Durability study and mechanism analysis of red mud-coal metakaolin geopolymer concrete under a sulfate environment

Abstract

The durability of construction materials in sulfate environments is extremely important for their service life. Geopolymer concrete, as a new type of construction material, has been subjected to very little research in sulfate environments and lacks real-time nondestructive testing methods. For this reason, in this study, the physical and mechanical properties of geopolymer concrete synthesized from red mud and coal metakaolin were tested and compared with ordinary silicate cement concrete. The physical phase composition, micro-morphology, porosity and functional group composition of the red mud-coal metakaolin geopolymer concrete were further analyzed using XRD, SEM-EDS, FT-IR and NMR. In addition, the electrochemical properties of the geopolymer were tested by electrochemical impedance spectroscopy and correlated with the diffusion coefficient of the material. The results show that the stability and compressive strength of geopolymer concrete is significantly better than that of ordinary silicate cement concrete in sulfate environment. Ettringite and gypsum type expansion damage were present in ordinary silicate cement concrete. In contrast, only sodium sulfate crystals were found in geopolymer concrete and no expansion products were found. This nearly indicates that the inert interaction between sulfate and hydration products is the main reason for its superior sulfate resistance. There is a significant correlation between the diffusion coefficient of red mud-coal metakaolin polymer concrete and the electrochemical parameter RCP/t and RDCP/t. Therefore, electrochemical impedance spectroscopy testing can be used as a real-time, non-destructive means of monitoring the performance of geopolymer concrete in sulfate environments.