Synthesis of wollastonite from AlF3-rich silica gel and its hardening in the CO2 atmosphere.

This work combines some aspects of eco-friendliness: consumption of toxic waste, cutback of energy consumption during the synthesis of the binding material, reduction of CO2 emission by using less CaCO3 in the raw meal, and consumption of carbon dioxide. In the study, the kinetics of two-step synthesis of wollastonite from CaO and AlF3 production waste, namely, silica gel, its carbonisation process and the mechanical properties of obtained samples were investigated. According to XRD and DSC data, the optimal temperature in the mixture with CaO/(Al2O3 + SiO2) = 1 for the hydrothermal synthesis of the wollastonite precursors is 130 °C: F--containing compounds were bound into katoite and cuspidine, and portlandite reacted completely within 8 h. The optimal temperature for wollastonite formation is 900 °C, but fluormayenite, cuspidine, and the traces of larnite form as well. During the curing in the CO2 atmosphere, wollastonite and larnite reacted completely and formed calcite, vaterite, and amorphous CaCO3. Cuspidine also participates in the carbonisation process and, in addition to amorphous SiO2, it releases fluorite, which contributes to the total compressive strength of the products. The values of the compressive strength (10-15 MPa) in the wollastonite-sand samples match the requirements for the belite and special low-heat cements.