Hydration of calcium phosphates under acidic conditions results in brushite (CaHPO4*2H2O) formation. One example for a brushite forming cement is the addition of H3PO4 to β-tricalcium phosphate (β-TCP), which reacts according to (1).
β-Ca3(PO4)2 + H3PO4 + 6 H2O → 3 CaHPO4*2H2O (1)
As this reaction proceeds very rapidly, retardation is necessary to obtain clinically acceptable setting times. Phytic acid (IP6), C6H6(OPO3H2)6, is a possible additive to achieve this. IP6 is additionally expected to improve the rheological performance of the cement pastes and the mechanical properties of the hardened cements.
In order to evaluate the general feasibility of creating brushite cement formulations modified with IP6, the hydration of such cements was systematically investigated by isothermal calorimetry and in-situ XRD. Crystalline β-TCP was mixed with H3PO4 solution of a proper concentration to achieve a molar ratio of β-TCP:H3PO4 = 1:1. The water to cement ratio was 0.5. IP6 concentrations of 5, 10, 12.5, 15 and 20 wt.% related to the β-TCP content were selected. Measurements were performed at 23 °C.
Calorimetry measurements proved a generally increasing retardation effect of IP6 with increasing concentration. By increasing the IP6 concentration from 10 to 12.5 wt.%, the sharp calorimetry peak indicating a rapid reaction disappears in favour of a broad heat flow event, indicating a slower and continuous reaction. In all cases, β-TCP reacted completely or at least nearly completely. While hydration of samples with 5 wt.% IP6 addition resulted in only brushite, higher IP6 concentrations led to mixtures of monetite and brushite.
Consequently, the studies proved that IP6 effectively retards β-TCP hydration and affects composition of the hardened cement, but does not impede hydration. Hence creation of IP6 modified brushite cement formulations is generally feasible.