•Properties of red mud in Vietnam's Tan Rai Alumina plant.•Dissolution ratio of SiO2 and Al2O3 in red mud, fly ash under different conditions.•Properties of red mud based alkali activated material after high temperature and autoclave curing.•Regression between the compressive strength and the total amount of dissolved SiO2. To produce the alkali activated material, red mud must be used with components containing active silicon dioxide because silicon dioxide in red mud does not dissolve in alkaline solution. The test results presented in this paper show that high pressure and high temperature treatment in an autoclave can dissolve part of the silicone dioxide in red mud and improve the properties of material. The compressive strength of red mud based alkali activated materials can reach 10.6 MPa with a softening coefficient over 0.70. Adding an active silicon dioxide supplement by replacing 26.3 wt% red mud with fly ash increases the compressive strength up to 20 MPa and the softening coefficient by more than 0.90. Furthermore, autoclave treatment reduces residual sodium hydroxide and lowers the pH value of material. As the result, red mud based and red mud-fly ash based alkali activated materials can be used for manufacturing of masonry units.