Reactions and Equi ibria in Magnesium Oxych oride Cements

Phase equi ibria in the system MgO‐MgC 2 ‐H 2 O at 2°±3°C were determined and the reactions by which the equi ibrium phases, 5Mg(OH) 2 MgC 2 8H 2 O and 3Mg(OH) 2 MgC 2 8H 2 O, deve op were studied by X‐ray diffractometry. As reactive MgO is disso ved by magnesium ch oride so utions, a thixotropic suspension is converted to a ge, which then crysta izes to form the ternary oxych oride phases. Insufficient y active MgO disso ves more s ow y so that, in an open system, bu k composition can shift by evaporation of water, resu ting in crysta ization of a nonequi ibrium phase assemb age, with residua magnesium ch oride so ution and unreacted MgO. Imp ications of these nonequi ibrium reactions for the performance of magnesium oxych oride cements are discussed.