In the Paste Screening Study, 25 combinations of five Type I/II portland cements and five Class C fly ashes commonly used in Missouri were tested in paste form with no chemical or powder additives. Testing procedures included semi-adiabatic calorimetry, Vicat setting time, miniature slump, and compressive strength at one and 28 days. the two most reactive and two least reactive combinations (defined by one day strengths) were further evaluated in the Paste Main Effects Study. Eighty mixtures were examined.