Influence of Polymeric Dispersants on the Dissolution Rate of Tricalcium Silicate and the Nucleation of Calcium-Silicate-Hydrate and Portlandite.

Polycarboxylate ether (PCE) and polyphosphate ether (PPE) dispersants (also known as superplasticizers) are indispensable components of modern concrete mix designs. They reduce the yield stress of cement suspensions and thereby facilitate the reduction of the water-to-cement ratio. However, PCEs and PPEs also cause retardation of the cement hydration as a secondary effect, which causes delayed concrete strength development. It is proposed that the retardation is caused by the polymer influence on the tricalcium silicate dissolution rate and the crystallization of the hydration products (e.g., calcium hydroxide Ca(OH) and calcium-silicate-hydrate or C-S-H). This study examines the effect of four different polymeric superplasticizers with carboxylate and phosphate groups on the reaction kinetics of the pure cement phase tricalcium silicate (in cement shorthand ). Our results confirm the strong retarding effect of the polymers on the hydration reaction of the paste. Based on model experiments, we studied the polymer influence on the dissolution of and the nucleation of the hydration products. We observed no dissolution-inhibiting effect of the polymers at the chosen concentrations and undersaturation. However, such an inhibiting effect may occur at solution conditions closer to the saturation limit. Titrimetric analyses indicate that all polymers suppress the nucleation of portlandite and C-S-H, with a higher polymer charge density leading to a more pronounced inhibition.