Mechanical properties and microscopic mechanism of active MgO-fly ash solidified saline soil in seasonal freezing areas

This paper presents an innovative method for improving the strength of salt-bearing clay under freeze–thaw cycles. The active magnesium oxide (MgO) and industrial waste fly ash were employed as primary solidifying materials in the seasonal freezing soil reinforcement process, and the reinforcement effect is achieved through carbonate crystals generated through carbonization tests. The solidification effect was investigated with varying ratios of solidifying materials, salt content, and freeze–thaw cycles. The experiment results revealed that the combination of active MgO and fly ash effectively enhances the strength of salt-bearing clay. Specifically, compared to unsolidified soil, the soil solidified with 8 % active MgO (8M0F) and a combination of 4 % active MgO and 4 % fly ash (4M4F) demonstrated a significant increase in ultimate strength by 14 and 12 times, respectively. Moreover, the number of freeze–thaw cycles exhibited a negative correlation with ultimate strength, whereas the salt content demonstrated a similar negative correlation. Microscopic analysis revealed that the 8M0F sample produced mainly magnesium carbonates and hydrogen and oxygen compounds, whereas the 4M4F sample generated primarily magnesium and calcium carbonates. The resulting compound crystals efficiently filled the internal pores and cemented the soil particles, leading to a substantial increase in strength. Overall, these results indicate that the active MgO combined with fly ash solidifying method can offer a viable and sustainable solution for improving the strength of salt-bearing clay in areas with seasonal freeze–thaw cycles.