Influence of moisture- and CO2-induced ageing in cement on the performance of admixtures used in construction chemistry

A commercial Portland cement was exposed to atmospheric humidity under controlled conditions as part of a project supported by the Nanocem Research Association to investigate the influence of moisture- and CO2-induced ageing in cement on the performance of admixtures used in construction chemistry. Pre-hydration of the cement was carried out at 35 ± 2C and 90 ± 3% relative humidity (r.h.) in a climatic chamber for 1 or 3 days. The cement was spread over a Plexiglas plate in a I mm thick layer. The pre-hydrated cement samples were analyzed by X-ray diffraction, infrared spectroscopy and thermogravimetry. All the cement pastes were mixed with a water/cement ratio of 0.55, which gives a flow table spread for the fresh cement of 18 cm. Apart from methyl cellulose, which was mixed into the cement in a dry state, the additives were dissolved in the mixing water. It was found that in cement that has been exposed to moisture the action of superplasticizers in pre-hydrated cement is generally less than in fresh cement.