Optimal design of ferronickel slag alkali-activated mortar for repair exposed to high thermal load

In this work, the optimal design of a mortar based on alkali-activated material technology is presented. Ferronickel slag, a byproduct of the ferronickel alloy industry, was used both as a binder component (in a finely ground form) and as fine aggregate in alignment with a circular economy approach. The proportions of binder, fine aggregate, and water were optimized using Design of Experiment Design of Mixtures. The performance indicators evaluated were flow, flexural and compressive strength both before and after high-temperature exposure, mass loss, and thermal shrinkage. Life cycle assessment was used to calculate the relative environmental cost of the studied mixes in comparison to a counterpart traditional Ordinary Portland Cement mortar. The optimal mix design exhibited high flexural strength (8.5 and 10.5 MPa, before and after high-temperature exposure, respectively), an unheated compressive strength equal to 69.5 MPa, and a post-heating residual one of 33.9 MPa, 7.7% mass loss and 3.4% thermal shrinkage. Mercury Intrusion Porosimetry along with Scanning Electron Microscopy with Energy Dispersive X-Ray Analysis were also performed on optimal mortar samples in order to link micro-structural heat-induced changes to residual (post-heating) macro-mechanical performances. Finally, when compared to OPC-based products, the optimized mortar mix resulted in 70% lower CO₂ emissions indicating great potential for the construction sector where concern about environmental impact keeps growing.