Effect of calcium sulphoaluminate cement on the early strength of one-part fly ash/slag-based geopolymers

Insufficient early strength remains a critical limitation for one-part fly ash/slag-based geopolymers, restricting their practical application in rapid construction scenarios. This study proposes a novel strategy by incorporating calcium sulphoaluminate (CSA) cement as a partial replacement for granulated blast furnace slag (GBFS), aiming to enhance early strength performance. The effects of CSA on workability, setting time and compressive strength were systematically evaluated, supported by XRD, FTIR, MIP and SEM analyses. Results show that CSA reduces flowability and significantly accelerates setting (down to 22 min at 100% replacement). While low CSA content impairs early and later strength, higher replacement levels (≥80%) markedly enhance 1-day strength (up to 8.6 MPa) and densify the matrix by promoting ettringite formation and refining pore structure (total porosity reduced to 28.95%). Meanwhile, CSA inhibits C-(A)-S-H gel formation by lowering alkalinity. Overall, the findings reveal a dual mechanism where ettringite-driven densification compensates for reduced geopolymer gel formation, offering a viable strategy to improve early strength in sustainable geopolymers. The optimal CSA replacement range is identified as 80–100%.