A contribution to understand the retarding effect of superplasticizers on tricalcium silicate hydration

The usage of superplasticizers for concrete production is beneficial in terms of workability and water reduction. Therefore, superplasticizers enable the production of concretes with increased flowability and high strengths. On the other hand, superplasticizers are also known to retard cement hydration. This is especially true for new polymers based on polycaboxylate ether grafted with side chains. In the present work it is investigated which reaction during the dissolution-precipitation process of cement hydration is mostly affected by these polymers. In order to simplify the complex multiphase cement system, tricalcium silicate (C3S) as main compound of ordinary Portland cement is utilized. Measurements of dissolution rate of C3S, nucleation time during pure crystallization of C-S-H phases and portlandite and calcium complexation capability of superplasticizers have been carried out.
Results show that compared to the reference sample superplasticizers modifies the dissolution rate of C3S in aqueous solution with low ion activity product. At increasing ion concentration during dissolution of C3S and thus increasing ion activity product the differences between samples without and with superplasticizers are dependent on polymer structure. Furthermore it is shown by homogeneous crystallization experiments that in the presence of superplasticizers the cause of retardation is more related to precipitation hindrance. Indeed, it is demonstrated that the pure crystallization of cementitious hydrate phases is strongly affected by superplasticizers. It is further shown that the observed effects are related to basic modes of interactions of superplasticizers with ions.

Author: T. Sowoidnich, C. Roessler, H.-M. Ludwig

Section: Science and production

Keywords: tricalcium silicate, superplasticizers, dissolution, crystallization