Amorphous calcium carbonate in carbon-mineralized cementitious materials: from phase transformation to binder design

Amorphous calcium carbonate (ACC) functions as a reactive intermediate in carbon-mineralized cementitious materials, bridging CO2 mineralization with carbonate polymorph selection, pore refinement, particle bonding and strength development. This review reorganizes ACC chemistry around the concept of controlled metastability, defined here as the regulation of ACC lifetime, hydration state, composition, spatial distribution and transformation kinetics so that carbonate precipitation remains sufficiently dispersed during processing but evolves toward a target phase assemblage within the required curing window. The review examines ACC formation through non-classical nucleation, polyamorphism and hydration structure; its regulation by ions, polymers, moisture, confinement and cementitious matrices; coupled phase evolution involving gel decalcification and secondary gel formation; and applications across calcium carbonate binders, recycled cementitious materials, industrial wastes and carbonation-cured systems. By linking these measurable metastability descriptors with processing variables and binder performance, this review identifies ACC control as a mechanistic basis for designing low-carbon, durable and carbon-sequestering cementitious materials.