In this work, Portland cement (OPC) is replaced by different ratios of aBlast Furnace Slag, Silica Fume(SF) and Cement Kiln dust (CKD). Compressive strength, combined water and phase composition of cement are measured after 1,3,7,28,90,180,270 and 360 days immersion in tape water.The effect of5%magnesium sulfate solutionon curingofthe hardenedblendedcement pastes was also examined.             Such replacement modified the strength, pore structure, and permeability of the hardened cement pastes. Sulphateattack on cement paste causes a serious deterioration. The aim of this investigation is to study the physico-chemical and mechanical properties of the prepared hardened blended cement pastes as well as study the effect of magnesium sulphate attack on these pastes. In addition, the effect of replacement of OPC by some additive was examined.