Micro-analysis of “salt weathering” on cement paste

Normally, concrete technologists attribute salt weathering, salt crystallization or physical attack to the deterioration of concrete that is partially exposed to sulfate environment. However, there are few convincing evidences supporting this view. The purpose of this paper is to check by means of extensive micro-analysis if traces of sulfate crystals are present in the paste. This would enable to verify in a direct way whether salt weathering really causes cement paste damage or not. In this research, cement paste and cement–fly ash paste specimens were partially exposed to sodium sulfate and magnesium sulfates solution under a constant storage condition (20 ± 2 °C, and 60 ± 5% RH) and a sharply fluctuating environment (40 ± 2 °C and 35 ± 5% RH for 24 h, then 10 ± 1 °C and 85 ± 5% RH, also for 24 h) respectively. ESEM and SEM images, combined with EDS and XRD analysis, were employed for micro-analysis. The experimental results indicate that Na2SO4 crystals or MgSO4 crystals could not be identified in the cement paste or the cement–fly ash paste, both under constant and fluctuating exposure conditions. Instead, large amounts of ettringite, gypsum and brucite were found and identified as probably one of the determining factors causing failure of cement paste. On the other hand, salt crystallization could be observed in the calcium carbonate crystals, the carbonated products of hydrated cement paste.