Influencing factors of the mixing performance of a near-nozzle continuous mixer for 3D concrete printing: An analysis based on spatial Lacey mixing index (SLMI)

3D concrete printing, as a vital construction method for future development, has lately received significant attention. Unlike classic pumped concrete material extrusion process, a near-nozzle continuous mixer reduces mixing energy by eliminating long conveying distances. However, there has been limited research on it mixing quality. In the study, the first mixing phase of an innovative near-nozzle continuous mixer is explored by full-scale numerical simulation. In order to improve the mixing process of additive 3D concrete printing, it is necessary to compare the mixing efficiencies and performances of continuous mixers with different configurations under various working conditions. Sixty cases with four parameters (sideward angle, the number of paddles, rotation speed, and mixture's composition ratio in different levels) are analyzed in the discrete element method (DEM) simulation. The influences of these parameters on overall and local mixing performances are determined with the modified Lacey mixing index, the spatial Lacey mixing index (SLMI). For unequal particles introduced in this study, SLMl is combined with the equivalent volume method. Qualitative and quantitative analysis results reveal that the four-15°-paddle conveyor mixer performs better above a certain rotation speed, regardless of the mixture's composition ratio. This paper increases the understanding of the mixing process for preparing well-mixed dry materials in a continuous conveying state, which is crucial for efficient near-nozzle mixing in 3D concrete dosing.