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As the main alkali and alkaline-earth oxides in red mud, the effects of Na 2 O and CaO on the physical properties, crystalline phases and microstructure of red mud based ultra-lightweight ceramsite (RUC) were systematically investigated using XRD, FTIR and SEM-EDS methods. Na 2 O and CaO promote the generation of glass phase and reduce the viscosity of liquid phase. A proper Na 2 O content accelerates the formation of internal large size pores and the expansion of ceramsite, but excessive Na 2 O makes the pores grow and connect to each other and then reduces the compressive strength of RUC. The presence of CaO produces the glass phase with high fluidity that can fill the internal pores of RUC, thereby reducing the expansion property and water absorption. High contents of Na 2 O and CaO accelerate the crystallization of nepheline and anorthite respectively, and the formation of anorthite is contributed to improve the strength of RUC. The bulk density and compressive strength of RUC can reach 365–485 kg/cm3 and 1.05–1.53 MPa when the contents of Na 2 O and CaO in RUC are controlled within 3% and 3.5%, respectively, which meet the requirement of GB/T 17431.2–2010. [ABSTRACT FROM AUTHOR]