Amorphous calcium carbonate (amorphous CaCO₃, ACC) is the amorphism of CaCO₃ with molecular formula of CaCO₃·nH₂O. It is termed as anhydrous ACC for n=0, which is unstable, and as hydrous ACC for n>0, which is stable to a certain extent. The value of n in hydrous ACC is also variable, leading to a series of amorphous CaCO₃ with different water contents, which is referred to as polyamorphism. ACC has been identified in diverse biological structures and synthesized in laboratory under certain conditions. Due to its thermodynamic instability, ACC serves as a crucial precursor for both geologic CaCO₃ (calcite) and biologic CaCO₃ (vaterite and aragonite). Particularly, it plays very important roles in biomineralization. ACC has been widely applied in the fields of calcium supplement, bone regeneration, biological medicine, drug carrier and protein adsorption. Despite decades of study, challenges persist regarding the preparation and conservation of ACC. Furthermore, comprehensive reviews on ACC remain insufficient, especially for the development in recent years. This paper is aimed to provide a comprehensive review on ACC, covering structural model, preparation routes and stability.

Several structural models of ACC were derived from simulations with different methods, such as Reverse Monte Carlo (RMC), Classical Molecular Dynamics (CMD) and Ab initio molecular dynamics (AIMD). However, it is still debatable on the real structure of ACC. ACC can be synthesized with two basic systems, Ca²⁺-H₂O-CO₃²⁻ and Ca²⁺-H₂O-CO₂, referred as to double decomposition reaction and carbonation reaction, respectively. The reaction between calcium salt and carbonate in aqueous solution is the most commonly used to synthesize ACC, in which high pH, high concentration and additive, e.g. Mg²⁺, PO₄³⁻, amino acid and protein, are the key factors to obtain ACC with desirable stability. In Ca²⁺-H₂O-CO₂ reaction system, the sources of CO₃²⁻ are diverse, including pure CO₂, CO₂ in air, CO₂ released from (NH₄)₂CO₃ or NH₄HCO₃, CO₂ released from organic matter, CO₂-storage materials and so on. Based on double decomposition reaction, the reaction between calcium salt and carbonate in alcohol-water binary solvent even in water-free solvent has been developed in the past decades. Moreover, the solid reaction between CaCl₂ and Na₂CO₃ via mechanochemical method has been employed to prepare ACC. ACC-polymer hybrid materials have also been synthesized based on the two basic systems. The crystallization of ACC occurs through two mechanisms: direct crystallization or dissolution-diffusion-crystallization. The first crystallization takes place in solid state, accompanied by the loss of structural water, while the second one occurs in aqueous media through two or three steps: dissolution (diffusion) and crystallization. In direct crystallization happens due to the dehydration of molecular water, induced by temperature or pressure. However, ACC powder will crystalize in humid environment. In aqueous reaction systems, the crystallization of ACC is inevitable. Therefore, reducing the water content of reaction system is a frequently used method to inhibit the crystallization of ACC and enhance its stability, even in a water-free system. In addition, some cations, e.g., Mg²⁺, some anions, e.g. OH⁻, PO₄³⁻ and organic molecules can be used to stabilize ACC in aqueous media.

The Ca²⁺-H₂O-CO₃²⁻ system with high pH or/and high concentration is the prevalent method to obtain ACC in early studies. The efficiency of this approach could be enhanced in the presence of organic additives, which is known as biological mineralization. The introduction of organic solvent in Ca²⁺-H₂O-CO₃²⁻ system can promote the formation and improve stability of ACC. However, the presence of water proves to be disadvantage for long-term stability of ACC. Water-free systems exhibit distinct advantages in both ACC preparation and long-term stability. The ADM method in aqueous medium has additional advantage in ensuring product purity by minimizing impurities in ACC. Beyond addressing structural water concerns, the introduction of Mg²⁺, PO₄³⁻, OH⁻ and organic molecules emerges as a strategy to enhance the stability of ACC. Furthermore, ACC has been formed on the template of polymers and proteins. Achieving long-term stability of ACC has been hot spot of research, leading to various innovative protocols in the past decades.