A prediction study of hydroxyapatite entrapment ability in concrete

The entrapment ability of phosphates containing OH −, such as hydroxyapatite (HAP) (Ca 5(PO 4) 3(OH)), was investigated as a means for chloride removal from a concrete pore network solution. The chloride uptake by HAP was found to be very low, of the order of 32–44 ppm after 28 days immersion in a 0.5 M sodium chloride (NaCl) solution at pH 9. A thermodynamic study was performed on the chemical equilibrium of HAP, whitlockite (Ca 3(PO 4) 2), and hilgenstockite (Ca 4O(PO 4) 2), indicating that the contribution of PO 4 3 - ion activity to the concrete is very low to justify the crystallization of chlorapatite (Ca 5(PO 4) 3Cl). In contrast, the precipitation of a solid phase such as FePO 4·2H 2O to act as a physical barrier inhibiting steel corrosion is thermodynamically favoured, which means that HAP and other phosphates may be used as a corrosion inhibitors.