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(4)O(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))(3)Cl). In contrast, the precipitation of a solid phase such as FePO(4)center dot 2H(2)O 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. (C) 2010 Elsevier Ltd. All rights reserved.