This paper reports an experimental study on the sodium sulfate resistance of three Portland cements with various cement compositions ($C_3A$ or silicate ratio) and blended cements incorporating 50% Ground Granulated Blastfurnace Slag (GGBS) or 20% Pulverized Fuel Ash (PFA). In order to evaluate effect of solution concentrations on the deterioration of mortar specimens, the test solutions used were 0.5%, 1.5% and 5% as sodium sulfate. The resistance of mortar specimens was evaluated using visual examination, expansion and compressive strength loss measurements. Additionally, in order to classify sulfate products formed during sulfate attack, microstructural analysis such as X-Ray Diffraction (XRD) and Differential Scanning Calorimetry (DSC) were also conducted on paste samples exposed to highly concentrated sulfate solution. Results confirmed that in additions to $C_3A$ content, Silicate Ratio (SR) of Portland cements is a critical parameter influencing sulfate resistance. Besides, the use of mineral admixtures such as GGBS and PFA led to the being beneficial in reducing the susceptibility to sodium sulfate attack. All the tests reported in the study were carried out at room temperature.