The hydration of C3S and ordinary Portland cement with relatively large additions of microsilica

The microstructure of pastes of ] part of C3S blended with 1·3 parts of precipitated silica, and 1 part of ordinary Portland cement (OPC) blended with 1·4 parts of silica fume, hydrated for two months at 25°C and three months at 40°C respectively, have been studied by analytical transmission electron microscopy and solid state NMR. The hardened pastes were found to consist, solely in the case of C3S/silica and principally in the case of OPC/silica fume, of silica particles embedded in CSH gel with a Ca/Si ratio significantly less than unity. The gel contained silicate units in chains of approximately 8·5 units in average length. Similar pastes prepared with alkali addition differed little in microstructure, but the OPC/silica fume paste with alkali addition contained isolated regions of low calcium and high alkali content with a distinctive morphology.