Sustainable clinker-free composites: Effect of mix proportion parameters on hydration and microstructure of steel slag-GBFS-phosphogypsum binder

Abstract

Steel slag (SS)-GBFS-phosphogypsum (PG) composites is a potential sustainable clinker-free binder. Three important mix proportion parameters were considered: the SS to GBFS ratio (S/G), PG content, and water to binder ratio (W/B). This study systematically explored effects of each parameter on the hydration and microstructure characteristics of the composites. It also evaluated their economic and environmental benefits. Results show that increasing the S/G decreases the exothermic rate. This leads to the formation of a more porous structure at each age. Consequently, the strength and flowability of the composites diminish. At the same time, the setting time and water absorption increase. A decrease in the W/B causes the main exothermic peak to occur earlier. Although the exothermic rate diminishes, this adjustment leads to a denser pore structure at each age. As a result, water absorption and flowability are reduced, the setting time shortens, and strength improves. An increase in PG content results in the main exothermic peak appearing earlier. This is accompanied by a decrease in peak value and an increase in the volume of harmful and more harmful pores at each age. Furthermore, higher PG content reduces flowability and shortens the setting time. Although compressive strength increases at all ages, flexural strength decreases in the later stages. Meanwhile, the water absorption rate rises significantly. After adjusting the three parameters, the hydration products of the composites were still C-(A)-S-H gel and AFt. However, their content decreased. Notably, compared to PC, the composites exhibit marked reductions in CO₂ emissions, production costs, and energy consumption.