We study the hydration and pozzolanic reactions of cement pastes made from Argentinian and Czech Portland cements as a function of age, using thermogravimetry. The measurements are done for 2, 7, 28, and 90 days cured samples in order to monitor the rate of hydration. The investigation is performed in the temperature range from 25 °C to 1000 °C with a heating rate 5 °C/min in an argon atmosphere. The mass change during the decomposition of calcium silicate hydrate gels, portlandite, and calcite are determined, and the changes in the portlandite amount are estimated in dependence on the time of hydration.