The hydration of a commercial Portland cement blended with calcium carbonate was studied by thermal analysis. TG, DTA and DTG analyses were performed on pastes prepared with a water to cement weight ratio of 0.5, after different setting times during the first 28 days of hydration. The analyses were run from 28 degrees C to 1000 degrees C with a 10 degrees C/min heating rate and 100 ml/min of air as the purge gas. All the analyses were performed after a drying step at 28 degrees C in the equipment itself, using the purge as the drying agent, resulting in a more effective and less time-consuming method than other conventional drying procedures. The original cement contains a partially decomposed dihydrated calcium sulfate, which is regenerated after 1 h of hydration and is consumed in about 16 h of setting. The results indicate that TG and DTG curves are faster and more precise tools than DTA curves to identify and to quantify all the main hydrated and carbonated phases which are present in the pastes, especially in the case of this cement blended with calcium carbonate, the total content of which cannot be estimated correctly by DTA as hydration proceeds.