Improving mechanical properties of manufactured sand concrete with high biotite content: Application of magnetic separation process and equipment optimization

Abstract

• Degradation mechanism of biotite in manufactured sand on concrete performance was presented. • An empirical formula of compressive strength for different biotite content was presented. • FEM-DEM coupling program was developed to optimize biotite separation process. Harmful components such as biotite in fine aggregates formed by natural rock crushing need to be rigorously assessed and effectively separated to reduce their potential damage to concrete structures. In this study, magnetic separation was used to remove excess biotite from manufactured sand and to realize the upgrade of manufactured sand, and a series of laboratory tests were carried out to study the influences of biotite content on the compressive strength and splitting strength of concrete. Experimental results show that the magnetic separation method can effectively reduce the biotite content in concrete from 10 % to 2 %, thus improving the mechanical properties of concrete. Through microscopic characterization, we found that the surface layer of biotite had a strong bonding effect with the early hydration product. This bonding effect contributes to the early strength of concrete, leading to a small difference in the strength values of manufactured sand concrete with different biotite contents. With increasing curing age, the strength contribution caused by this bonding effect gradually decreases. In addition, FEM-DEM coupling simulation was used to optimize the operation parameters and magnetic system structure of the magnetic separator to further achieve efficient separation of biotite from manufactured sand. Numerical prediction indicates that the dry magnetic separator has an extrusion magnetic system structure, which makes the magnetic induction and magnetic induction gradient at the guide disk higher and provides a much greater magnetic force than other competitive forces for biotite particles. To efficiently recover low-grade manufactured sand (low biotite content) from manufactured sand, the feed rate t , conveyor belt speed V and magnet-to-steel disk thickness ratio μ should be kept at 0.8 kg/s, 0.85 m/s and 3, respectively. This study can provide potential solutions and broad application value for the problem of manufactured sand with high biotite content in practical engineering.