Long-term dissolution behavior of amorphous aluminosilicate in sodium hydroxide solution for geopolymer synthesis using circulating fluidized bed combustion fly ash

Abstract

The dissolution behaviors of silicon (Si) and aluminum (Al) in the alkaline solution is vital for revealing the reaction mechanism during geopolymer synthesis from fly ash. Most of the relevant reported studies reported were performed under constant agitation for a short period (several hours), which is insufficient because during geopolymer formation, the Si and Al are generally dissolved slowly and continuously in the static state with no agitation. In this study, the circulating fluidized bed combustion fly ash (CFBFA) was immersed in NaOH solutions in the static state for specified times (1–35 d), and the dissolution behaviors of Si and Al were investigated. The effects of the NaOH concentration, curing temperature and liquid-to-solid ratio (L/S) on the dissolution behavior were investigated. The phase and structure changes of CFBFA before and after dissolution were revealed by X-ray diffraction and nuclear magnetic resonance spectroscopy. The obtained results revealed that the dissolution of Si was slower in the static state than that under the continuous agitation, whereas the dissolution of Al under these two conditions was similar. The dissolution activity of amorphous Si in CFBFA in the NaOH solution was significantly higher than that of amorphous Al. Increasing the NaOH concentration promoted the dissolution of Si more than that of Al. The dissolved Si and Al combined easily to form some precipitates when the temperature was above 60 °C or the L/S was 10:1 mL/g. The formation of precipitates was not conducive to the further dissolution of Si and Al. The rate-determining step for the dissolution of amorphous Si and Al in the static state was the diffusion of OH– ions and the reaction products. The order of the dissolution activity of amorphous aluminosilicates in the NaOH solution was Q4(0Al) > Q4(1Al) > Q4(2Al). These results provide reference information for the evaluation of active Si and Al amounts in CFBFA during geopolymer synthesis.