The mechanism of stabilization of highmagnesia Portland cements by reactive silica under autoclave conditions

Abstract

Synopsis Factors which play important roles in the mechanism of stabilization of high-magnesia Portland cements by active silicate materials under autoclave conditions have been considered. Among these, reduction in the rate of hydration of uncombined MgO, formation of a magnesium silicate hydrate similar to the natural mineral deweylite and development of substantial strength in autoclaved products are thought to be the principal factors which control the volume expansion of these cements. It is proposed that strength development in these materials is consequent upon the formation of large amounts of the well crystallized 11 Å tobermorite phase. The extra strength generated in autoclaved mixtures of cement when they incorporate active silica may be sufficient to contain the expansive forces generated by the hydration of perielase into brucite. Much Mg(OH)2, very little Ca(OH)2 and some unhydrated MgO are present in the final products in addition to tobermorite. Some information on the microstructure of the stabilized cement as seen with a scanning electron microscope is given.