A prediction study of hydroxyapatite entrapment ability in concrete

摘要

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.