The influence of sodium and magnesium sulphate on the penetration of chlorides in mortar

Abstract

Marine environments are very aggressive to concrete, mainly due to the presence of chlorides and sulphates. The influence of sulphates on chloride penetration in mortars was investigated by immersion in combined test solutions containing 165 g/l NaCl and 33.8 g/l SO42− (as MgSO4 or Na2SO4) at temperatures of 5, 20 and 35 °C. After immersion periods ranging from 7 weeks up to 37 weeks, chloride profiles were measured by means of potentiometric titrations, XRD analysis and electron probe micro analysis. In general, chloride ions penetrate much deeper into the mortar than sulphate ions. Nevertheless, chloride penetration is clearly influenced by the presence of sulphates in the environment. Sulphate ions compete with chloride ions to bind to aluminate phases. Therefore, the presence of sulphates initially increases chloride diffusion. When magnesium sulphate is present the formation of Mg-related reaction products such as brucite additionally influences the chloride penetration. Later, up to 37 weeks of immersion, a decreasing chloride diffusion is noticed compared to samples exposed to a single chloride solution, due to pore blocking products of the sulphate reaction. Contrarily, immersion periods longer than 37 weeks in combined solutions result in increasing chloride diffusion due to sulphate induced cracking at the outermost layers. Notwithstanding the reciprocal influence of chlorides and sulphates, the magnitude of the effect of sulphate on the chloride diffusion coefficient was limited. Chloride diffusion generally increases with increasing temperature. The presence of sulphates decreases chloride binding even more significantly at 5 and 35 °C than at 20 °C.