Investigation of the hydration process of a wollastonite-based brushite cement

Wollastonite-based brushite cements are mainly used for refractory material applications, but they may also offer new prospects for the solidification/stabilization of hazardous wastes. Most studies on these binders have been focused on the characterization of the final products, on the associated microstructure or on the functional properties of the resulting materials. This work provides new insight into their setting and hardening process, by characterizing the evolution of both the liquid and solid phases with ongoing hydration. The investigated binder was a two-component system, consisting of wollastonite and of a phosphoric acid solution containing borax and metallic cations (Al$^{3+}$, Zn$^{2+}$). The hydration process was investigated using a cell allowing the simultaneous measurement of the elastic modulus and the electrical conductivity during setting. The phase assemblage was characterized by X-ray diffraction, scanning electron microscopy and $^{31}$P and $^{27}$Al MAS-NMR. Hydration was a multi-step process which yielded several products amorphous silica, monocalcium phosphate monohydrate (Ca(H$_2$PO$_4$)$_2$.H$_2$O) which precipitated transiently during the first stage of hydration, and brushite (CaHPO4.2H2O) which crystallized at higher pH. In addition, elemental mapping by SEM-EDS showed the precipitation of an amorphous phase containing phosphate, aluminum and zinc, which tended to get richer in calcium with ongoing hydration. Its structure was investigated by NMR spectroscopy (one pulse $^{31}$P and $^{27}$Al and REDOR $^{27}$Al/$^{31}$P). It mainly contained hexavalent aluminum bonded to orthophosphate moieties. This amorphous phase was shown to play a key role in the consolidation process of the material and its final strength.