The cement industry is the second-largest manufacturing emitter of CO2, and the manufacture and placement of concrete accounts for 7-8% of anthropogenic CO2 emissions. There is increasing pressure on the cement industry to develop a sustainable cement to support rapid global urban development without the associated emissions of cement production. The calcination reaction required to manufacture Portland cement is the prime source of emissions, so eliminating the release of CO2 liberated during calcination of limestone provides an excellent opportunity to produce a more sustainable cement. Calcium carbonate (CaCO3) cement is a novel alternative, which can eliminate the calcination process or close the loop on CO2 emissions during cement production. In this study, a vateritic CaCO3 is synthesized from the double decomposition reaction of equimolar CaCl2.2H2O and Na2CO3 at room temperature using a continuous stirred-tank reactor (CSTR). The morphology of precipitated vaterite displays nano-aggregated micron-sized spherical particles. The dissolution of vaterite and recrystallization to a stable polymorph (calcite and/or aragonite) provides the necessary cementing reaction. Mg2+ and/or Sr2+ ions control the transformation kinetics and the abundance of the stable phase. However, the low concentration of Mg2+ (0.05 M) was insufficient to inhibit calcite formation and promote aragonite formation. Aragonite containing hardened cement exhibits 10 times higher compressive strength than a calcite containing one. The hardened cement exhibits a pH neutral microporous structure with relatively low compressive strength; the highest compressive strength was measured at approximately 1.11 MPa. By capitalizing on the unique properties of CaCO3 cement, it can be developed as an ideal cement to support environmental restoration projects, such as oyster reef restoration along the Intracoastal Waterway in Florida, where the neutral pH of CaCO3 cement and its chemical composition provide it with enhanced properties for oyster recruitment compared to Portland cement
