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Due to the high CO 2-footprint of ordinary Portland cement (OPC), the search for alternative binders is now in a full swing. Rankinite-which is a hydraulically inactive material and low in calcium, is a real alternative to OPC, as it absorbs the harmful greenhouse gas from the air through carbonation hardening. Nevertheless, the carbonation hardening has not yet been fully clarified and sufficiently investigated. In this study we show that rankinite achieves a final strength exceeding 100 MPa at 45 °C and 24 h, whereby the binder is only ~ 50% carbonated. The reaction is diffusion limited while a dense layer of carbonation products around the rankinite grains hinders a higher degree of carbonation. The carbonation reaction could be fully characterized by spatially resolved microanalysis such as LA-ICP-MS, NMR and XRD. Finally, durability tests show the excellent suitability of the rankinite binder for a wide range of applications, making it an attractive material not only from an environmental point of view. Concrete is the second largest processed commodity after water consumed annually by the population of Earth 1. Due to such vast demand for the building materials, ordinary Portland cement (OPC) industry is responsible for over 5% of global anthropogenic greenhouse gas emissions, with almost equal amount of CO 2 emitted to the atmosphere after production of one tonne of cement 2-4. Accordingly, the scientific community is struggling to find the solutions for greenhouse gas mitigation and reduction of the negative effect of the cement production. Even though, in the past decades many options to alleviate the adverse effect of OPC production to the environment were proposed 5 , however, recent studies have shown that strategies like clinker substitutions, alternative fuels and/or improved energy efficiency alone will not be sufficient enough to meet the target CO 2 reductions 6. Thus, finding alternative cementitious materials with lower CO 2 footprint than OPC is one the major challenges for the building material industry and the scientific community. At the moment, one of the most promising approaches is the production of low-lime calcium silicate cement (CSC) 7-11. This type of binding material not only requires lower amounts of limestone but also has lower production temperature, thereby resulting in much lower CO 2 emissions 12. Moreover, such binders are environmentally amicable not only due to lower CO 2 emissions, but also for the ability to permanently store CO 2 in the concrete structure in their carbonation hardening process 13. Implementation of such efficient carbonation technologies can potentially lead to cementitious materials becoming one of the largest global CO 2 sequestration sectors 14. Rankinite-Ca 3 Si 2 O 7-is one of such low lime calcium silicates that can be used as an alternative binder that gained more interest recently 7,15,16. Since the CaO/SiO 2 ratio of rankinite is almost twice lower than of ordinary cement, thus it requires lower amounts of calcareous raw materials. The fuel and...

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