Influence of ageing of binder systems on the performance of additives

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Chemical aging of the cement takes place during its exposure to atmospheric air. Generally, cement is very hygroscopic and promptly reacts with water and CO2. The aging can take place already during the manufacture and storage of cement. Such aging affects its physical, chemical and mechanical properties such as hydration, stiffening and development of compressive strength. Chemical admixtures such as superplasticizers, water retention agents and accelerators highly depend on the adsorption or composition of the pore solution of the cement to become effective. In this project effectiveness of superplasticizers with aged cement and cement clinker phases was investigated. For this study, methacrylic acid ester and allyl ether-based polycarboxylates (PCEs) were used. Additionally, the influence of aging on the effect of a retarder (sodium gluconate) was examined. It was found that the change of surface charge, resulting from aging, and subsequent agglomeration of the binder particles are the decisive factors for the different behavior of aged cement. These effects are caused by the formation of nanoscale hydrate phases (aging products) which strongly affect adsorption of superplasticizers. In the new project we plan to focus on mortar systems which are particularly rich in admixtures. Another goal is to achieve a deeper understanding of the interaction of admixtures and the influence of aging on the reaction equilibrium in complex binder systems. The results should allow to improve the robustness of mortars under various storage conditions.