Mitigating natural aging in hemihydrate phospho-calcium sulfate with sodium tripolyphosphate: Sustaining long-term cementitious performance

Hemihydrate phospho-calcium sulfate (HPS) is a promising binder for mine backfill due to its excellent cementitious properties. However, its susceptibility to natural aging during storage reduces cementitious activity, weakening backfill performance. To address this issue, this study first elucidates the aging kinetics of HPS based on kinetic fitting analysis, demonstrating that the process follows a diffusion-controlled first-order kinetic mechanism, which promotes the formation of CaSO4·2 H2O and results in a decline in cementitious activity. A strong negative correlation between crystallization water content (CWC) and strength is established, confirming CWC as a reliable macroscopic indicator of cementitious activity. Building upon this foundation, the study systematically investigates the retarding mechanism of sodium tripolyphosphate (STPP) using a multiscale approach that integrates macroscopic performance assessment, microstructural characterization, and molecular-level analysis. Macroscopic results show that STPP effectively delays HPS aging at dosages exceeding 0.3 %, with the spray method improving dispersion uniformity. Optimal retardation is observed under acidic conditions and temperatures below 45°C, whereas excessive solution resting time (>8 h) reduces STPP's efficiency due to possible hydrolysis and structural changes. Microstructural and molecular-level analyses indicate that the retardation mechanism of STPP primarily stems from its strong chemical adsorption onto HPS surfaces. The P–O functional groups in STPP coordinate with Ca2⁺ ions, inhibiting nucleation and subsequent aging reactions. This coordination interaction slows the transformation of HPS into CaSO4·2 H2O, thereby mitigating the decline in cementitious activity. These findings provide a comprehensive multiscale understanding of STPP's retarding mechanism and offer valuable insights into controlling the aging process of HPS, which is crucial for its effective utilization in mine backfill applications.