Tri-Magnesium Phosphate as a Candidate Biocompatible Retarder for Magnesium Potassium Phosphate Cement: Setting Behavior, Mechanical Properties, and Microstructure

Magnesium potassium phosphate cement (MKPC) is a promising bone repair material but suffers from excessively rapid setting time (typically within minutes) that limits clinical application. This study systematically investigates trimagnesium phosphate (TMP) as a candidate retarding additive for MKPC. TMP was used to partially replace dead-burned magnesium oxide at replacement levels of 0%, 10%, and 15% by mass. The effects of TMP content, water-to-cement ratio (0.17–0.23), and magnesium-to-phosphate molar ratio (4–10) on setting time, fluidity, hydration kinetics, compressive strength, and microstructure were comprehensively evaluated. Results show that TMP effectively extends the setting time from 9–13 min (without TMP) to 10–19 min, providing a working window that may be suitable for biomedical applications requiring extended handling time. Notably, 10% TMP incorporation enhances early compressive strength, with 1-day strength reaching 35.2 MPa compared to 28.5 MPa for control samples. Hydration heat analysis reveals TMP moderates the acid-base reaction kinetics through its slower dissolution rate compared to MgO. Microstructural characterization shows TMP promotes the formation of denser K-struvite crystals with refined microstructure. The optimal TMP dosage of 10% achieves a balanced performance, extending setting time while improving early strength and microstructural densification. These findings establish TMP as an effective retarder for developing MKPC-based materials with potential for biomedical applications, pending further biological validation.