For every ton of portland cement that is manufactured, approximately half a ton of carbon dioxide is released from calcining limestone. One method of reducing the carbon dioxide from portland cement production is to reduce or eliminate the use of limestone through replacement with calcium oxide-bearing waste materials. In this study, portland cement clinker was synthesized using minimal limestone content and maximal waste material content, specifically fly ash and blast furnace slag. The synthetic cements were characterized using X-ray diffraction, scanning electron microscopy, and isothermal calorimetry. Results show that portland cement clinker can be successfully synthesized from a maximam of 27.5% fly ash and 35% slag. The synthetic cements possessed early-age hydration behavior similar to a commercial Type I/II portland cement. However, the presence of sulfur impurities contained in waste materials significantly affected phase formation in portland cement clinker.