Impact of chloride on the mineralogy of hydrated portland cement systems

Chloride ion is in part bound into ordinary Portland cement paste and modifies its mineralogy. To understand this a literature review of its impacts has been made and new experimental data were obtained. Phase pure preparations of Friedel's salt, Ca4Al2(Cl)1.95(OH)12.05·4H2O, and Kuzel's salt, Ca4Al2(Cl)(SO4)0.5(OH)12·6H2O, were synthesized and their solubilities were measured at 5, 25, 55 and 85 °C. After equilibration, solid phases were analysed by X-ray diffraction while the aqueous solutions were analysed by atomic absorption spectroscopy and ion chromatography. The solid solutions and interactions of Friedel's salt with other AFm phases were determined at 25 °C experimentally and by calculations. In hydrated cements, anion sites in AFm are potentially occupied by OH, SO4 and CO3 ions whereas Cl may be introduced under service conditions. Chloride readily displaces hydroxide, sulfate and carbonate in the AFm structures. A comprehensive picture of phase relations of AFm phases and their binding capacity for chloride is provided for pH ~ 12 and 25 °C. The role of chloride in AFt formation and its relevance to corrosion of embedded steel are discussed in terms of calculated aqueous [Cl-]/[OH-] molar ratios.