Use of Basic Oxygen Furnace Slag (BOFS) in geopolymer mortar for modification of concrete in hydraulic structures: stabilization properties and reduction of volume changes

Introduction. Increasing demands for sustainability and environmental safety in construction materials have encouraged the use of industrial by-products such as Basic Oxygen Furnace Slag (BOFS) and fly ash (FA) as alternative components in geopolymer binders. This study aims to evaluate the physico-mechanical properties, micro- and nanostructure, and resistance to alkali-silica reaction (ASR) of geopolymer mixtures based on BOFS and FA, which are particularly relevant for hydraulic engineering applications. Methods and Materials. BOFS and Class F FA were used as aggregates and pozzolanic components. Compressive strength was measured at 3, 7, 28, and 56 days. Micro- and nanostructural analyses were performed using Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS), and X-ray Diffraction (XRD). Expansion tests were conducted according to ASTM C 1260 in both water and 1 M NaOH solution at 80 °C. Results and Discussion. The optimal compressive strength (up to 50 MPa at 28 days) was achieved with 75% replacement of sand by BOFS and an alkali activator ratio of Na2SiO3/NaOH = 1. SEM and EDS analyses revealed the formation of a dense gel-like structure with N-A-S-H and CaCO3 phases. All samples exhibited expansion below 0.1% in water, meeting the criterion for non-reactive aggregates. In alkaline solution, expansion reached 0.25% in some compositions. A correlation was observed between microstructure, the content of reactive phases (Si, Al, Ca), and mechanical performance. Conclusion. The results demonstrate that BOFS and FA can be effectively utilized in geopolymer mixtures, providing adequate strength, dimensional stability, and ASR resistance. Carbonation curing and proper selection of alkali activators enhance BOFS stabilization and reduce its expansive behavior. These findings support the potential use of such mixtures in sustainable and durable construction systems, especially for hydraulic structures.