Effects of hydrothermally synthesized xonotlite and tobermorite intermediate phases on wollastonite formation for thermal insulator

The effects of calcium silicate hydrate intermediate phases tobermorite and xonotlite synthesized hydrothermally under varying Ca/Si ratios, water/solid (w/s) ratios, reaction temperatures, and durations on the wollastonite formation through calcination at 800 oC were investigated in detail. TGA-DTA results revealed that materials synthesized at 200 oC exhibited lower weight losses and sharper exothermic peaks indicative of beta-wollastonite formation, suggesting a more efficient transformation into the final phase. FTIR analyses demonstrated that well-ordered xonotlite-like crystalline structures were formed at 200 oC, whereas tobermoritelike phases were observed at 150 oC. Prolonged reaction times were found to increase the accumulation of amorphous C-S-H, thereby adversely affecting crystallinity of the intermediate phases. Higher w/s ratios enhanced ionic transport and silicate solubility, thereby promoting the formation of xonotlite. The controlled selection of temperature, Ca/Si ratio, and w/s ratio was determined to be a key factor in the formation of crystalline precursor phases and their subsequent transformation into wollastonite. Although alpha-wollastonite formation was detected at 900 oC, the wollastonite structure was found to retain its thermal stability up to 1100 oC.