High temperature cements

A method of designing a cement composition comprises determination of the temperature to which the composition will be exposed in situ; determination of a stable, thermodynamic equilibrium composition of a CaO-Al2O3-SiO2-H2O (CASH) mineral system in the [xonotlite/wollastonite]-grossulaire-anorthite or grossulaire-anorthite-quartz triangles of the Si-Ca-Al phase diagram with a possible contribution of iron and/or magnesium, analogous to the cement when set, at the determined temperature; determining proportions of cement and mineral oxides required to provide a mixture having the determined composition; and defining a series of particulate materials of predetermined particle sizes and densities, comprising cement and mineral oxides in the determined proportions.