Since natural sand for concrete is running short globally, due to the increasing influence of environment protection legislation, and because accessible reserves are dwindling, we are obliged to improve the quality of manufactured sand, and use it in greater quantities as a substitute for the natural product. However, producing well-shaped and properly graded sand also generates micro-fine particles as an inevitable byproduct. In this study, the specific size range which has the greatest detrimental effect to concrete has been successfully discovered, and hence a method of using micro-fines most effectively can be proposed.