Water: its influence on the formation and crystallization of amorphous calcium carbonate

Nature produces CaCO3-based materials that display fascinating mechanical properties. These properties are a result of the exquisite control over the formation of CaCO3 crystals that nature possesses. Such a high level of control is likely achieved, at least in parts, by controlling the crystallization of CaCO3 via a transient phase, amorphous CaCO3 (ACC), with certain soluble additives and insoluble organic scaffolds present in living organisms. Inspired by the excellent properties of natural CaCO3-based materials, a lot of research has been devoted to the fabrication of synthetic counterparts with similar properties. Yet, we are far from obtaining the same degree of control over the properties of these synthetic materials. A contributing reason for this inferior control is our incomplete understanding of the influence of the processing conditions, soluble additives, and insoluble scaffolds on the formation of CaCO3 and thereby the composition, structure and properties of resulting materials.

This thesis is devoted to gaining a better understanding of the early stages of the CaCO3 formation. To achieve this goal, we introduce a new, organic solvent-free method to quench this process with a high temporal resolution. We produce ACC particles using a microfluidic spray-dryer and characterize them with different techniques. This method allows us to demonstrate that the amount of mobile water contained in ACC particles increases during their growth. As a result of the higher amount of mobile water, larger particles display a lower kinetic stability against the temperature-induced crystallization than smaller counterparts. We also reveal that certain additives reduce the amount of mobile water contained in ACC particles, thereby increasing their kinetic stability if exposed to elevated temperatures. To study the influence of additives on the kinetic stability of ACC against crystallization in more detail, we expose ACC particles functionalized with different additives to a humid environment or elevated pressures. We monitor the evolution of the structure and degree of hydration of these ACC particles during their crystallization. We show that the humidity-induced crystallization of ACC follows a distinctly different pathway than the pressure-induced one. In both pathways, the amount of water and its mobility that depends on its interaction strength with additives can influence the crystallization kinetics of ACC. The resulting CaCO3 crystals display a difference in their size, morphology, orientation, and structure. To process ACC particles into bulk CaCO3-based materials that possess similar mechanical properties to natural ones, insoluble organic scaffolds that offer a control over the local composition and structures of these materials across multiple length scales are typically required. To open up new possibilities of achieving this goal, we develop a new method to fabricate two dimensional (2D) structured hydrogel sheets. These 2D structured hydrogel sheets are composed of self-assembled, crosslinked hexagonal prismatic hydrogel particles. Using this method, we can vary the microscale structure and composition of these hydrogel sheets with the size, composition and arrangement of individual particles, their surface morphology with the polymerization conditions, and their mechanical properties with the crosslink density.