AbstractThe expansion and changes in phase composition of dilute paste and mortar bars of normal and sulphate‐resisting Portland cement exposed to various sulphate solutions were studied over a period of time. Although strong expansion was usually associated with the presence of considerable amounts of calcium sulphoaluminate, there was no quantitative correlation between expansion and the concentration of crystalline calcium sulphoaluminate. Similar quantities of this compound were found in corresponding samples of normal and sulphate‐resisting cement. It was inferred that, on continued corrosion, calcium sulphoaluminate partially passes into a gel phase which may determine the properties of the cement.In the presence of magnesium ions the bars disintegrated before excessive expansion occurred, since these ions reacted with hydrating cement with the precipitation of magnesium hydroxide, which tends to block the pores and channels, thus preventing penetration of the solutions. Chloride ions increased expansion by forming calcium chloroaluminate. Under the static conditions of the experiments protective surface films develo