Effect of Ca/(Si plus Al) on red mud based eco-friendly revetment block: Microstructure, durability and environmental performance

Red mud with high Na+ and alkali content seriously limits its resource utilization in the field of building materials. In this work, red mud, fly ash, blast furnace slag and desulfurization gypsum were used as main raw materials to prepare eco-friendly revetment block (RERB) based on the synergistic effect of alkali and sulfate on aluminosilicate minerals. The effect of Ca/(Si + Al) on the microstructure, durability and environmental performance of RERB was investigated. The results showed that the red mud increased the alkalinity of the RERB system, which accelerated the dissolution of active substances and produced C(N)-A-S-H gel. The degree of polymerization of hydration products and the content of [AlO4](-) in the structure of C(N)-A-S-H gel were obviously affected by Ca/(Si + Al). The properties of RERB were inverted V-shaped with the decrease of Ca/(Si + Al). When Ca/(Si + Al) was 0.96, RERB had the highest degree of polymerization, and its hydration products contained more C(N)-A-S-H gel and a certain amount of ettringite. The interaction between C(N)-A-S-H gel and ettringite enabled RERB to have compact microstructure, good durability and environmental performance. Most importantly, the hydration products of RERB contained more Na+, which indicates Na+ played a role in the charge balance of the system. This work clearly demonstrated that the properties of the RERB system can be significantly affected by the change of Ca/(Si + Al).