Enhanced Workability and Hydration Control of α-Calcium Sulfate Hemihydrate via Sodium Hypophosphite–Modified PCE

This study investigates the performance of a phosphate-modified polycarboxylate superplasticizer (P-PCE) synthesized using sodium hypophosphite as the phosphorus source in α-calcium sulfate hemihydrate (α-HH) systems. The effects of conventional PCE (C-PCE) and P-PCE on the flowability, hydration kinetics, mechanical strength, and microstructure of α-HH paste were systematically evaluated through minislump tests, setting time measurements, calorimetry, conductivity, inductively coupled plasma-optical emission spectrometry (ICP-OES), and scanning electron microscopy (SEM). Notably, compared with C-PCE, P-PCE increased the maximum spread from 265 to 280 mm, and prolonged the initial and final setting times by 26% and 16%, respectively. Our results substantiate that P-PCE demonstrates superior dispersing and retarding properties compared with C-PCE, leading to enhanced workability and improved mechanical performance of α-HH. The strength enhancement is primarily attributed to the significant reduction in the water-to-hemihydrate ratio enabled by the superplasticizer, which is further complemented by microstructural densification arising from its higher anionic charge density and calcium ion complexation facilitated by its phosphonate and phosphinate groups. These findings provide valuable insights into the design of advanced superplasticizers for sustainable gypsum-based materials.