Experimental Study of Dipotassium Hydrogen Phosphate Influencing Properties of Magnesium Phosphate Cement

A magnesia-phosphate cement (MPC) paste based on magnesia and potassium dihydrogen phosphate (${\mathrm{KH}}_2{\mathrm{PO}}_4$ or PDP) was prepared with the additions of dipotassium hydrogen phosphate (${\mathrm{K}}_2{\mathrm{HPO}}_4$ or DHP) and borax as set-retarders. The resulting materials were characterized in terms of their setting characteristics, strength properties, hydration products, and microstructures. The replacement of PDP by DHP was found to increase the setting time and reduce the intensity of the exothermic reactions during the initial setting and hardening stages. When the replacement percentage did not reach 50%, substitution of dipotassium hydrogen phosphate for potassium dihydrogen phosphate in the MPC paste could strike a good balance between the speedy strength gains and delayed setting time. The setting time of the MPC system increased from 5 min (without DHP addition) to 180 min with PDP replaced by DHP completely. The addition of borax also led to an extension of the initial hardening period, as indicated by reductions in the early values of compressive and flexural strengths. The phase of hydration product analysis indicated that no new phase appeared with the addition of DHP, only that more unreacted magnesia appeared with increasing DHP contents. The microstructural characteristics of the materials produced were strongly dependent on the level of DHP and borax employed.