Influence of magnesia surface on the setting time of magnesia-phosphate cement

The following paper presents the influence of magnesia reactivity in magnesia-phosphate cement. When water is added to cement, monoammonium dihydrogen phosphate (NH4H2PO4 or MAP) goes in solution till saturation while magnesia (MgO) is wetted and starts to dissociate. This dissociation only depends on MgO surface, except when magnesium carbonate is present. In this case, it accelerates the dissolution process, independently of the surface state. For magnesia; the higher the amount of surface defect sites, the higher the MgO wetting. Wetting and nucleation are promoted by a large interface between MgO and MAP, because adsorption probabilities are more important. Therefore, grinding a powder allows a better reactivity. On the contrary, calcination, in a first step, enhances the surface state and then reduces MgO reactivity due to the melting of grains, which reduces the total developed surface. (C) 2002 Elsevier Science Ltd. All rights reserved.