Mechanistic insights into two-stage expansion of concrete under external sulfate attack

Despite extensive academic research on durability issues caused by sulfate attack, deterioration reasons and expansion mechanisms of concrete remain controversial, with descriptions of expansion development still focusing on phenomenological observations. The present study analyzes the macro-micro property changes of concrete and establishes a time-varying expansion prediction model considering environmental and material parameters. The micropore evolution in different pore categories is investigated, and the primary type and precipitation location of expansive products, as well as the expansion mechanism of concrete, are clarified. The results indicate the possibility of thaumasite sulfate attack (TSA) in concrete at low temperatures. The expansion development of concrete under sulfate attack can be divided into two stages: the expansion latency stage explained by crystallization pressure theory, and the significant expansion stage associated with volume increase theory. The expansion level at the inflection point of the expansion rate was solely related to the intrinsic micropore properties of concrete.