Effect of fineness of coal-bearing metakaolin on sulfate corrosion resistance of metakaolin-cement solidified soil

This study investigated the effect of coal-bearing metakaolin (CMK) with two fineness levels on the sulfate resistance of cement solidified soil. Specimens with total binder (CMK + cement) contents of 6%, 8%, and 10% were immersed in a 5% sodium sulfate solution for 60, 90, and 120 days. Sulfate resistance was evaluated by visual observation and unconfined compressive strength (UCS) tests, while the deterioration mechanisms were investigated using scanning electron microscopy, X-ray diffraction, and mercury intrusion porosimetry. The results showed that CMK significantly improved the sulfate resistance of cement-stabilized soil compared with the control specimens. Under the investigated conditions, the finer CMK increased the UCS after sulfate exposure by 9.3%–15.6% compared with the coarser CMK, although this advantage gradually diminished with increasing immersion time. Microstructural analyses indicated that CMK refined the pore structure and promoted the formation of non-expansive C-A-S-H phase, thereby enhancing the sulfate resistance of the solidified soil.