This paper presents an investigation into the sodium sulfate resistance of three portland cements of various compound compositions and 24 blended cements produced by replacing 10%, 20%, 30% and 40% of these cements with a fly ash and a natural pozzolan. Sulfate expansion of mortar specimens was evaluated using ASTM C1012 test procedure. No acid titration was made to change pH of the sulfate solution. In addition to 5% sodium sulfate concentration prescribed in the standard, 18 selected cements were also exposed to sodium sulfate of 3% concentration. A mathematical expression was developed to estimate the sulfate susceptibility of cements regarding their C(3)A, C3S/C2S ratio, mineral additive content as well as concentration of sulfate solution. (c) 2005 Elsevier Ltd. All rights reserved.