Feasibility analysis of the preparation of geopolymers from different types of coal based ash: reaction, synthesis, and properties

This study compared the feasibility of circulating fluidized bed boiler fly ash (CFBFA), pulverized coal furnace fly ash (PCFA), and coal gasification slag (CGS) for the preparation of geopolymers by investigating the Si and Al dissolution reaction of the raw materials, product structure, and the development of the mechanical properties of the geopolymers. The results indicated that the reaction mechanism for preparing geopolymers were the same for the three kinds of coal based ash, i.e., the depolymerization and repolymerization of Si-O and Al-O bonds in raw materials. The loose and porous microstructure of CFBFA resulted in a high dissolution ratio of amorphous Si and Al, and its self-hardening properties generated ettringite crystals. This resulted in a high initial strength of the CFBFA based geopolymer (CFBFAG). The active Si and Al in PCFA and CGS were released slowly during the curing process, which was conducive to an enhanced compressive strength of the geopolymer in the later stage. The compressive strengths of the CGS based geopolymer (CGSG) after curing for 90 days reached 52.1MPa, which was 139% and 11.4% higher than that of CFBFAG and PCFAG for the same curing time respectively. A microstructure analysis showed that the CGSG had a denser microstructure and more ordered geopolymer gel network than the other geoploymers. Compared with the CFBFA and PCFA, CGS was found to have a greater application potential in the preparation of geopolymers.