Sodium consolidation in red mud simultaneously the mechanical properties enhancement of cementitious material by aluminosilicate solid wastes

Construction material utilization represents a sustainable disposal approach for red mud (RM). While the key challenge in reaching RM utilization lies in achieving sodium consolidation and enhancing activity, this research selected fly ash (FA) and steel slag (SS), as related and urgent aluminum silicate solid wastes, to solve the RM “problem”, achieving dual objectives of sodium consolidation and activity enhancement. Results demonstrated that R1F1-1000 °C (RM:FA = 1:1, calcined at 1000 °C) could obtain cementitious materials with low soluble sodium (Na) content of 1.08 mg/g and enhanced compressive strength of 44.98 MPa. Under the optimal condition, the residual rate of Na reached 94.66%, wherein there existed 95.79% insoluble Na. Moreover, most of the insoluble Na in R1F1-1000 °C was present as NaAlSiO4 and NaAlSi3O8, as verified by thermodynamic simulation. In addition, the compressive strength of mortar reached 46.37 MPa when the optimal condition (R1F1-1000 °C) was 20%. The mechanism of “killing two birds with one stone” can be concluded as: amorphous SiO2 and Al2O3 within FA tend to preferentially combine with Na in RM, resulting in the formation of high-temperature-stable silico-aluminates known as NaAlSiO4 and NaAlSi3O8. The insoluble Na compounds convert into Ca3NaSi9Al7O32 during the hydration process, forming C (N)—A—S—H gel and considerably enhancing the strength of the cementitious material.