the damaged matrix under repeated internal stresses.

In summary, the freeze-thaw damage process observed at the micro level provides a fundamental explanation for the macroscopic performance. It is worth mentioning that the significant spalling of fly ash and the overall mass loss during the freeze-thaw process may exceed that of certain conventional fly ash-based geopolymer stabilization materials. This could be attributed to the incorporation of aeolian sand, as the 50% content of aeolian sand far exceeds the number of fine aggregates typically used in conventional base layer designs. The weakened interlocking ability between the aggregates is a key factor contributing to the decline in various properties (Liu et al., 2023).

Figure 13:

Microstructure evolution of specimens with different freeze-thaw cycles