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

Abstract

This work focuses on the carbonation reaction of calcium silicate hydrates (CaO)(SiO 2 )(H 2 O) (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 CO 2 , according to a process that is commonly based on the use of minerals such as natural wollastonite (CaSiO 3 ). CSH and wollastonite powders were placed in a gas stream consisting either of pure CO 2 or of a 15 vol % CO 2 −85 vol % air mixture, saturated with water, at atmospheric pressure and 40 °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 α 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 °C, α = 75% for CSH against 12% for CaSiO 3 . Furthermore, carbonation promotes the formation of calcite with only 15 vol % CO 2 , allowing the direct use of cement plant fumes.