Effect of calcium to silica ratio on the synthesis of calcium silicate hydrate in high alkaline desilication solution

Abstract High alkaline desilication solution (DSS), a high volume byproduct from the pretreatment of high‐alumina fly ash, was used as low‐cost mother liquor for the synthesis of calcium silicate hydrate (C‐S‐H). Through the combined analysis of X‐ray diffraction, thermogravimetric analysis, X‐ray fluorescence, 29 Si MAS NMR, and Brunauer‐Emmett‐Teller, the relationship between chemical composition and structure of C‐S‐H synthesized under Ca/Si of 0.83:1 to 2.0:1 was investigated. Silicon conversion and yield of product have a positive correlation with Ca/Si. Sodium uptake in C‐S‐H is inhibited as Ca/Si increases. The formation of sodium in C‐S‐H transfers from “bound Na” to “mobile Na” and aluminum from tetrahedrally coordinated Al (IV) to octahedrally coordinated Al (VI). The increase of Ca/Si leads to shortening of silicate chain and formation of more dimers, which causes more water bound in C‐S‐H. The mechanism of calcium addition on silicate chain obtained from DFT calculation primarily results from more interlayer calcium occurrence to affect bridging tetrahedron and cationic bounding states reorganization. Reasonable control for Ca/Si momentously contributes to the adjustment for composition and structure of C‐S‐H synthesized in DSS.