On February 28, 2023, Natalia Pires Martins successfully defended her PhD thesis entitled “Use of calcium sulfoaluminate binder chemistry for sustainable cement”. Natalia did her PhD at the Chair of Sustainable Construction of the ETH Zurich and was one of the Early Stage Researchers of the MSCA-ETN SULTAN project.
Natalia carried out her PhD in Civil Engineering under the supervision of Prof. Guillaume Habert, from the ETH Zurich, and co-supervision of Prof. Ruben Snellings, from VITO, Belgium. Her PhD research was part of the H2020-MSCA-ETN SULTAN project, the EU Training Network targeting the remediation and reprocessing of sulphidic mining wastes. Natalia continues her career at VITO, as a researcher in in the Waste Recycling Technologies team.
The summary of the PhD research along with list of publications are available below; the full text of the thesis will soon be made publicly available.
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 the doctoral thesis was to explore the opportunities offered by the CSA binder chemistry to produce environmentally sustainable CSA cements and binders. Focus was 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 were X-ray diffraction (XRD) and thermogravimetric analysis (TGA), which were complemented by several others across the different studies. Three experimental studies were undertaken, the first two being centered on the valorization of sulfidic mine tailings in cement. First, sulfidic tailings were used as an alternative raw material for CSA clinker production. On a second study, the tailings were used as a partial replacement for cement. The third study focused 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 were evaluated.
In the first study, clinkers were produced from different raw meal formulations and underwent detailed mineralogical and chemical characterization. The results showed that part of sulfur from the raw materials is permanently lost to the gas phase during clinkering (as SOx emissions), but strategies to enhance sulfur retention in the solids were identified. Iron was 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. This 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 investigated in paralell, i.e., PC and Calcium aluminate cement (CAC), CSA showed an enhanced ability to immobilize certain heavy metals. However, only a limited contribution to cement hydration was observed. Modifications on the phase assemblages of CSA and CAC hydrated pastes caused by minor constituents present in the tailings were identified.
In the last study, a low-tech CSA binder was designed by blending the appropriate proportions of natural pozzolan, slaked lime, and gypsum. The binder formulation was done based on the measured and simulated hydrated phase assemblage. The optimum formulations achieved enhanced performance while keeping a low carbon footprint. It is also possible to show that setting time can be controlled by adjusting the formulation and/or using an additive (sucrose).
The results of the doctoral thesis highlight the promising potential of the CSA binder chemistry to produce sustainable cements and multi-component binders with unique properties. Such binders are able to incorporate sulfur-containing mineral waste streams as secondary raw materials or binder components and are capable of providing safe containment of potentially hazardous metal(loid)s within their hydrated matrix.
List of peer-reviewed publications in scientific journals
- P. Martins, S Srivastava, FV Simão, H Niu, P Perumal, R Snellings, M. Illikainen, H. Chambart, G. Habert. Exploring the potential for utilization of medium and highly sulfidic mine tailings in construction materials: a review (2021). Sustainability 13 (21), 12150. https://doi.org/10.3390/su132112150
- P. Martins, B. Çiçek, C. Brumaud, R. Snellings, G. Habert. Beyond efficiency: Engineering a sustainable low-tech cementitious binder for earth-based construction (2022). Cement and Concrete Research 162, 106973. https://doi.org/10.1016/j.cemconres.2022.106973