Influence of moisture- and CO2-induced ageing of cement on performance of chemical admixtures used in concrete and mortar

The purpose of this paper was to obtain a more in-depth understanding of the influence of ageing on the interaction of cement with chemical admixtures. The exposure conditions tested were 35°C and 90% relative humidity for 1 and 3 d, respectively. As admixtures, three superplasticizers (naphthalene sulfonic acid formaldehyde polycondensate NSF, polycarboxylate ether PCE and casein), one water-retention agent (methyl cellulose MHEC) and two accelerators (Ca formate and amorphous Al2O3) were investigated. The results show that the ageing of cement can lead to a significant change of the flow, water retention and setting behaviour patterns. As a consequence of this partial surface hydration, the specific surface area and the surface charge of cement are altered, substantially affecting the interaction with chemical admixtures. The following correlations were found: 1) The effectiveness of superplasticizers in aged cement is generally less than that in fresh cement. The dispersing power of biopolymer-based superplasticizer like casein is slightly affected by moist storage. The dispersing power of polycarboxylate sample and the NSF admixture is reduced; 2) The aged cement exhibits an improved water retention capacity with a higher water demand to achieve the workability required in practice. Consequently, the dosage of methyl cellulose to reach a high water retention in aged cement is lower than that in fresh cement; and 3) The accelerating effect of Ca formate and amorphous Al2O3 in aged cement is decreased. After ageing for 3 d, they may even retard cement hydration.