Amorphous and crystalline CaCO₃ phase transformation at high solid/liquid ratio – Insight to a novel binder system

The transformation reactions in aqueous calcium carbonate systems are relevant for biomineralization processes, functionalised materials design and recently for the potential development of inorganic binders. The so-called ‘calcium carbonate cements’ are obtained by mixing amorphous (ACC) and crystalline CaCO₃ (herein the anhydrous minerals calcite, aragonite and vaterite) with low amounts of water. The mechanisms behind the setting and hardening of these binders are still not clear. In the present study, the dissolution and precipitation reactions in low water ACC systems with and without crystalline CaCO₃ have been investigated by in-situ techniques, Raman spectroscopy and optical pH measurements, coupled with reaction time resolved mineralogical and chemical analyses of solid and liquid samples. The results clearly indicate the presence of initial crystalline CaCO₃ polymorphs to control the reactions kinetics and the resulting individual microstructure. Vaterite, with a considerably higher surface area than calcite and aragonite, produces more bonded and connected material. In the ACC-vaterite system the transformation of vaterite to calcite takes places in two dissolution-precipitation stages, accompanied by typical pH in- and decreases. Time-resolved analyses and hydrochemical modelling clearly reveal the solid transformation behaviour to be governed by the presence/absence of Mg²⁺ in the solution and the impurities, such as Na⁺, liberated from the educts to the reactive solution. The control and adjustment of the identified influencing parameters could allow for promising future development of CaCO₃-based binders.