Natural rubber (NR) has been widely used in the rubber industry owing to its excellent elastic properties. However, it requires reinforcing fillers to improve its mechanical properties for the manufacturing of rubber

products. Generally, calcium carbonate (CaCO₃) is employed as a non-reinforcing filler. This work aimed to synthesize spherical-shaped CaCO₃ at a submicrometric scale without and with surface treatment with olive soap and explore its utilization as a reinforcing filler in NR composites. The mechanical properties of various amounts (0 to 60 phr) of CaCO₃-filled NR composites were explored. As a result, the NR/treated CaCO₃ composites provided higher tensile strength than the NR/untreated CaCO3 composites and pure NR at all filler loadings. This may have been due to the improved interfacial interaction between NR and CaCO₃ with the improved hydrophobicity of CaCO₃ after treatment with olive soap. The optimal filler loading was 20 phr for the highest tensile strength of the rubber composites. Evidence from SEM and FTIR revealed the vaterite polymorph and shape stability of CaCO₃ particles in the NR matrix. The results demonstrate that the particle size and surface treatment of the filler have essential effects on the mechanical property enhancement of the rubber composites. The synthesized spherical CaCO3 could be a potential reinforcing filler with broader application in polymer composites.