The mechanism and rate of attack of sodium sulphate solution on cement and cement/pfa pastes

A study of the mechanisms and rates of attack of a sodium sulphate solution on four ordinary Portland cement (OPC) pastes, and four OPC/pfa pastes is presented. Accelerated experiments were carried out on mortar cubes, and samples from different depths were analysed by X-ray spectrometry and X-ray diffraction.

The mechanism of attack was similar for the different cements: initial attack on the hydrated calcium aluminates produced AFt and AFm phases, and subsequent attack on calcium hydroxide resulted in the formation of gypsum. Incorporation ofpfa reduced the rate of attack, due to the finer pore size distribution of the paste, and the formation of Afm in preference to Aft. Subsequent attack on calcium hydroxide was greatly reduced.