Crystallographic research on the hydration of Portland cement. A first report on investigations in progress Available to Purchase

The results of investigations carried out at Birkbeck College since 1948 are described.

(a) Anhydrous cement compounds. The crystal structure of tricalcium silicate (C₃S) has been determined. The crystal is built from Ca⁺⁺, (SiO.)^4-, and O^-- ions; co-ordination of the Ca ions by oxygen is unsymmetrical. In atite, the form present in cement, some Si is replaced by AI, and Mg ions are present in otherwise vacant sites. Possible relationships between structure and cementing properties are discussed.

(b) Hydration Products. At room temperature, action of water on C₃S can yield, under appropriate conditions, either of two compounds, calcium silicate hydrates (I) and (II). These differ in composition but are closely related in structure. The first has been detected in a sandtime block, and one of them, probably the second, is present in set C₃S pastes. Direct detection of the hydrated compounds in set cement has so far been hindered by experimental difficulties, but it is probable that one or other of these substances is present. Hydrothermal reactions in the CaO-SiO₂H₂O system have been studied; it is probable that steam curing of cement at 100°C does not yield silicate hydration products very different from those formed at room temperature, but that at 180°, basically different compounds may be produced.

Calcium silicate hydrate (I) has a layer structure, in which entry or removal of water molecules, due to changes in temperature and/or humidity alters the inter-layer spacing. The layers themselves are elongated in one direction, giving rise to a fibrous structure. These facts may explain the cementing power of the substance, and may also account for the volume changes observed in set cement.