Influence of curing duration on thaumasite formation of Portland-limestone cement pastes

Abstract

Extensive researches have been carried out on the conventional sulfate attack, while it has been found that the thaumasite form of sulfate attack (TSA), sulfate attack at low temperature, has just been discovered and its mechanism is not well understood so far. In this study, the sulfate attack of cement paste incorporating 30% mass of limestone powder was investigated. After 20 °C water cured for 7, 14, and 28 days, respectively, 20 mm cube specimens were exposed in a 5% magnesium sulfate solution at (6 ±1) °C for periods up to 240 days. Their appearance change, compressive strength development were examined at different storage time, and selected paste samples were examined by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The results indicate that all Portland-limstone cement pastes suffer from appearance deterioration to some extent. The compressive strength of cement paste initially increases and after 120 days decreases with increasing exposed period. In addition, the cement paste with short curing time is more susceptible to sulfate attack, which directly leads to the formation of non-binder thaumasite crystal accompanied by the formation of ettringite, gypsum and brucite, and becomes a white, mushy, and incohesive matrix. Additionally, the extent of sulfate attack is greater and the formation of thaumasite is observed earlier for shorter curing time.