Strengthening the bulk properties of cement mortar through promoted in-situ formation of hydroxyapatite: Feasibility and perspectives

Hydroxyapatite (HAP) formation could be a convincing surface treatment for marble/limestone and cement-based materials, leading to augmented properties, while the effects on the bulk material properties have not been verified. In this study, in-situ HAP formation was promoted by completely immersing cement mortar specimens in aqueous diammonium hydrogen phosphate (DAP) solutions with different molar concentrations (0.10 M to 2 M) and treatment reaction time (1–28 days). The assessment properties included compressive strength, ultrasonic pulse velocity, voids, and water penetration. The outcomes demonstrated that compared with the untreated (UT) specimens, compressive strength augmented significantly (6–45 % higher than UT) for all specimens treated with DAP solution in all molar concentrations and reaction times, which aligned with registered higher ultrasonic pulse velocity (UPV) values (2–13 % higher than UT). In contrast, dramatic reduction in voids (1–16 % lower than UT), water absorption (7–25 % lower than UT), and water penetration were reported for treated specimens than UT ones. Comprehensive analyses including TGA, FTIR, XRD tests and thermodynamic modeling revealed that formation of HAP was the main mechanism, despite the limited amount detected in the studied specimens. DAP penetration depth and slow reaction kinetics of HAP formation were considered the major bottlenecks for more robust bulk property improvement.