Preparation of high-performance silico-aluminophosphate geopolymers using fly ash and metakaolin as raw materials

Abstract Silico-aluminophosphate (SAP) geopolymers were synthesized by using metakaolin (MK) and a high content of fly ash (FA) for the first time. With the increase in the FA mass fraction in the raw materials from 50% to 100%, the setting time of the SAP geopolymers was shortened and their compressive strength was decreased. The effects of the FA content on the SAP geopolymers composition and microstructure were investigated. The results showed that a high FA proportion caused the lack of active aluminum, which led to excess phosphate acid reacting with the calcium from FA. The fast formation of amorphous calcium phosphate (ACP) shortened the initial setting time through the consumption of water. However, after long-term aging, ACP transformed into needle- and slice-like nanoparticles, which decreased the compressive strength of the resulting geopolymers. Moreover, MK supplied active aluminum to increase the water-containing -Si-O-Al-O-P- network (SAP gel). The SAP gel contributed to the compressive strength enhancement and prevented the ACP transformation; however, it prolonged the final setting time, due to the slow polycondensation of SAP gel. These results indicate that a desirable setting time and compressive strength of the SAP geopolymers can be achieved by adjusting the FA and MK contents.