Effect of Surface Coating Strengthening on Humidity Resistance of Sodium Silicate Bonded Sand Cured by Microwave Heating

The microwave heating sodium silicate bonded sand (SSBS) process is regarded as the most likely molding sand to realize green casting, owing to its low sodium silicate addition, rapid hardening speed, high strength, and excellent collapsibility. However, SSBS can absorb water easily in the air at room temperature. A surface coating strengthening method was used to improve the humidity resistance of SSBS. The properties of SSBS treated by the surface coating strengthening method were compared with that without any treatment. The experimental results indicated that humidity resistance had been improved greatly using the surface coating strengthening method, and comparing to SSBS without any treatment, the compressive strength (σ_6h) of the treated SSBS increased 2.32 times and the moisture absorption rate (w_6h) can be decreased by 45%. The morphology results revealed that there was a surface coating around the treated SSBS. The linear scanning indicated that the nonhygroscopic coat components presented on the boundary of the treated SSBS, and the surface phase analysis manifested that the boundary of the treated SSBS emerged new three phases, including Al₂TiO₅, PbTiO₃, and NaAlO₂, which reduced the free sodium ion content and improved the humidity resistance of SSBS.