The ettringite – superplasticizer interaction and its impact on the ettringite distribution in cement suspensions

Synopsis: In terms of workability of cement based materials, ettringite formation is of major importance. Thus, the interaction between polycarboxylate-ether based superplasticizers (PCE) and various pure cement clinker and hydrate phases is investigated by means of rheological, adsorption and zeta potential measurements. Microstructural analysis of fresh pastes is carried out using cryo-microscopic methods. The rheological results reveal that the ettringite phase is dominating the workability properties of cement suspensions as the PCE shows high affinity to adsorb onto ettringite surfaces. The dispersive effect of PCE in cement suspensions is mainly governed by electrostatic and/or steric repulsive forces evoked by the PCE molecules. This effect can be observed by cryo-microscopy on high-pressure frozen cement suspensions. Without PCE, ettringite tends to agglomerate with the tricalciumsilicate rich clinker particles due to their opposite zeta potential. Therefore, the ettringite is inhomogeneously distributed in the OPC suspension. This distribution pattern changes completely in the presence of PCE, where the ettringite crystals now are mainly present in the interstitial pore space. These results underline that the ettringite phase and its dispersion in the fresh cement paste is acting as the major key mineral concerning workability issues for cement applications.