Effects of alkali activation and hydrothermal processes on the transformation of fly ash into Al-substituted tobermorite fiber

Abstract

By choosing two sample fly ash with different chemical compositions and calcium-bearing mineralogical phases, this article explored the influence of calcium-bearing minerals in fly ash and hydrothermal methods on the synthesis of tobermorite fiber. It was found that fly ash with more than 20% calcium oxide content easily decomposed and transformed into 9 Å and 11 Å tobermorite fiber, with the tobermorite content exceeding 30%, under the conditions of hydrothermal solution without alkali and 1 h reaction time. It was demonstrated by FTIR spectrogram and solid NMR analysis results that calcium-bearing mineral is a significant factor which can trap most of Al2O3 in the fly ash, preventing it from soaking up into the lattice of tobermorite, and also inhibiting the substitution of Si by Al. A thermal activation method was designed to enhance the transformation process of fly ash with lower calcium concentration by a similar mechanism to calcium action.