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To explore the influence of thermal activation and particle size on cementitious activity of bauxite tailings, in this study, the raw bauxite tailings were classified into coarse bauxite tailings (CBT) and fine bauxite tailings (FBT) by a powder separator, and then the effect of activation temperature on cementitious activity of CBT and FBT was investigated. XRD, TG, and FTIR were used to study the phase and structure changes of CBT and FBT during the process of thermal activation. The results show that the main mineral phases of CBT and FBT, diaspore and kaolinite, begin to remove a large amount of hydroxyl groups at 500 ℃ and convert into corundum and metakaolin, respectively. The diaspore and kaolinite have completely removed the hydroxyl groups at 600 °C and 700 °C, respectively. With the increase of activation temperature, the particle size of CBT and FBT are first gradually decreased, when the activation temperature exceeds 700 ℃, as the activation temperature continues to arise, the particle size of CBT and FBT are gradually increased due to the occurrence of sintering. The activity index of bauxite tailings is increased with the increase of fineness, and the optimum activation temperature of CBT and FBT is 700 °C. When the activation temperature is the same, FBT has higher pozzolanic activity than CBT. [ABSTRACT FROM AUTHOR]

• 3D printing mortar is prepared by using bauxite tailings as fine aggregate instead of natural sand. • The addition of bauxite tailings can greatly improve the buildability of 3D printing mortars. • The mechanical properties of the 3D printing mortar are the best when the bauxite tailings replacement ratio is 35%. • Using bauxite tailings to make 3D printing mortar is feasible and environmentally friendly. Using tailings to replace natural resources to prepare building materials is not only effectively alleviate the pressure of natural resource shortage, but also has important significance for environmental protection. This study aims to explore the applicability of bauxite tailings as fine aggregate instead of natural sand to prepare 3D printing mortar. The workability, rheological properties, buildability, hydration properties and mechanical properties of 3D printing mortar using bauxite tailings instead of 15%, 25%, 35% and 45% of natural sand were determined. The results show that the incorporation of tailings reduces the fluidity, slump and setting time of the 3D printing mortar, but delays the hydration process of the cementitious material. The rheological analysis reveals that the incorporation of tailings can significantly improve the static yield stress, dynamic yield stress, plastic viscosity and thixotropy of 3D printing mortar. With increasing tailings content, the macroscopic deformation and structural deformation rate of 3D printed components are gradually decreased. The compressive strength and flexural strength of 3D printing mortar are increased first and then decreased with increasing tailings content. Based on the test results, it is determined that the optimal ratio is that the mass ratio of tailings to replace natural sand is 35%, which can make the 3D printing mortar obtain good workability and high mechanical strength. Overall, it is feasible to use bauxite tailings to prepare 3D printing mortar, which optimizes both buildability and mechanical properties. [ABSTRACT FROM AUTHOR]

• Hydration process including NG, I and D of composite binder was simulated according to hydration kinetics model. • Tailing size affected the hydration kinetics of composite binder. • Adding tailings reduced the hydration heat. • Bauxite tailing content exceeding 30% substantially impacted later hydration. To explore the influence of bauxite tailings particle size and content on the hydration process of cement-tailings composite binder, the hydration kinetics of blended cement containing activated bauxite tailings was investigated. Hydration heat of cement-bauxite tailings composite binder was measured at 20 °C. The Krstulovic–Dabic kinetics model was used to explore the kinetics of blended cement containing bauxite tailings. The hydration process of blended cement was analyzed by the X-ray diffraction (XRD). Results demonstrate that the induction period of the composite binder increased with increase in the bauxite tailings content. The hydration heat increased with increase in powder selection frequency of the tailings. The kinetics model could effectively simulate the hydration process of cement- bauxite tailings composite binder. With the increase of the tailings content, the value of α 1 decreased and that of α 2 increased. Calcined activated bauxite tailings could react with calcium hydroxide owing to the pozzolanic reaction. The calcium hydroxide content of the composite binder decreased compared with pure cement and decreased with increase in the bauxite tailings content, particularly for bauxite tailings incorporations exceeding 30%. [ABSTRACT FROM AUTHOR]