Experimental study on preparation of fly ash-based geopolymer blended with recycled calcium source

A large amount of waste eggshells raises environmental issues. This study uses eggshells as recycled calcium source to synthesize fly ash and eggshell geopolymer (FAEG). The effects of eggshell content, liquid-solid ratio, NaOH/Na2SiO3 ratio, and NaOH solution concentration on the strength of the FAEG were evaluated by unconfined compressive strength (UCS) test. The geopolymerization products and microstructure characteristics of the FAEG were analyzed by XRD, FTIR, TGA, and SEM test. The results show that eggshells acted as recycled calcium sources are capable of significantly enhancing the UCS of the FAEG. The optimal content of eggshells is 15%. The NaOH/Na2SiO3 ratio has a significant effect on the UCS of the FAEG, while the influence of liquid-solid ratio and NaOH concentration is slight. The optimal NaOH/Na2SiO3 ratio is 50:50 and the liquid-solid ratio is recommended to be 0.55. Eggshells take part in the geopolymerization by acting as a precipitating and seeding element in the reaction and favoring the generation of the geopolymeric gels in the FAEG.