Superplasticisers and ettringite formation: what calcium sulfoaluminate systems reveal about Portland cement hydration

Early hydration of calcium sulfoaluminate (CSA)-type and Portland cement (OPC) cements is primarily characterised by the formation of ettringite. During the initial minutes of hydration, small quantities of ettringite are formed, which impacts the workability of the cement pastes. Therefore, retarders such as polycarboxylate ether (PCE) superplasticisers are frequently employed to delay this early ettringite formation and improve the workability. In this study, the influence of two distinct, well-characterised PCEs on the early ettringite formation in pure ye’elimite–calcium sulfate and C3A–calcium sulfate mixtures were investigated by isothermal calorimetry. The results were complemented by Brunauer–Emmett–Teller surface area analysis, X-ray diffraction and thermogravimetric analysis. The results demonstrate that both PCEs and increasing dosages significantly suppress the initial ettringite formation in ye’elimite-based binder systems. Consequently, it can be concluded that the dispersing effect of PCE-type superplasticisers is attributable not only to the physical dispersing action of the PCEs but also to their ability to suppress early-age ettringite formation. This behaviour contrasts sharply with the influence of PCE superplasticisers on ettringite formation from C3A, where highly charged PCEs tend to promote higher ettringite formation by delaying the passivation of the C3A surface at an early age reducing workability.