Setting and hardening process of a wollastonite-based brushite cement

Wollastonite-based brushite cements are used for refractory material applications, but they may also offer prospects for the solidification/stabilization of hazardous waste. These binders are formed by the reaction of wollastonite with an orthophosphoric acid solution containing borax and metallic cations (Al3+, Zn2+). This work provides new insights into their setting and hardening process using a panel of characterization techniques (XRD, NMR, rheology...). A multi-step hydration process is evidenced by the formation of several products: amorphous silica, monocalcium phosphate monohydrate (Ca(H2PO4)(2).H2O or MCPM), that precipitates transiently during the first stage of hydration, and brushite (CaHPO4.2H(2)O) that crystallizes at higher pH (3). In addition, an amorphous phosphate phase containing aluminum, calcium and zinc massively precipitates at the beginning of hydration, and gets richer in calcium as hydration progresses. Setting of the cement paste occurs when brushite starts to form. The successive formation of MCPM and brushite is well predicted by thermodynamic modelling.