Chemical and mechanical origin of ettringite morphology and packing properties

The contribution of early hydration products is often neglected in rheological studies of cementitious systems, particularly with respect to the quantitative evaluation of their influence on particle packing. Ettringite, owing to its predominance as an early-age hydration product and its frequently elongated morphology, is the most relevant phase to consider in this context. This study investigates the factors controlling ettringite morphology in the absence of admixtures and evaluates their impact on packing behaviour. Ettringite was synthesised under controlled solution conditions and analysed via microscopic techniques to assess the effect of ion concentration on morphology. In addition, the packing properties of ettringite samples of different morphology were studied by applying controlled compressive stress through centrifugation experiments. Results show that ettringite morphology is defined by solution supersaturation with respect to ettringite and mechanical stress: reduced supersaturation (i.e. dilution of the system) favours the formation of longer, thinner crystals, while stress induces breaking, with the resulting aspect ratio of the crystals dictated by the magnitude of the applied stress. Ettringite crystals were found to exhibit poor packing properties compared to those of common building materials. Moreover, a clear correlation between aspect ratio and packing properties was identified, consistent with predictive models for elongated macroscopic particles. These findings have significant practical implications, as understanding and controlling ettringite morphology can provide an effective means to tune early-age rheology in cement-based materials. We finally suggest that these findings are specifically relevant for low-clinker content binders where supersaturation is expected to be lower than traditional binders.