Phase and density changes during the ''conversión'' reaction of high alumina cement

Abstract

The basis of the conversion reaction, in which high alumina cements develop hydrogarnet subsequent to their set, is weel-established and its mineralogical features accord with phase equilibrium relations in the CaO-Al₂O₃-H₂O system. What is less well understood is the role of silica in the conversion. Silica may substitute into hydrogarnet, which has the formula Ca₃Al₂Si_3-XO_12-4x(OH)_4x. Moreover the density of the hydrogarnet phase increases rapidly with the extent of silica substitution. Nomographs showing the variation of density and X-ray d spacing with composition are presented. Although HAC contains relatively little SiO₂, examination of a high alumina cement warm cured at 85ºC and containing 2.46 wt% SiO₂, shows that this hydrogarnet concentrates silica. Iron is, however, present mainly as amorphous hydrated iron oxide. Implications for cement proportioning to avoid conversion are discussed.