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.