The nickel industry in Indonesia produces approximately 13 million tons of ferronickel slag (FNS) annually, most of which remains underutilized and poses potential environmental concerns. This study investigates the mechanical and deformation characteristics of concrete incorporating FNS as a sustainable replacement for natural coarse aggregate. Experimental tests included workability, compressive strength, ultrasonic pulse velocity (UPV) and digital image correlation (DIC) analyses to evaluate strain development and elastic properties. The inclusion of FNS reduced workability due to its angular particle shape and absorption capacity but enhanced concrete density and strength. The optimum mixture, with 50% FNS (FNS-50) replacement, achieved the highest compressive strength 38.0% greater than the control (FNS-0) and the highest modulus of elasticity at 32.2 GPa, representing a 25.6% increase compared with FNS-0. DIC observations revealed delayed strain localization and fewer columnar cracks in FNS-40 and FNS-50 specimens, indicating superior deformation resistance. A positive correlation was observed between compressive strength and elastic modulus, confirming the structural efficiency of FNS concrete. The findings demonstrate that FNS can serve as an effective and environmentally friendly coarse aggregate substitute, contributing to sustainable concrete technology and waste valorisation in the nickel industry. Copyright © 2026 Jessica Sjah et al. Journal of Engineering published by John Wiley & Sons Ltd.