Correlation between setting, heat evolution, and deformations of cementitious binder systems depending on type and amount of superplasticizer

Today polycarboxylate ether based superplasticizer (PCE) is commonly used in concrete technology when high flow properties and water reduction are specified. The ionic strength of the polymers’ backbones determines the adsorption behaviour of polymers on clinker and early hydration products. The amount of required polymers for specified flow properties and the performance over the time of casting is thus determined by the molecular structure of the superplasticizer. The time depending consumption of polycarboxylate ether polymers strongly affects the reaction of aluminates and sulphate ions as well as the hydration process in general. Hence, the choice of polymers for particular flow properties greatly affects the very early properties of cementitious materials such as setting, heat evolution and autogenous deformations. In order to better understand how PCEs influence the early properties, mixes from cement, limestone filler, viscosity modifying agent and water were varied with a high and a low charge density superplasticizer in differing amounts. Results are presented from measurements with an automatic Vicat device, an isothermal heat flow calorimeter, and shrinkage cones. Tests were conducted at 5, 20, and 30 °C. It is shown that in presence of PCE the final set correlates well with the inflexion point of the heat flow curve, which emphasises the interrelation between C-S-H formation and setting. No such clear correlation can be found for the initial set, which is attributed to the fact that the initial set is rather a rheological than a structural phenomenon, so that other effects overlap with C-S-H formation. The results demonstrate that for a given polymer concentration low charge density polymers yield earlier setting than high charge polymers. However, this influence is overridden by the influence of the total amount of polymers in a cementitious system. Since PCE is typically added according to rheological specifications, and low charge PCE typically requires higher amounts of polymers than high charge PCE for comparable flow performance, low charge PCE retards setting more than high charge PCE. The paper furthermore points out that there is no significant influence of the polymer type or amount on the early deformations. Since type and amount strongly affect the hydration, it is demonstrated that early setting causes higher strain after the final set. It is hence concluded that higher PCE solid contents reduce the risk of early cracks that occur at time of setting, when a solid structure has already been formed but without resistance against cracks yet. Originality The paper focuses on early hydration processes and the way they are affected by chemical admixtures. In terms of hydration history, the focus is placed on the period between fresh concrete and hardened concrete. The paper thus mainly contributes to the main topics of the 13 th ICCC “Properties of fresh and hardened concrete”, considering the effects of chemical admixtures on rheology, hydration, setting and deformations. However, since the effects cannot be extracted from micro-structural developments, the paper also contains numerous links to the main topic “Hydration and microstructure”. Chief contributions To date most research that observes cement superplasticizer interactions focuses on rheological properties. Only few published papers deal with the properties after casting until final set. Typically general differences of mixtures with polycarboxylate ether superplasticizers are investigated versus mixtures without. Aspects of particular molecule types and respective ionic backbone charge densities are less focused in research. The results of this paper help improving mixtures with regard to early hydration effects and sensitivity against early crack formation.