The discarded mill tailings behind the dam will cause disasters at any time. A solution to this problem may be to utilize those wastes, e.g. mill tailings from magnetite, as the Manufactured Fine Aggregates (MFA) for preparation of concretes. The mill tailings from magnetite can be considered as new resources of sand. However, in production of MFA from mill tailings, the particles smaller than 75 μm were strictly eliminated. The degranulation results in not only lower utilization of the resources but also worse micro grading of MFA. The micro fines, i.e. powdery mill tailings from magnetite, supplements powder content in concrete systems in w/c>0.40 to modify the ratio of water to total powder (w/p). The perfect micro grading for good workability can be reached in the case of optimal w/p ratio. The filling effect owing to perfect micro grading of powdery mill tailings makes concretes more compact and the pozzolanic constituents in micro particles of the tailings contribute to the strength of concretes.