Early-Age Behaviour of Calcium Sulfoaluminate Cement Blended with Fly Ash-Containing Portland Cement: Hydration, Workability and Strength Development

Calcium sulfoaluminate cement (CSA) offers potential performance and environmental benefits to cementitious mixes. Regardless, the joint behavior of CSA cement together with fly ash blended portland cement (PC) remains insufficiently understood. Therefore, this document presents an experimental investigation of the effect of simultaneous variations of CSA as PC replacement and water to binder (w/b) ratios on the hydration, fresh state, and early-age mechanical properties of paste and mortar mixes. Laboratory tests were based on statistical experimental design with CSA percentages varying from 25 to 75% and w/b ratio from 0.35 to 0.52. Findings indicate that increasing the CSA content significantly reduces the setting time while w/b ratio variations result in minimal effects. Hybrid mixes exhibited slump and slump retention time (workability window) reductions as CSA content increased. Compressive and flexural strength decreased with higher w/b ratio, but CSA cement content increments promoted compressive strength with time while flexural strength evolution was affected by higher CSA cement content. Among the investigated compositions, the mixture containing 25% CSA cement exhibited the most favorable balance between hydration kinetics, fresh-state behavior and strength development, indicating its potential as an effective PC–CSA mixture.