Dissolution behaviour and mechanism of fly ash in acid activators

Clarifying the dissolution behaviour and mechanism of fly ash in acid activators is essential to understand the properties of fly ash-based silico-aluminophosphate geopolymer. This paper investigated the in-situ dissolution behaviour of fly ash (FA) in aluminium dihydrogen phosphate (MAP), phosphoric acid (PA), citric acid (CA), oxalic acid (OA), and tartaric acid (TA) using optical microscopy and electron probe microscopic analysis (EPMA). The phase and elemental changes before and after dissolution were further investigated using quantitative X-ray Diffraction (QXRD) and 2D-Fourier transform infrared (FTIR) spectrometry. In addition, the changes in dissolved elements were elucidated from a liquid phase perspective. The results showed that the capacity of each acid to dissolve FA was CA>MAP>PA>TA>OA. Ca-containing phases in FA were preferentially dissolved in all acids. The main contributor to FA dissolution in acid was the amorphous phase, and the SiOSi bond in quartz was more sensitive than other chemical bonds to acid. When FA was dissolved in OA and TA, new crystalline phases—calcium oxalate and calcium citrate—formed on the FA surface, inhibiting further dissolution.