Sulfate attack and the role of internal carbonate on the formation of thaumasite

The sulfate attack is known to influence the durability of concrete. In general, significant damage due to sulfate interaction results in the structural breakdown of the concrete structure. However, the precipitation of thaumasite due to sulfate concentration, leaching and the role of internal carbonate in case of limestone filler in cement is still not understood in detail. This work has adopted a combination of experimental and modeled data to investigate the formation of thaumasite under different exposure conditions. Experiments were carried out by using the progressive equilibrium approach (PEA). Further microstructural properties such as porosity, sulfate uptake, phase composition and phase transformation reactions during sulfate attack were investigated. The results show that thaumasite formation is favoured at lower temperatures (8 °C) independently of the type of cement clinker (high and low C3A content) used. Thaumasite was found to form only in cement systems which contained a source of carbonate and at high sulfate contents in the cement paste, i.e. the molar SO3/Al2O3 ratio exceeded 3. Leaching, the reduction of alkalies and portlandite, had no significant influence on the stability of thaumasite. It slightly reduced the amounts of thaumasite formed. Thaumasite formation was found not to be a form of the initial sulfate attack. The phenomenon occurs at the late (last) stage of sulfate attack. The initial sulfate induced deterioration is caused by ettringite formation. The thermodynamic approach used to investigate the chemical aspects of sulfate attack turned out to be a good tool for simulating external sulfate attack. However, in the experiments, the predicted equilibrium conditions have not been reached after 9 months. Limestone addition of a few percent in Portland cement increased the compressive strength and reduced the porosity, especially the capillary porosity. Thereby, also the resistance of Portland cement systems against sulfate attack was increased. Furthermore, the hydration of Portland cement was influenced by limestone addition and monocarbonate was formed instead of monosulfate as stable AFm phase.

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