Abstract: During underground engineering construction in gypsum-enriched formations, the reaction between  gypsum and groundwater leads to the dissolution of Ca2+ and SO42-, which enhances the erosion effect on the concrete foundation of underground structures and affects the safety of underground engineering. In order to investigate the erosion characteristics and mechanisms of concrete under different  formation environmental conditions, commonly used C20 and C30 concrete in underground engineering construction were studied. The study focused on the gypsum-containing formation  conditions in the southern part of Chengdu basin and explored the erosion characteristics and mechanisms of concrete in both gypsum-containing and non-gypsum-containing formations under changing underground temperature conditions. The results show that: 1) The erosion degree of C20 and C30 concrete in gypsum-containing formations is significantly greater than in non-gypsum-containing formations. Under the erosionby different geological groundwater, the  maximum mass change rates of C20 and C30 concrete are 1.80% and 0.87%, with the maximum compressive strength reduction rates of 20.14% and 12.80%, respectively. C30 concrete exhibits a smaller attenuation in compressive strength under different groundwater conditions, making it more suitable as a construction material for underground engineering. 2) Increasing temperature enhances the erosion effect of SO42- on concrete, leading to a decrease in compressive strength. Among them, the maximum compressive strength reduction rates of C20 concrete under Quaternary and Guankou  Formation groundwater erosion reach 6.89% and 3.89%, respectively. 3) Concrete erosion occurs from the outside to the inside, where Ca2+ and SO42- in groundwater recrystallize to form gypsum on the  external surfaces of the concrete and then diffuse along cracks towards the interior, resulting in the formation of ettringite crystals, expansion forces, crack formation, and a decrease in compressive strength. Through correlation analysis, a relationship equation between the mechanical properties of concrete and temperature was established, achieving quantitative calculation of long-term changes in the mechanical properties of concrete in gypsum-containing formation engineering construction.

Key words: concrete, gypsum, erosion, compressive strength, dissolution, groundwater