<p>Amorphous silica is usually precipitate in geothermal power plant and become solid waste in large amounts. Silica is common reinforcing filler in the polymer composites. To give reinforcement in composites application, the geothermal sludge needs further treatment. Our previous research has been successfully synthesized nanoparticle silica from geothermal solid waste. This work investigated the effect of using nanoparticle geothermal silica (NGS) as a reinforcing filler in rubber composites. The synthesis of silica aggregate was modified using the Oswald ripening mechanism by two-step sodium silicate addition. The ratios of primary and secondary sodium silicates were varied by 1:0, 1:0.5, 1:1, and 1:1.5. The resulted NGS was characterized by Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), and BET surface area analyzer to determine its chemical structure and morphology. We also analyzed the mechanical properties, surface morphology and vulcanization kinetics of rubber compounds reinforced with NGS, using two available commercial silica nanoparticles as references. The extent of interaction that can be formed by the filler and rubber was analyzed using bound rubber content (BRC) approach. The results showed that NGS had primary particle sizes between 20 and 50&#xa0;nm and a surface area of 168–209&#xa0;m²/g. Additionally, the silica and rubber interaction affected the total modulus of rubber vulcanisate and the kinetics of vulcanization reactions. However, NGS that have primary and secondary sodium silicates ratio of 1:1 provided a good balance between filler–rubber interactions, vulcanization characteristics, and mechanical properties of the rubber composite.</p> Graphical abstract <p></p>