Enhancing the reactivity and strength of fly ash-calcium sulfoaluminate cement using gypsum modifier

Abstract

The CO$_2$ emissions in the production of ordinary Portland cement (OPC) calls for an eco-friendly alternative binder in concrete. Fly ash is a by-product from coal-fired power plants that is gaining interest as a supplementary cementitious material. Fly ash is being utilized by blending with OPC in blended cements, but the strength of this blended cement still relies in OPC. Another way of using fly ash as a binder is by alkali activation, but it only develops strength under special conditions that are impractical outside the laboratory. Calcium sulfoaluminate (CSA) cement is another material considered to be an eco-friendly alternative to OPC because of its lower CO$_2$ emission in production. In this study, the blending of fly ash with CSA and the addition of gypsum is investigated and compared with the blend of fly ash and OPC. The physical properties such as strength development and pore size characteristics are evaluated by compressive strength tests and mercury intrusion porosimetry (MIP). The hydration products formed are examined using different microstructure analysis techniques such as X-Ray diffraction (XRD), thermogravimetry analysis (TG), Fourier-transform infrared spectroscopy (FT-IR), and scanning electron microscopy (SEM). The results are consistent with previous studies that increasing the percentage of fly ash in blended cements leads to a decrease in compressive strength. But when gypsum is added in fly ash-CSA blends, it greatly improves the strength performance. A 40% fly ash mass percentage blended with CSA and an additional 5% gypsum-modifier showed the highest compressive strength from all the blends and exceeded the performance of fly ash-OPC blended cement with the same fly ash mass percentage.