Formation of chloroaluminates in calcium aluminate cements cured at high temperatures and exposed to chloride solutions

Abstract

The formation of hexagonal chloroaluminates in mortar specimens pre-cured at 20, 40 and 60°C for two weeks and stored in a 0.5 M NaCl solution for up to 255 days has been studied. The appearance of this phase as a function of time has been monitored by X-ray diffraction. In addition, its microstructure has been observed by means of backscattering electron microscopy. The chemical composition was studied by X-ray microanalysis. The formation of chloroaluminate phases in reinforced concrete is related to the immobilization of chloride ions penetrating through the concrete to the reinforcement. Thus the formation of stable chloroaluminates could lower the risk of corrosion. In order to check this point, corrosion rate measurements were performed throughout the experiment. In spite of the high capacity of aluminous hydrates to react with chloride ions to form chloroaluminates, the remaining chloride ions in the pore solution leads over time to reinforcement corrosion. The presence of hexagonal phases in the cement paste ensure a better resistance against the penetration of chloride ions than when cubic phases are present. This effect was attributed to the denser microstructure exhibited, by samples containing the hexagonal phases.