Investigation of solid and aqueous chemistry of lO-year-old Portland cement pastes; with and without silica modifier

Mature cement specimens, cured in controlled conditions, provide a valuable source of data which can be used to verify or develop models, based on thermodynamic equilibrium. Four 10-year-old cement pastes were subject to detailed examination of their chemistry: OPC, OPC + 15% microsilica, HAPC (high alkali Portland cement), and HAPC + 15% microsilica. The solid phases were analysed by analytical electron microscopy (AEM), thermogravimetry and X-ray diffraction. The aqueous phase was characterizedfor its content of Na, K, Ca, Mg, Si, Al and SO₄. AEM showed considerable variation in the Ca/Si ratio of C-S-H within paste samples, indicating persistent compositional disequilibrium. Ca/Si ratios were noticeably lower in the silica-modified pastes, although the mean values were not statistically different; 1·54 ± 0·21 in OPC, compared to 1·36 ± 0·19 in OPC + 15 % microsilica. The presence of microsilica also favours the formation of AF_t at the expense of AF_m, some of this AF_t being relatively Si-rich. Soluble alkali is considerably reduced in the silica-modified pastes owing to the increased amounts of low ratio C-S-H from pozzolanic reaction. Thermodynamic modelling of the aqueous phase shows, in general, good agreement between observed and predicted solubilities. Two areas of weakness were identified: solid solution phenomena may significantly alter the solubility properties of host phases; there is also a lack of a robust thermodynamic model for C-S-H in cement pastes.