Effect of Omani Limestone Waste as a Reinforcing Agent on the Mechanical Properties of Scrap-Based Aluminum Matrix Composites

This study utilizes Omani limestone waste powder (CaCO3) sourced from the mountain ranges of the Sultanate of Oman as an economical natural reinforcement for scrap-based aluminum matrix composites (AMCs) using stir casting. The recycling of aluminum alloy from end-of-life automotive engine cylinder blocks was strengthened with limestone at volume fractions of 2.5%, 5.0%, and 7.5%. Mechanical characterization was conducted in accordance with ASTM standards (E8/E8M, E18, E23). Statistical significance (p < 0.05) was calculated using one-way ANOVA. The optimum 5.0 vol.% reinforcing fraction showed tensile strength, Rockwell hardness and Charpy impact energy of 130.9 MPa (+16.9%), 91 HRF (+19.7%) and 7.2 J (+28.6%) compared to the unreinforced scrap alloy (112.0 MPa, 76 HRF, 5.6 J). The results of Scanning Electron Microscopy (SEM) research showed that the composite of 5.0 vol.% had a uniform distribution of CaCO3 particles and very low porosity, while the composite of 7.5 vol.% had a significant porosity (2–8 μm), interconnected microcracks and particle agglomeration. The porosity was increased with the increase of the content of the reinforcement as shown by the density experiments based on Archimedes’ principle. The maximum deviation of the experimental density from the predicted one was at 7.5 vol. % reinforcement. X-ray diffraction (XRD) confirmed the stability of the aluminum matrix structure as well as stable CaCO3 phases without any evidence of harmful interfacial reaction products (Al4C3 or CaAl2O4). The findings confirm the optimal reinforcement ratio of 5.0 vol.% of Omani limestone, tackling both the environmental load of limestone quarrying waste and the expensive synthetic reinforcements, in accordance with the circular economy goals of Oman Vision 2040.