Quantitative Determination of Free Lime Amount in Steelmaking Slag by X-ray Diffraction

Abstract

Steelmaking slag is a by-product of steel making. It has been reused as a material for civil engineering, pavement construction etc. For such applications, it is extremely important to evaluate the amount of free lime, which has the chemical form of calcium monoxide (CaO) and can cause the failure of mechanical properties by volume expansion through absorption of water or carbon dioxide gas. So far, various chemical analyses of the free lime content have been employed that are frequently combined with ethylene glycol extraction. Unfortunately the method does not always distinguish the chemical form of calcium. In the present paper, we propose the use of X-ray diffraction. The technique can clearly show the existence of different chemical forms of calcium, such as CaO, Ca(OH)₂, CaCO₃, and calcium silicates such as Ca₂SiO₄ and other forms as different peaks. As the technique is non-destructive, one can also use other chemical analyses for the same samples. The amount of free lime can be determined by the standard addition method. In our study, the content of free lime in slag powder of under 32 micron dia was determined as 11.5 mass%. It was also found that the amount of free lime can be effectively decreased to 3–5 mass% by hot steam and/or carbonation processes.