Salt-Frost Resistance of Magnesium Oxychloride Cement with Added Highland Barley Straw Ash

Magnesium oxychloride cement (MOC) exhibits excellent resistance to salt–brine erosion; however, its poor water resistance remains a significant drawback. This study investigates the use of highland barley straw ash (HBSA) as an active admixture to improve the salt–frost resistance of MOC mortars (MOCMs). Salt–frost erosion tests were conducted on MOCMs containing varying amounts of HBSA. The macroscopic properties, microscopic pore structure, and chemical composition of HBSA-doped MOCMs were analyzed across multiple scales in a salt–frost erosion environment to determine the damage characteristics and degradation mechanisms. The findings indicate that HBSA significantly affects the salt–frost resistance of MOCMs. As the HBSA content increases, the salt–frost resistance initially improves but then decreases. After 60 cycles of salt–frost erosion, the MOCM with 10% HBSA exhibited a 22.7% improvement in compressive strength retention compared to the control sample. The pore structure of the MOCM containing 10% HBSA was found to be more refined, with a reduced proportion of harmful pores. This refinement helps minimize osmotic and expansion pressures during salt–frost erosion. Furthermore, a significant amount of M-S-H gel was formed within the microstructure of the MOCM containing 10% HBSA, enhancing its microstructural integrity and resistance to salt–frost damage.