Study of dehydration and rehydration processes of portlandite in mature and young cement pastes

The hydration process of the cements induces the formation of different kinds of hydration products. The main products of hydration are C-S-H gel and portlandite [Ca(OH)(2)]. The C-S-H gel is an amorphous compound that is discomposed progressivity with the temperature until approximately 1,000 A degrees C, while the portlandite is discomposed between 450 and 550 A degrees C. Also, calcium carbonate can be formed as a consequence of the portlandite carbonation. All of these processes can be analysed and quantified by simultaneous differential thermal analysis and thermogravimetric analysis. And by X-ray diffraction it is possible to identify the crystalline phases. Some authors have corroborated that the portlandite can be rehydrated, after dehydration processes due to thermal exposition of the cement paste. But all of these experiments have been made with young cement pastes or at temperatures lower than 650 A degrees C. In this work the behaviour of young and mature cement pastes have been studied in relation with the portlandite decomposition and the possibility of the rehydration of it in water presence. We found that young pastes and old pastes, stored at laboratory conditions, and later burned, show a certain grade of rehydration, specially the pastes burned at 650 A degrees C (with a parts per thousand 80% of reformation of portlandite) with respect to the pastes burned at 1,000 A degrees C (between 20 and 40%). It is corroborate that the rehydration process is directly related to the formation of CaO during the burning. Also, a formation of unstable portlandite is detected in young pastes burned at 650 A degrees C, which can be rehydrated easily. Although, the mature pastes that have been burned initially and stored under laboratory conditions cannot be rehydrated, due to the formation of stable products during the storage.