THE EFFECT OF SILICA FUME ADDITIONS ON THE DURABILITY OF PORTLAND CEMENT MORTARS EXPOSED TO MAGNESIUM SULFATE ATTACK

摘要

The deterioration of concrete structures due to the presence of sulfate in soils, groundwater and marine environments is a well-known phenomenon. The use of blended cements incorporating materials such as natural pozzolans, fly ash or silica fume have an important role in the long-term durability of concrete exposed to sulfate attack. In this work, the partial replacement (mass fractions 2-15 percent) of commercially blended Portland cement (Type CEM IIA-S 42.5N) with silica fume (SF) was investigated in terms of the resistance of SF mortars to a magnesium sulfate (MgSO_4) solution (omega(SO_4)~(2-) = 2.5 percent; exposure time up to 6 months at 20 deg C). The changes in the length of the mortars due to expansion, as well as the changes in the elasticity modulus and mechanical strength loss as a function of silica fume replacement were determined. Structural changes in the mortars due to magnesium sulfate attack were also evaluated by differential thermal analysis (DTA) and by X-ray diffraction (XRD) analysis. The results show that the replacement of SF in most cases has a positive effect on the magnesium sulfate resistance. Mortars containing more than 8 percent of mass fraction of SF replacement are characterized by a good sulfate resistance and show lower expansion than the control mortar on the basis of the sulfate-resisting Portland cement, due to the absence of the gypsum and ettringite, detected by the XRD analysis. It is also shown that the elastic modulus is proportional to the compressive strength, but higher compressive strength does not necessary correlate with a better resistance to sulfate attack.