Performance and Mechanism of Polycarboxylate Superplasticizer in Red Mud Blended Cementitious Materials

The utilization of red mud by blending it into cement paste is still facing poor workability issues due to the finer particle size and higher water absorption of red mud, which can be solved by the addition of polycarboxylate superplasticizer (PCE) to effectively maintain the working performance. However, the specific mechanisms by which different topologies of PCEs, in terms of water-reducing (WR)- and slump-retaining (SR)-type PCEs, influence red mud blended cement paste require further clarification. This research investigates the effect of WR-PCE and SR-PCE on the rheological properties, mechanical properties, and microscopic morphology of red mud blended cement paste under different red mud contents. The results demonstrated that at saturated dosages of 0.5% WR-PCE and 0.75% SR-PCE, both types of PCEs improved paste fluidity and reduced plastic viscosity and shear stress. Moreover, the time-dependent fluidity loss rate of the SR-PCE-incorporated paste was lower to that of the WR-PCE-incorporated paste at 30 and 60 min. With 0% and 25% red mud contents, the compressive strengths at 1, 3, 7, and 28 days were higher for WR-PCE than for SR-PCE due to the enhanced hydration of C₂S and C₃S. Furthermore, hydration products in the WR-PCE-incorporated paste were more uniformly distributed compared to the SR-PCE-incorporated paste. However, a 50% red mud content negatively impacted paste strength, likely due to the high alkalinity destabilizing the PCE. This study aims to elucidate the mechanistic relationship between PCE topology and the improved performance of red mud blended cement paste.