Prevention of sulfate-induced thaumasite attack: thermodynamic modeling in BaCO3-blended cement

Abstract

Thaumasite, an expansive salt, remains stable over a wide range of compositions in the CaO-SiO₂-Al₂O₃-CaCO₃-CaSO₄-H₂O system. Despite its slow formation, it constitutes a risk for the integrity of underground structures such as foundations and tunnels that are in contact with sulfate-containing soil or groundwater. Sulfate-resistant Portland cements, which pursuant to the existing legislation are manufactured with clinker containing 0-5% of C₃A, prevent ettringite- but not thaumasite-mediated concrete deterioration. The present study used thermodynamic modeling to explore the viability of a new type of BaCO₃-blended Portland cement able to resist thaumasite formation. The results of sulfate attack (44 wt% Na₂SO₄ solution), simulated with the GEMS geochemical code in cements with 5 or 20% BaCO₃, or 2.5, 5, 10 or 20% CaCO₃ at 8 ºC, showed that less thaumasite precipitated and at higher sulfate/cement ratios in the presence than in the absence of Ba. Particularly at the higher replacement ratio, Ba proved to be able to immobilise sulfates in the medium via the precipitation of BaSO₄, a highly insoluble salt, and hamper the precipitation of thaumasite. The study also showed that a higher BaCO₃ content in the system hindered thaumasite formation even in the presence of greater amounts of carbonates. At 5% BaCO₃, thaumasite started to precipitate after 53 g of Na₂SO₄ were added per 100 g of cement, while at 20%, the sulfate content threshold was higher, at 70 g per 100 g of cement, and smaller quantities of the salt formed.