Microstructural insight into the deterioration mechanism of the mortar subject to the combined action of external sulfate attack and cyclic wetting-drying

Combined action of external sulfate attack (ESA) and cyclic wetting-drying (CWD) results in the severe durability issue of cement-based materials. A comprehensive microstructural investigation of deterioration mechanism of mortar exposed to ESA and CWD is scarce until now. By means of multiple techniques, the phase evolution, microstructural evolution, pore structure variation and thus the deterioration mechanism of mortars with different water-to-cement ratios (0.3 and 0.5) exposed to ESA and CWD, are investigated in this study. It is found that the amount of portlandite declines fast at the first two wetting-drying cycles and decrease gently in the following cycles, the loss of OH- into external sulfate solution plays a dominant role over uptake of Na+ from external sulfate solution during combined action of ESA and CWD; the evolution of pore size distributions of mortars with different water-to-cement ratios exhibit distinct patterns; decalcification of C-S-H leads to the shrinkage of C-S-H and collapse of interlayer space and gel pore, responsible for the decreasing amount of interlayer/gel pore at the first 10 wetting-drying cycles and the increase of the amount of interlayer/gel pore at the following 8 cycles; only a few gypsum and thenardite/mirabilite are detected, ettringite is the principal cause of deterioration of mortar subject to ESA and CWD.