Hydration reactions in autoclaved DSP cements

DSP (densified with small particles) cement pastes, made from white Portland cement and silica fume, were autoclaved at temperatures of up to 240°C. The hydration products were characterized using X-ray powder diffraction, ²⁹Si nuclear magnetic resonance spectroscopy and scanning electron microscopy. The influences ofCaO/SiO₂ ratio, autoclave temperature and time of reaction were studied. Semi-crystalline calcium silicate hydrate was the major product at low temperatures, with xonotlite, tobermorite, gyrolite, kilchoanite and 8CaO·5SiO₂ present at higher temperatures. The nature of the crystalline phase depends on the initial CaO/SiO₂ molar ratio; at 240°C, a distinct change in chemistry appears to occur between CaO/SiO₂ ratios of 1·3 and 1·1. NMR spectroscopy has proved to be an effective means of studying the polymerization of silicates.