Often, when proportioning “green” concrete mixtures, the use of Supplementary Cementitious Materials (SCMs) is deemed to be favorable. While appearing more in the literature, it is still not commonplace that design strength is considered in assessments of environmental impacts. When mechanical or material properties are incorporated into environmental impact assessments, comparisons are drawn based on the property attainable for any given mixture, most commonly, compressive strength. However, in structural applications, compressive strength is typically specified in the design and there are currently no means for specifying the best concrete constituents for a set compressive strength to reduce greenhouse gas emissions from production. In this research, concrete mixture proportioning based on water-to-binder content and supplementary material-to-Ordinary Portland Cement ratios are examined. For these parameters, mathematical models are developed to perform optimization of GHG emissions for four groups of concrete (those containing varying levels of fly ash, ground granulated blast furnace slag, natural pozzolans, and limestone filler as a means to reduce Ordinary Portland Cement content). For the particular binary blended binders examined in this work, optimal ratios of supplementary material use were highly dependent on the type of alternative material and were consistently below the highest replacement level considered. The equations developed will facilitate the green engineering of concrete when a specified strength is required.