Why OPC and GGBS combination is Preferred? As per ASTM C989-09 (Sulfate resistance); the use of slag will decrease the C₃A content of the cementing materials and decrease the permeability and calcium hydroxide content of the mortar or concrete. This implies that it will be better to use a cement type that has a higher percentage of C₃A content (Type I), compared to a cement type that has a lower percentage of C₃A content (Type V). Again as per ASTM C989-09 (Sulfate resistance); the combination of slag and Portland cement, in which slag content was greater than 60 to 65%, had high sulfate resistance, always better than the Portland cement alone, irrespective of Al₂O₃ content of the slag. The improvement in sulfate resistance was greater for the cements with higher C₃A contents. This again implies that it will be better to use a cement type that has a higher percentage of C₃A content (Type I), compared to a cement type that has a lower percentage of C₃A content (Type V). As per ACI 223R-03; the dosage of admixture is based on total cementitious material, a given amount of retarder will have a greater retarding effect as the proportion of slag cement in concrete is increased. The increased retardation is particularly noticeable with Portland cements having low C₃A contents and alkali levels. This implies that Type V cement will be the most retarded compared to Type I cement when used in combination with GGBS at equal replacement levels. Thus, the use of SRC with GGBS is not prohibited; but the points above justify the use of OPC and GGBS blend rather than a SRC/MSRPC and GGBS blend in concrete.