Kilogram scale synthesis of C3A polymorphs and their hydration reactions

Abstract

Studies on the properties of pure C₃A phases are often limited to methods requiring small sample amounts due to the lack of a convenient laboratory synthesis yielding sample amounts exceeding 100 g. Here, we report a simple and large scale lab method for the synthesis of C₃A polymorphs with yields of up to 500 g per batch. Commercial calcium aluminate cement (CAC) was used to prepare cylindrical green bodies of CaCO₃ and Al₂O₃ (and NaNO₃ for orthorhombic and monoclinic polymorphs). The green bodies were sintered at 1300 °C and 1400 °C respectively. The chemical and mineralogical compositions of the obtained C₃A polymorphs were analyzed by X-ray powder diffraction and X-ray fluorescence spectroscopy. The reactivities of these C₃A polymorphs were compared to conventionally synthesized C₃A (using mechanical powder compaction prior to sintering) via in-situ isothermal heat flow calorimetry. Additionally, we demonstrate that synthetic C₃A retains its reactivity over one year if stored appropriately. As the new synthesis protocol yields hundreds of grams of C₃A, it enables experimental methods such as slump flow testing with pure phases, which is also reported for all polymorphs.