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

This paper analyzes the use of Omani limestone, collected from the mountain ranges of the Sultanate of Oman, a major global exporter of limestone, as a natural reinforcement to improve the mechanical properties of recycled aluminum alloys obtained from automotive engine cylinder scraps. The recycled aluminum was melted and mixed with limestone powder at volume fractions of 2.5%, 5%, and 7.5% using the stir-casting technique. Mechanical test results showed progressive, statistically significant improvements (one-way ANOVA, p < 0.05) with increasing reinforcement content. At the optimal 5.0 vol.% fraction, tensile strength increased by 16.9%, surface hardness improved by 19.7%, and impact resistance increased by 28.6% relative to the unreinforced alloy. Scanning Electron Microscopy (SEM) confirmed uniform particle distribution and microstructural densification at 5.0 vol.%, but severe porosity, particle agglomeration, and microcrack initiation at 7.5 vol.%. X-ray diffraction (XRD) analysis confirmed the stable presence of CaCO₃ compounds and positive interaction with the aluminum matrix. The 5.0 vol.% fraction is identified as optimal, delivering the best balance of mechanical enhancement and microstructural soundness. These results suggest promising prospects for Oman's limestone waste in advanced engineering applications with significant environmental and economic benefits.