Use of calcium sulfoaluminate binder chemistry for sustainable cement

Concrete production is one of the main contributors to manmade global CO2 emissions. In concrete, most of the emissions come from the production of its binding component, Portland cement (PC). Among other potential solutions, the replacement of Portland cement with lower CO2 binders is considered a promising strategy to reduce the environmental impact of concrete. That is the reason why alternative cements and binders such as Calcium sulfoaluminate (CSA) have received considerable attention in the last decades. CSA cements can be produced in traditional cement plants at lower temperatures than PC, resulting in energy savings. In addition, they have a lower limestone requirement, which results in lower direct CO2 emissions during clinkering. Other attractive characteristics of CSA cement are the requirement for sulfur-containing raw materials and their high theoretical potential for waste immobilization due to its ettringite-rich hydrated matrix. In this thesis, such characteristics are regarded as opportunities for the valorization of underutilized waste streams such as sulfidic mine tailings in the production of CSA cements. Sulfidic tailings are fine mineral wastes that result from mining activities. They are associated with high management costs and severe environmental impact due to metal(loid) leaching and soil/water acidification. The utilization of sulfidic tailings in CSA production could contribute to the circularity and stabilization of tailings while decreasing the need for the sourcing of natural raw materials for CSA. The global aim of this doctoral thesis is to explore the opportunities offered by the CSA binder chemistry to produce environmentally sustainable CSA cements and binders. Here, the focus is given to the study of the mineral phase composition of materials (clinkers, hydrated cement/binders) and the links between mineralogy and specific material properties. The main analytical techniques used are X-ray diffraction (XRD) and thermogravimetric analysis (TGA), which are complemented by several others across the different studies. Three experimental studies are undertaken, the first two being centered on the valorization of sulfidic mine tailings in cement. First, sulfidic tailings are used as an alternative raw material for CSA clinker production. Second, sulfidic tailings are used as a partial replacement for cement. The third study focuses on the use of CSA chemistry as an inspiration for the production of a clinker-free low-tech CSA binder. Across the different studies, properties such as SO2 retention, metal(loid) leaching, compressive strength, and setting time are evaluated. In the first study, clinkers are produced from different raw meal formulations and undergo detailed characterization. The results show that part of sulfur from the raw materials is permanently lost to the gas phase during clinkering, but strategies to enhance sulfur retention in the solids are identified. Iron is found in different types of calcium ferrites and as an impurity in reactive clinker phases. Regardless of raw meal composition and clinker mineralogy, the clinkers displayed an outstanding ability to immobilize metal(loid)s present in the raw materials. That ability was also observed in the second study, in which the tailings were used as a partial replacement for cement. In fact, compared to the other cement chemistries analysed in this study, i.e., PC and Calcium aluminate cement (CAC), CSA shows an enhanced ability to immobilize certain heavy metals. In terms of the tailings contribution to cement hydration, however, only a limited effect was observed. In the last study, a low-tech CSA binder is designed by blending the appropriate proportions of natural pozzolan, slaked lime, and gypsum. The binder formulation is done based on the measured and simulated hydrated phase assemblage. The optimum formulations achieve enhanced performance while keeping a low CO2 footprint. It is also possible to show that setting time can be controlled by adjusting the formulation and/or using an additive (sucrose). At the end of the thesis, the main findings of the three studies are summarized and the general conclusions are presented. Last, the limitations of the present study are mentioned and opportunities for future work are identified