Mechanical properties, 3D printing and environmental performance of Ordinary Portland Cement (OPC) mortars containing calcite rich residue from stabilised basic oxygen furnace slag

The metallurgy industry generates slags, and their recycling is crucial for promoting a circular economy and addressing environmental management concerns. This study investigated the potential for upcycling a calcite-rich stream, derived from vanadium recovery from Basic Oxygen Furnace (BOF) slag, as a Supplementary Cementitious Material (SCM) in concrete. Blended pastes and mortars were prepared by partially replacing cement with the residue. The effects of cement substitution were evaluated using isothermal calorimetry, rheology and consistency tests, X-ray diffraction (XRD), and Scanning Electron Microscopy coupled with Energy Dispersive X-ray Spectroscopy (SEM-EDS). The results showed that the residue consisted mainly of calcite and aragonite, with a randomly shaped morphology and a median particle size of around 2 μm. The residue exhibited moderate reactivity as a SCM, assessed by the Rapid, Relevant, Reliable (R3) test. After 28 days, the compressive strength of samples with up to 30% cement replacement satisfied the 75% strength activity index requirement. Environmental leaching tests conducted on the prepared mortars demonstrated significant immobilization of heavy metals from the residue within the concrete matrix. Preliminary 3D printing tests indicated that the residue enhanced printability by reducing concrete segregation, due to its finer particles. These results highlight the potential for valorising calcium-rich side streams and similar waste materials, contributing to improved circular economy practices and reduced environmental impact in both the metallurgy and construction industries.