Combined Influence of Reagent Concentrations and Agar Hydrogel Strength on the Formation of Biomimetic Hydrogel–Calcite Composites

We report results of CaCO3 crystallization experiments by counter diffusion in agar gel with two different solid contents (0.5 and 2 wt %) and two solute concentrations (0.1 M CaCl2, 0.1 M Na2CO3; 0.5 M CaCl2, 0.5 M Na2CO3). Solute concentration and hydrogel strength influence the characteristics of the gel−mineral composite formation. High reagent solution concentrations give rise to high supersaturation and high growth rates. When combined with a light gel, single crystal composites form; in a dense gel, the aggregates are mosaic crystal composites. Low reagent solution concentrations result in low supersaturation and low growth rates; when combined with a light gel, single crystal composites form; in a dense gel, the precipitate is a co-oriented polycrystal composite. Gel occlusion within the mineral increases with gel density. Gel distribution inside the mineral is homogeneous for high growth rates. For low growth rates, the gel accumulates locally in the precipitates. Light gels are pushed ahead by the growing crystals, and gel occlusion into the mineral is decreased; at low reagent solution concentrations, slightly more gel gets occluded. In conclusion, agar gel solid content determines the amount of gel occlusion and calcite orientation organization; reagent solution concentration influences the mode of gel distribution inside the mineral/gel composite aggregates