To improve luster, heighten an additional value and enlarge the use by applying a surface treatment to steelmaking slag by mutually rubbing the steelmaking slag by stirring the steelmaking slag having a specific remaining expansion factor.
The steelmaking slag having ≤3.5% remaining expansion factor is used. When the stirring is carried out by a rotary dram mixer, the stirring is continued for the time (t) (min) satisfying the formula (t)*(r)=75 [(r) is the number of revolution (rpm)] or more. The treatment can be also carried out by stirring the slag for the time (t) (min) satisfying the formula (t)*(r)=50 or more and dipping the stirred slag in water. When the stirring is carried out by irradiating ultrasonic wave, the slag is dipped in the water and the ultrasonic wave is irradiated for the time (t) (min) satisfying the formula (t)*W/V=600 [W is a power consumption (W)]; V is the weight of the slag (kg)] or more. When the ultrasonic wave is irradiated while stirring the slag by the rotary dram mixer, the ultrasonic wave is irradiated for the time (t) (min) satisfying the formula (t)*(r)*W/V=50,000 or more while stirring the slag in the state clipped in the water by the rotary dram mixer.
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