Reactions of sulphate-resistant Portland cement and its blends with silica fume and aqueous magnesium sulphate

Mixtures of sulphate-resistant Portland cement, Ca(OH)2 and silica fume were moist cured at 55°C for approximately twelve months and subsequently reacted with 0·5 M MgSO4 solution at 85°C for six months. Marker experiments in conjunction with electron microprobe analyses were used to elucidate reaction mechanisms. Two distinct patterns of mineral zonation were observed in altered cement paste: sharply banded and irregularly banded. Reasons for the differences in microstructure are explained in terms of differing reaction mechanisms, which are primarily related to matrix permeability. The mineralogy of the degraded cement was determined; besides gypsum and an M-S-H phase, two phases hitherto unreported in Portland cement occurred: magnesium hydroxysulphate, Mg6(OH)10SO4·3H2O, and, in one blend composition, a clinohumite-like phase. Observations are made on the probable durability of conventional concretes in contact with MgSO4(aq).