Effect of phosphate additions on the hydration of Portland cement

New cementitious compositions have been sought by reactions involving ordinary Portland cement and sodium and calcium phosphates. The phosphate-modified cements produce more heat and exhibit much faster hydration rates than does OPC. The phosphate-containing products formed at room temperature are X-ray amorphous, but scanning electron microscopy shows that the hydration of OPC modified by different phosphates can result in morphologically distinct hydration products. Hydrothermal treatment at 160°C of OPC modified by CaHPO4 leads to phase transformations of a poorly crystalline phosphate containing product phase into crystalline hydroxyapatite. Generally, the presence of sodium and calcium phosphates resulted in improved flexural strengths; an exception was when (NaPO₃)_n·Na₂O was used. The effects of different factors on the mechanical properties in the OPC-phosphate system are discussed. These factors include the effects of different sodium and calcium phosphates, processing by warm pressing, w/c ratio, and curing conditions.