Effect of Hydrophobic Fumed Silica on Bending Strength of Sodium Silicate-Bonded Sand Cores

Inorganic sand cores involving sodium silicate binder and microsilica have environmental advantages during the casting process of aluminum alloy. Nevertheless, the bending strength of sodium silicate-bonded sand (SSBS) needs to be further improved. In this research, the effect of hydrophobic fumed silica on the bending strength of sand cores was studied. The experimental results revealed that hydrophobic fumed silica with the addition of 0.050 wt.% can be adopted as an optimal modifier to enhance the bending strength of SSBS. According to scanning electron microscope and spectroscopy techniques, dense bonding bridges and a complex Si–O–Si network containing specific silicon molecules with a silicon atom bonded to three other silicon atoms contribute to the excellent bending strength, with a 53.1% increase in cold strength (24 h) compared to a commercial sample of a modified sand core. Meanwhile, the newly formed Si–O–Al chemical bond plays a crucial role in increasing the bending strength of sand cores.