Concrete exposed to groundwater, soil and seawater is often subject to sulphate attack. Partial replacement of Portland cement by fly ashes from coal combustion process has been proven to be effective in reducing sulphate attack. This paper shows the results of studies of sulphate resistance of mixtures incorporating fly ashes from co-burning bituminous coal and biomass. The sulphate resistance of mortars was examined by determination of changes of linear expansion and compressive strength of specimens immersed in Na_2SO_4 solution. The results show that blends of Portland cement PN-EN 197-1 Type I incorporating fly ashes from co-burning bituminous coal and biomass decreased sulphate resistance of the mortars as compared with those made with Portland cement incorporating fly ashes from coal combustion process. The studies of microstructure of cement pastes curing in sulphate solution were carried out by scanning electron microscope equipped with energy dispersive spectrometer (EDS).
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