Although field cases reported the presence of chloride in situations in which concrete suffered from the thaumasite form of sulfate attack (TSA), few laboratory studies have been carried out into the impact of chloride ions on TSA. In fact the literature contains contradictory results with some studies indicating that chloride reduces TSA, while others show the opposite and, moreover, no published experimental data have been found that address the role of chloride ions on the extent of thaumasite formation, the effect of thaumasite formation on cement chloride binding capacity or chloride induced corrosion of steel reinforcement in conditions conducive to thaumasite formation. Thus, the aim of this study was to investigate these issues with respect to the performance of standard Portland Cement (PC) and Portland Limestone Cement (LF) mortars and other binders based on blends of PC with pulverised fuel ash (PFA) or ground granulated blast furnace slag (GGBS) that potentially may be more resistant to TSA. Siliceous fine aggregate and a water to binder ratio of 0.6 were used to cast specimens that, following curing, were exposed to various solutions containing sulfate and / or chlorides at 5°C and at approximately 20°C for a period of up to 630 days.
The performance of the samples was assessed by regular visual inspections and mass changes together with measurements of length, porosity and oxygen permeability. The pH and chemistry of the test solutions were monitored and the deterioration products were investigated using XRD, IRS and SEM/EDX techniques. The effect of chloride concentration on the solubility of calcite and gypsum was also investigated. Water soluble chloride was evaluated in order to measure free chloride of powdered samples taken from different depths into the specimens. Linear polarization resistance and visual observations were used to monitor the corrosion of steel reinforcement in the experiments and the degree of carbonation in selected specimens was also evaluated.
All specimens stored at 5°C, except those stored in the combined sulfate and 2.0% chloride solution, suffered from TSA, where the greatest deterioration occurred to LF mortars. The extent of thaumasite degradation was concentration sensitive, where damage was accelerated at 0.5% and mitigated at 2% in the conditions investigated, where this mitigation effect is attributed to pH increase, Friedel’s salt deposition, increased gypsum solubility and reduced calcite solubility. At low concentrations, corrosion risk increases due to the reduction of chloride binding capacity of thaumasite-affected areas of the cement.
The use of slag and fly ash as cement replacements delayed sulfate attack, probably due to the consumption of calcium hydroxide and improved pore structure. However, mortars made with these cements and exposed to DS4 (BRE Ground aggressivity Class) magnesium sulfate at low temperature suffered conventional sulfate attack. The presence of chloride in solution led to further enhancement of sulfate resistance of fly ash mortars, probably due to the positive effect of high chloride binding capacity. However, it enhanced lime leaching in GGBS mortar which would provide more calcium ions required for gypsum precipitation. At high chloride concentration (2.0%) and due to high gypsum solubility, no gypsum was formed.
The non chloride binding capacity of thaumasite means that where TSA occurred, the reinforcement was increasingly vulnerable to damage. An additional implication of poor binding capacity of thaumasite is that because C-S-H can be
transformed to thaumasite, physically adsorbed chloride on C-S-H would be released into the pore solution which would tend to increase corrosion risk.
