Portland cement compositions for use in high-temperature, high pressure wells are designed such that the lime-to-silica molar ratio is between 0.5 and 1.0, and the alumina-to-silica molar ratio is between 0.05 and 0.10. After curing and setting at temperatures between 85° C. and 300° C., the cement compositions form tobermorite as an initial and permanent calcium silicate hydrate phase.