Understanding the setting and hardening process of wollastonite-based brushite cement. Part 1: Influence of the Ca/P ratio and H₃PO₄ concentration of the mixing solution

Wollastonite-based brushite cement is prepared by mixing wollastonite with a phosphoric acid solution containing metallic cations and boric acid. This work investigates simplified systems comprising wollastonite and H₃PO₄ solutions only, in order to clarify the influence of the H₃PO₄ concentration, Ca/P and l/s ratios (this latter parameter being correlated to the two others) on the setting and hardening process. At constant H₃PO₄ concentration, increasing the Ca/P ratio, and decreasing the l/s ratio, accelerates the early stages of hydration but limits its progress at 7 d. At high Ca/P ratio, more basic calcium orthophosphates form in addition to dicalcium phosphate. At constant l/s ratio, increasing the H₃PO₄ concentration in the range 7-10 mol.L^-1 retards cement hydration, and inhibits the setting at concentrations ≥ 12 mol.L^-1. A good compromise is obtained for H₃PO₄ concentrations between 9 and 10 mol.L^-1: hydration is not too fast and yields the maximum amount of dicalcium phosphate. (authors)