A computer simulator predicts the performance of neat Portland cement systems, or Portland cement systems containing supplementary cementing materials (SCM). Parameters to be entered into the simulator include the cement blend composition, the Bogue composition of the Portland cement, the Blaine surface area of the Portland cement, the absolute density of the Portland cement, the SCM densities, the SCM chemical activity indices, the curing-temperature profile and the curing-pressure profile. The predictions may be verified by laboratory testing before using the cement system during a cementing treatment.