Phosphate treatment has attracted particular interest in rehabilitation and service-life extension of aging concrete structures. However, the effects and underlying mechanisms of important phosphate precursors, e.g., diammonium hydrogen phosphate (DAP) and dipotassium hydrogen phosphate (DPP), on cementitious systems remain poorly understood. This study conducts a fundamental investigation into these effects on hardened Portland cement pastes across a range of water-to-cement (w/c) ratios (0.3–0.7) treated in 0.1 M and 1.5 M solutions. The resulting changes in compressive strength, water absorption were determined, and the underlying microstructural evolution was systematically evaluated using TGA, FTIR, XRD, SEM, and XCT. Compared to DPP treatment, this study found that DAP treatment consistently resulted in greater compressive strength gains and water absorption reduction, particularly in denser pastes (lower w/c). Microstructural analysis suggests that DAP treatment favors the formation of more stable, nanocrystalline hydroxyapatite (HAP), which appears to mitigate excessive calcium leaching from the cement matrix. Conversely, DPP treatment seems to yield more amorphous calcium phosphate phases and may induce more significant decalcification of calcium silicate hydrates (C-S-H). The initial w/c ratio of the hardened pastes influenced the treatment outcome considerably, with low w/c pastes showing the highest absolute strength gains and high w/c pastes exhibiting the largest relative reduction in water absorption. These findings elucidated unexplored distinct chemical pathways of different phosphate precursors and highlighted the complex mechanisms, providing a critical basis for future development of these treatments. • Phosphate treatments could enhance cementitious materials via mineral precipitation. • DAP treatment promoted the formation of stable, nanocrystalline hydroxyapatite. • DPP treatment produced more soluble, amorphous calcium phosphate phases. • The treatment effect hinged on a delicate balance between two competitive mechanisms.
Wei et al. (Thu,) studied this question.