Effect of polycarboxylate superplasticiser on the early-age behaviour of OPC-CSA mortar

Ordinary Portland cement–calcium sulfoaluminate (OPC-CSA) blends are gaining attention as low-carbon binders for fast-setting and high early-strength applications. However, controlling their workability and rheology is critical for modern construction processes. This study examines the effect of polycarboxylate ether-based superplasticiser (SP) on the hydration behaviour and performance of OPC-CSA mortars. In 100% OPC systems, increasing SP dosage prolonged the dormant period and significantly delayed the setting time, without affecting the intensity of the acceleration peak. A more pronounced renewed aluminate peak indicated preferential adsorption of SP on aluminate phases. In OPC-CSA blends, the rapid formation of ettringite diminished SP’s dispersing efficiency, resulting in a rapid loss of flowability. The ettringite-related heat flow peak was slightly delayed and intensified with higher SP content, leading to marginal setting delays. At 2 h, compressive strength increased to 8.95 MPa with 1 wt% SP. While early-age strength (1 and 3 days) was slightly delayed due to retarded silicate hydration, all mixes showed comparable strength at 28 days. The results demonstrate that appropriate SP dosage can enhance both fresh and hardened properties of OPC-CSA systems, despite the complex interplay of hydration reactions.