The influence of chloride on the thaumasite form of sulfate attack in mortars containing calcium carbonate
Chlorides have been reported to be present in several field cases where thaumasite attack has occurred. However, no published systematic research dealing with the role of chloride in the thaumasite form of sulfate attack could be found in the literature. This research project has been designed through a comprehensive experimental programme to address this issue. This investigation studies the following: the formation of thaumasite in long-term exposure of carbonated systems to sulfate environment; whether or not the presence of chloride affects the thaumasite form of sulfate attack (TSA); the effect of long-term and short-term carbonation on the precipitation of thaumasite; the composition of thaumasite formed in chloride containing solutions; the chloride binding capacity of thaumasite-affected cement matrix; and the use of metakaolin to prevent TSA in carbonated mortar. Mortar samples were cast using siliceous sand and Portland cement replaced by different amounts (0,5 and 15%) of limestone filler, and by 10% metakaolin. Mortar cubes were subsequently stored in deionised water, magnesium sulfate solution, combined sulfate and sodium chloride solution and simulated seawater at 5°C and 20°C. Long-term specimens consisted of Portland cement mortars containing 15% limestone filler, which were exposed to atmospheric carbonation at 5 and 20°C for 5 years, were also immersed in these salt solutions at both temperatures. The mortar cubes were examined regularly every month, and the results of visual assessment recorded. The mineralogy of the deteriorated products was determined by x-ray diffraction (XRD), infrared spectroscopy (IRS). The pH of the solutions was also measured periodically. The composition of the thaumasite and the deteriorated cement matrix was assessed by means of the determination of the unit cell parameters of the crystal, by quantitative infrared spectroscopy (IRS); scanning electron microscopy (SEM); backscattered electron image (BEI) and chemical analyses by energy dispersive X-ray and quantitative x-ray microanalyses. The results indicate that the effect of chloride on sulfate attack is affected by temperature, because it affects the solubility of some minerals and increases the activity of carbonates. At 20°C, the presence of chlorides appears to mitigate sulfate attack by combining with the aluminates of the cement forming Friedel's salt. At this temperature, the main carbonate phases were calcite and aragonite, whereas thaumasite was predominant in all samples at 5°C, which caused damage that increased with increase in the chloride concentration in solution and the carbonate content, as evidenced by increased loss of mass and pH. Because of the demand for an alkaline environment, thaumasite did not precipitate within the long-term carbonated surface areas, but immediately underneath. The formation of thaumasite was not prevented in carbonated specimens after short-term. In the presence of chloride, thaumasite precipitated with lower lattice parameter c, indicative of a higher carbonate to sulfate ratio, and its composition shifted towards the thaumasite end-member of the solid- solution series. Thus, chloride does not seem to enter into thaumasite crystal structure. In addition, the chloride binding capacity of thaumasite-affected areas of the cement is reduced as the residual CSH phase both decalcifies and loses its silicon to give place to the deposition of thaumasite. The performance of metakaolin containing mortars in salt solutions was improved by the pozzolanic effect, but also by an increase in the chloride binding capacity, which reduced the deleterious effect of the chloride on the formation of thaumasite. However, metakaolin-containing samples in magnesium sulfate developed some signs of damage that were detected after 18 months. The interaction between chloride and other ionic species on thaumasite formation in the cement matrix is very complex, and can have some implications to the CaC03 threshold for durable concretes under TSA prone environments and also to the steel corrosion of concrete reinforcement.