The Effect of Mineral Admixtures on the Sulfate Resistance of Limestone Cement Concrete

摘要

Limestone has been widely used as filler or as main cement constituent for the last ten years, especially after the standardization of Portland-limestone cements (EN 197-1, 2000). However, concrete made from limestone cement may exhibit a lack of durability due to the formation of thaumasite. Considering the numerous technical and economical advantages of limestone cement, the “problem of thaumasite” is of great interest. In this work the effect of natural pozzolana, fly ash, ground granulated blastfurnace slag and metakaolin on the thaumasite formation in limestone cement concrete is examined. In addition, the effect of the type of sand (calcareous or siliceous) and the curing temperature is investigated. A limestone cement, containing 15% w/w limestone, was used. Concrete specimens (B=350kg/m3, W/B=0.5) were prepared by replacing a part of limestone cement with the above minerals. The specimens were immersed in a 1.8% MgSO4 solution and cured at 5oC and 25oC. The status of the samples for a storage period of 3 years was reported based on visual inspection, strength tests and X-Ray diffraction. The use of specific mineral admixtures improved the resistance of the limestone cement concrete against sulfate attack. Fly ash, ground granulated blastfurnace slag and metakaolin showed the best behavior, while natural pozzolana presented only a limited improvement of concrete sulfate resistance. Concrete specimens incorporating calcareous sand showed better resistance than specimens containing siliceous sand. Finally, no damage was observed in the specimens exposed to sulphate solution at 25oC indicating that conventional sulphate attack, at ambient temperature, is much slower than thaumasite-kind of sulphate attack, at low temperature.