Carbonation of Calcium Silicate Hydrates as Secondary Raw Material from the Recovery of Hexafluorosilicic Acid

This work focuses on the carbonation reaction of calcium silicate hydrates (CaO)(SiO2)(H2O) (CSH) powders synthesized by an aqueous process using fluorosilicic acid waste and lime as main reactants. We investigated whether these CSH could constitute a secondary raw material to produce building materials that harden at low temperature by reaction with CO2, according to a process that is commonly based on the use of minerals such as natural wollastonite (CaSiO3). CSH and wollastonite powders were placed in a gas stream consisting either of pure CO2 or of a 15 vol % CO2-85 vol % air mixture, saturated with water, at atmospheric pressure and 40 degrees C. Based on thermogravimetric analysis, X-ray diffraction, and scanning electron microscopy of the products, we proved the occurrence of the carbonation reaction and we calculated the carbonation degree alpha for various reaction times. The structural water contained in the CSH played a beneficial role in the carbonation reaction since their carbonation degree is much higher than that of wollastonite: for a reaction time of 120 min at 40 degrees C, alpha = 75% for CSH against 12% for CaSiO3. Furthermore, carbonation promotes the formation of calcite with only 15 vol % CO2, allowing the direct use of cement plant fumes.