Stacking faults of the hydrous carbonate-containing brucite (HCB) phase in hydrated magnesium carbonate cements

Abstract

Recently, a hydrous carbonate-containing brucite (HCB) with an approximate composition of MgCO3·35 Mg(OH)2·H2O was postulated as hydration product of binders based on mixtures of reactive magnesia and hydromagnesite (Mg5(CO3)4(OH)2·4H2O). X-ray diffraction showed that the 001 reflection of this phase was split into two reflections at 20 °C, whereas at 60 °C only one reflection occurred. Rietveld refinement revealed that the X-ray pattern of the HCB-phase could be fitted well when a random displacement (stacking faults) and a movement of the layers in x, y and z direction were allowed. The presence of water and/or carbonate leads to different distances between the layers, thus causing the splitting of the 001 reflection. At 60 °C, only carbonate is included in the brucite structure, leading to a similar distance between all brucite layers. The assignment of the additional reflection appearing at 20 °C to a separate, unknown hydrated magnesium carbonate, as suggested in earlier studies, can be excluded.