Among the various mechanisms affecting the durability of concrete, external sulfate attack was first identified in the early 1900s, and led to the discovery of sulfate resistant Portland cement (SRPC). Besides SRPC, another way of coping with the problem of sulfate attack is the use of supplementary cementitious materials either as an admixture to concrete or in the form of blended cements. This book presents an investigation on the sulfate resistance of blended cements containing different amounts of natural pozzolan and/or low-lime fly ash compared to ordinary Portland cement and sulfate resistant Portland cement. The effect of permeability as controlled by w/c on the sulfate resistance is also investigated. In addition, this book includes a thorough review of the literature for mechanism, types of sulfate attack (internal, external, physical, chemical), and factors affecting the sulfate attack.