The hydration of a tricalcium silicate paste has been examined from very early stages, i.e. during the induction period, up to two and half years using both scanning and transmission electron microscopy as well as solid state nuclear magnetic resonance. This latter technique can be used to reveal the degree of hydration of the sample and the nature of the silicate units which make up the C-S-H gel. Solid state NMR shows that during the induction period a small amount of material containing hydrated monomeric silicate units is formed, although this is difficult to identify using SEM. However during the acceleratory period SEM shows the development of the typical fibrillar C-S-H material. Backscattered electron imaging shows the cores of anhydrous material surrounded by the denser inner product.
Thermogravimetric analysis has been used to find the C/S ratio of this material, which is approximately 1.8, whilst solid state NMR shows that the hydrated silicate units are largely dimeric at this stage. As hydration proceeds, the fracture surfaces become increasingly dominated by large calcium hydroxide crystals, although some fibrillar material can be identified even after over two years. The C-S-H contains progressively more middle units in silicate chains at the expense of the end units. A sample of tricalcium silicate hydrated for 26 years has also been examined.