This paper presents an experimental investigation on the sulfate resistance of blended cements containing various amounts of natural pozzolan and/or Class-F fly ash. The performance of blended cements was monitored by exposing the prepared mortar specimens to a 5% Na2SO4 Solution for 78 weeks. For comparison, an ordinary Portland cement (produced with the same clinker as blended cements) and it sulfate resistant Portland cement (produced from a different clinker) were also used. In addition to the cement chemistry, water-cement (w/c) ratio of mortars was another parameter selected that will presumably affect the performance of mortars. The experimental results of expansion measurements showed that the effect of w/c ratio was more pronounced for the low sulfate resistant cements with higher C(3)A amounts, while the blended cements were less affected by an increase in the w/c ratio. (C) 2006 Elsevier Ltd. All rights reserved.