The tough life of an ettringite crystal and its consequences on packing and rheology

The morphology of early hydration products, and ettringite in particular, is potentially key to understanding and addressing the rheological challenges faced by modern cement and concrete technologies. By employing synthetic ettringite as a simplified model system, this study isolates three key controlling factors governing ettringite morphology: the ionic availability in the pore solution, the physical and stress environment, and the interactions with surrounding compounds. More specifically, ion dilution was found to favour the formation of thinner and longer ettringite crystals. However, their final length is strongly constrained by crystal breaking induced by mechanical stress during processes such as mixing or sample preparation. In systems where ettringite coexists with non-elongated particles, representative of cementitious materials, the applied stress and the restrictions imposed by the surrounding particle network jointly limit the final crystal length. In turn, ettringite morphology exerts a significant influence on particle packing efficiency, with crystal diameter emerging as a key parameter. To reduce the negative effect of ettringite on packing, and hence on rheology, strategies aimed at thickening the crystals offer a potential route. Although chemical admixtures are shown to influence ettringite morphology, commonly used PCE-based superplasticisers promote the formation of thinner crystals, producing an effect opposite to that required for improved packing. Despite the limitations of simplified model systems, the mechanisms identified here highlight the need for alternative strategies to control ettringite morphology and mitigate its adverse impact on rheology, providing new insights for the design of modern and sustainable cementitious systems.