Degradation behavior of early-age frozen mortar under the combined effects of sulfate attack and freeze-thaw cycles

This paper studies the deterioration of mechanical properties and microstructure of early-age frozen mortar (EAFM) under the combined effects of sulfate attack and freeze-thaw cycles (SFCs). The focus is on the influence of freezing onset time, freezing duration, and fiber content on the performance of EAFM. The results show that as SFCs increase, the mechanical properties of mortar decline and microstructural damage become more severe. An earlier freezing onset time (4 h) and a longer freezing duration accelerate deterioration. The addition of 0.2 % basalt fiber (BF) and polyvinyl alcohol (PVA) fibers improves resistance to SFCs damage. When both are used at 0.1 %, the synergistic effect is strongest. At the end of erosion, the compressive strength, flexural strength, and mass loss rate only decreased by 13.71 %, 11.17 %, and 2.18 %, respectively. A prediction model for relative dynamic elastic modulus and damage layer thickness was developed and showed good accuracy. Microscopic analysis reveals that with more SFCs, the porosity and the proportion of large pores increase, while Ca(OH) 2 content and tortuosity decrease. Among all test groups, ESFCA6 showed the most severe deterioration. This study provides theoretical and technical support for durability design and protection in cold-region engineering, and can also offer a theoretical basis for the durability assessment of structural engineering in cold regions.