A prediction study of hydroxyapatite entrapment ability in concrete

Abstract

The entrapment ability of phosphates containing OH-, such as hydroxyapatite (HAP) (Ca5(PO4(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 (NaCI) solution at pH 9. A thermodynamic study was performed on the chemical equilibrium of HAP, whitlockite (Ca 3(PO4J2), and hilgenstockite (Ca 4O(PO4). indicating that the contribution of PO 3 4 ion activity to the concrete is very low to justify the crystallization of chlorapatite (Ca5(PO4)3Cl). In contrast, the precipitation of a solid phase such as FePO42H 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. © 2010 Elsevier Ltd. All rights reserved.