Effect of sodium nitrate on improvement in the properties of ceramsite using red mud

Based on analyzing the chemical composition,mineral composition and thermal stability of red mud,the preparation of ceramsite was investigated with calcite itself in red mud or added sodium nitrate as a foaming agent.Main components of the obtained ceramsite were analyzed by X-ray diffraction(XRD) and the internal structure was observed by scanning electron microscopy(SEM).The results show that with calcite itself as a foaming agent,the ceramsite with a bulk density of 1.43g·cm^-3,a water absorption rate(WAR) of 1.23% and a cylinder compressive strength(CCS) of 22.14 MPa is prepared when the mass ratio of red mud/waste glass/bentonite is 74:15:11.However the bulk density is obviously decreased by adding sodium nitrate.The ceramsite with a bulk density of 1.28g·cm^-3,a WAR of 1.54% and a CCS of 12.03 MPa is prepared when the mass ratio of red mud/waste glass/bentonite/sodium nitrate is 70.0:14.2:10.2:5.6.Crystalline phases present in the obtained ceramsite are mainly gehlenite,different from calcite in red mud based on XRD data.SEM images indicate that the majority of pores in the obtained ceramsite are sealed,and when sodium nitrate is used as a foaming agent,the porosity is higher in comparison with calcite in red mud,and the internal pore size is more uniform.