Mechanisms of phosphate treatments for cementitious materials under thermal and pressure gradients: Implications for subsurface applications

Phosphate treatments inducing in-situ hydroxyapatite (HAP) formation offer a promising consolidation strategy for durability-critical subsurface infrastructure, such as wellbore seals for carbon storage and geothermal systems. However, the impact of elevated temperature and pressure-gradient-driven transport remains poorly understood. This study investigates diammonium hydrogen phosphate (DAP) treatment of cement pastes spanning five water-to-cement (w/c) ratios under 75 °C and vacuum-imposed pressure gradients. The results reveal a critical competition between HAP precipitation and calcium leaching. In dense pastes (w/c ≤0.4), temperature and vacuum synergistically accelerate HAP formation and penetration, yielding substantial compressive strength gains. Conversely, porous pastes (w/c ≥0.6) deteriorate under identical conditions, exhibiting strength loss and increased water absorption. Microstructural analyses (XRD, SEM, TGA, FTIR) show that thermal/vacuum conditions promote specific HAP morphologies (e.g., needle-like clusters) while accelerating portlandite dissolution and C–S–H decalcification in highly porous matrices. We propose a “safe operating window” for phosphate consolidation: strong thermal and pressure gradients benefit dense systems, whereas milder conditions are required for porous substrates to avoid leaching-induced destabilization. These findings provide a mechanistic basis for optimizing DAP-based consolidation in subsurface cementitious applications.