Crystal Growth during the Hydration of CaSO₄.½H₂O

Abstract

THE CaSO₄.½H₂O – H₂O system has long been investigated as a simple system of cementitious material^1,2. Though the net volume of a CaSO₄.½H₂O – H₂O paste decreases by 7 per cent, the gross volume increases by about 0.5 per cent during hardening. There is no satisfactory theory to explain this anomaly. During an examination of the hydration of Portland cements³, it was seen that gypsum at first precipitates as very fine needles and at a later stage some of these needles have grown while others have disappeared. It was thought that this might also occur during the hydration of CaSO₄.½H₂O. To check this hypothesis two batches of CaSO₄.½H₂O were made by heating gypsum (analytical reagent quality, B.D.H.) at 120° C. One batch was heated to constant weight, the other much longer, and both were cooled over fused calcium chloride in a desiccator. The batch heated for a long time proved to be slower in reaching a given hydration stage, but the course of the reaction was otherwise the same. These CaSO₄.½H₂O batches were then ground to pass a 200 mesh Brit. Stand. sieve. Six pastes were made, using three different water/solid ratios. In all cases more than sufficient water was present for complete hydration. 24 specimens from these pastes were examined electron-optically using techniques described previously⁴. Figs. 1–3 show different stages of hydration of CaSO₄.½H₂O. At first very fine crystals of gypsum were formed (Fig. 1). The amount of these very fine gypsum crystals formed and the length of time they lasted depended on the water/solid ratio and the reactivity of the solid. Fig. 2 represents the stage of hydration of CaSO₄.½H₂O paste at the beginning of set. By this time the crystals had become longer and in most cases much thicker, although a significant number of very thin crystals were present. A considerable amount of CaSO₄.½H₂O was left unreacted at this stage. Fig. 3 represents the structure of completely set paste. At this stage the paste is a mixture of large and very small crystals. Most of the CaSO₄.½H₂O has been hydrated by this time.