Quantitative analysis of crystalline and amorphous phases in pulverized coal fly ash based on the Rietveld method

High proportions of the amorphous phases with minor and variable amounts of crystalline phases (most commonly mullite and quartz) are the main components of pulverized coal fly ash (PCFA). As for the amorphous phase, it can be further subdivided into amorphous aluminum‑silicon matrix (AASM) and amorphous silicon (AS), which show different reaction activity during subsequent applications. In the present work, a more comprehensive approach to evaluate the relative content of crystalline and amorphous phases in PCFA was proposed and verified. The results showed that the AS in PCFA can be removed by sodium hydroxide solution under certain leaching conditions, such as NaOH(aq) concentration (˂15%) and temperature (˂80 °C). The terminating point of the removal of AS was marked by the forming of hydroxysodalite (Na8Al6Si6O24(OH)2(H2O)2) in desilicated PCFA (De-PCFA). Based on the Rietveld method, the quantitative analysis of crystalline and amorphous phases in PCFA and De-PCFA was performed in turn; then the relative content of crystalline (mullite, quartz) and amorphous (AS, AASM) phases in PCFA can be obtained by calculation.