Precipitation of carbonated apatite in the cement system α-Ca3(PO4)2-Ca(H2PO4)2-CaCO3

Powder mixtures of alpha-Ca-3(PO4)(2), Ca(H2PO4)(2), and CaCO3 were investigated to assess their potential for the precipitation of a carbonated apatite suitable for cement applications from a supersaturated solution. Of particular interest was the requirement of carbon dioxide release during the setting reaction to obtain Ca9-x(HPO4)(1-1.5x) (PO4)(5-2.5x)(CO3)(3.5x)(OH)(1+1.5x) with a fixed carbonate content. Results indicated that a compromise is necessary with respect to the maximum permissible level of CaCO3 addition to the powder mixture as against appropriate setting and hardening properties. Cementlike materials with good setting and hardening properties were obtained when alpha-Ca-3(PO4)(2) was the main reactant using Ca(H2PO4)(2) and CaCO3 as additives in relative proportions of <10 wt % corresponding, approximately, to the stability field area (x,lambda) contained within the lines lambda = -1.5x, x = 0.3, and lambda = -1. Desirable workability and setting properties were lost as the parameters x and -lambda increased. Finally, it was also found to be virtually impossible to obtain a predetermined chemical composition for the setting apatite precipitate, since to increase the amount of carbon dioxide released from the initial powder mixture during setting, the solution pH would need to be decreased, and in doing so the nature of the precipitate could be altered to fulfil the thermodynamic conditions of the solubility diagrams of calcium phosphates. (C) 1999 John Wiley & Sons, Inc.