Sulfate Resistance of Clay-Portland Cement and Clay High-Calcium Fly Ash Geopolymer

This paper examines the short-term resistance ability of two different cementitious systems prepared using silty clay as a major component against 5 wt% sodium sulfate and 5 wt% magnesium sulfate solutions. The two cementitious systems are clay–portland cement and clay-high calcium fly ash (FA) geopolymer. The 28-day strength of the clay-FA geopolymer is up to 1.2 times higher than that of clay-cement. The results show that the physical performance of clay-FA geopolymer when exposed to sulfate solution is better than that of clay-cement. While the geopolymer phase is prominent in the clay-FA geopolymer system, gypsum, and ettringite phases are present in both systems especially in the clay-cement system. The strength reduction in clay-cement with duration of sulfate exposure is caused by the formation of ettringite and crystallization of gypsum (observed by scanning electron microscope and X-ray diffraction) as well as the decalcification of CSH phases. There is no major change in the microstructure and pH of clay-FA geopolymer when exposed to sulfate solutions, and hence the strength reduction is insignificant. The exposure to magnesium sulfate solution causes more degradation in both clay-FA geopolymer and clay-cement system than the exposure to sodium sulfate because the CSH phases react with magnesium sulfate solution to form ettringite. The disappearance of CSH phases in both systems is observed by XRD analysis. Overall, clay-FA geopolymer show better resistance to sulfate attack than clay-cement. The clay-FA geopolymer can be considered as a green construction and building material in sulfate-rich areas.