Abstract: In tunnel construction under sulfate-rich environment, attention should be paid to the erosion resistance of concrete structures. Based on the tunnel project of Mouyuan Expressway in Yunnan Province, this study used the method of dry-wet cycle sulfate erosion to explore the effects of different mineral admixtures (fly ash, limestone-based micropowder, silicon-based micropowder) on the apparent morphology, mass and compressive strength of concrete. The microstructure and product changes of specimens were analyzed by means of scanning electron microscopy and X-ray diffraction. The results show that limestone-based micropowder and silicon-based micropowder have good filling effect and pozzolanic activity. After curing in water bath at 50 ℃ for 7 d, additional hydrated calcium silicate (C-S-H) gel can be generated, which can reduce the removal of calcium phase, reduce the damage degree of apparent morphology, mass loss and compressive strength loss of specimens. After 75 dry-wet cycles, the mass of pure cement, 10% fly ash, 10% limestone-based micropowder, 10% silicon-based micropowder, 5% fly ash and 5% silicon-based micropowder decrease by 21.7%, 32.2%, 16.9%, 12.1% and 20.1%, respectively. The strength after erosion accounted for 79.3%, 49.9%, 101.5%, 95.4% and 84.5% of the initial value, respectively. When the C-S-H gel wrapped with calcium hydroxide (CH) crystal in the specimen is seriously eroded, the dry-wet cycle conditions make the CH crystal frequently contact with CO₂ in the air and react to generate a large number of calcium carbonate crystals.

Key words: mortar, mineral admixture, sulfate erosion, microstructure, product analysis