Understanding the CaCO₃ phase transition of carbonated wollastonite composites caused by sodium tripolyphosphate: From amorphous to crystalline

Developing new cementitious materials through mineral carbonation attracts increasing attention for reducing carbon emissions. However, the role of CaCO₃ phase transition in the strength development of carbonated composites is not clear. In this study, new carbonated wollastonite composites are prepared and sodium tripolyphosphate (STPP) is used as a phase-controlling additive for the phase transition evolution of CaCO₃ polymorphs during the carbonation process. Moreover, mechanical performance, microstructure, and carbonation mechanism are investigated. Results show that STPP is effective in enhancing mechanical performance by controlling CaCO₃ phase transition. Specifically, STPP prolongs the phase transition of amorphous calcium carbonate (ACC) until 72 h later (the control binder at 1 h), allowing more opportunities for structural rearrangement. Besides, the introduction of STPP results in the formation of more stable ACC, vaterite, and aragonite, causing a compact microstructure and a lower carbonation degree. More importantly, STPP concentration within 0.3 M strongly improves the cementitious performance of all carbonated products (2.65–4.14 MPa/%), contributing to compressive strength growth (11.10–83.71%). The 0.1 M STPP-containing binder exhibits the highest compressive strength of 75.59 MPa. Our results contribute to unique pathways toward understanding the carbonation mechanism and a more sustainable cement industry.