Durability of the hydrated limestone-silica fume Portland cement mortars under sulphate attack

The effect of silica fume, emerging as a by-product in production of ferros ilicon, on corrosion resistance to sulphate attack of Na2SO4 and MgSO4 solu tions has been studied in Portland cement mortars containing limestone and mortars containing no limestone. Expansion and changes in the elasticity mo dulus of mortars as a function of silica fume content have been investigate d. The phases formed and the microstructural changes in the mortar exposed to sulphate corrosion have been determined by differential thermal analysis (DTA), X-ray diffraction, scanning electron microscopy, and transmission e lectron microscopy. The presence of monocarbonate and the absence of monosu lphate were detected in the mortars containing limestone. The addition of s ilica fume results in less CH formed by the hydration process and consequen tly less gypsum and ettringite during the sulphate immersion of mortars. Mo rtars containing more than 5 mass % of SiO2, or simultaneously limestone an d more than 2 mass% of SiO2, are characterised by a good sulphate resistanc e and show lower expansion than the control, the sulphate-resisting mortars independent of the type of the sulphate solution. (C) 1999 Elsevier Scienc e Ltd. All rights reserved.