Study on the effect of calcium and sulfur content on the properties of fly ash based geopolymer

Fly ash based geopolymer is a green cementitious material with the potential to replace portland cement. Due to the changes of raw coal and coal combustion technology in recent years, the fly ash often has high CaO and SO3 contents. The action mechanism of CaO and SO3 on the mechanical properties and dimensional stability of fly ash-based geopolymers is not clear, which restricts its application. The influence of CaO and SO3 content on the flexural strength, compressive strength, water immersion expansion rate and drying shrinkage rate of fly ash based geopolymer were studied, and X-ray diffractography (XRD), scanning electron microscope (SEM), energy dispersive spectrometer (EDS) and 29Si nuclear magnetic resonance (NMR) were used to study the influence mechanism of calcium and sulfur on the geopolymer. The results show that the mechanical properties of fly ash based geopolymers increase initially and then decrease with the increase of CaO and SO3 content. When the contents of CaO and SO3 are 11% and 4% respectively, the properties of geopolymer prepared at ordinary temperature (20 celcius) are the optimum. The 28d flexural strength and compressive strength can be increased by 17.91% and 15.56% respectively, and compared with the unmodified fly ash based geopolymer, the drying shrinkage is reduced by 60.65%. Microscopic test results show that the addition of CaO and SO3 promoted the geopolymer reaction, increased the gel products and generated ettringite, which enhanced the strength of the matrix and compensated the shrinkage of geopolymer.