Impact of the charge density on the behaviour of polycarboxylate ethers as cement dispersants

Abstract

The charge density of polycarboxylate ethers (PCEs) is the driver for adsorption of these polymers on the surface of hydrating Portland cement, which is the necessary step for the PCE to disperse and plasticise Portland-based materials. The charge density of a PCE is commonly considered the result of a certain molecular architecture and the complement of the sidechain density, regardless the synthetic route chosen to produce the comb polymer. Hence, its effect is normally masked by the impact of different structures. In this work, three sets of three polymers each have been prepared by esterification of three similarly sized but different in constitution polyacrylic acids. Within each set, the reaction conditions have been tuned to promote a more or less accentuated acid function loss. The resulting PCEs all bear the same structure and conformation but substantially differ in charge density. The so-isolated molecular feature has been studied through its impact on adsorption extent and rate, chemical stability, calorimetric and rheological characteristics of cement paste. It is shown that the charge density has a wide impact on the performance of PCEs as superplasticisers. Its modification heavily affects the adsorption behaviour and can turn the same structure to behave as either strong water reducer or workability extender, with a limited effect on the heat signature of the cement paste. Furthermore, some insights are disclosed regarding reactivity of polyacrylic acids towards esterification, selectivity of ester formation and resulting chemical stability.