Cement is used for concrete in construction and architectural structures. Clinker, which is an intermediate material for cement, is produced by mixing and calcinating cement raw materials such as limestone, clay and silica in a rotary kiln at a high temperature. The unreacted calcium oxide remaining after calcination of the clinker is called free lime (f.CaO).
When calcination in a rotary kiln is insufficient, limestone, the main raw material, does not react sufficiently with silicon dioxide, aluminum oxide, etc., and the amount of free lime increases, resulting in the cement not meeting the expected composition. Furthermore, free lime changes to calcium hydroxide and calcium carbonate by reacting with moisture and carbon dioxide in the air, causing volume expansion. Thus, process control of clinker is important because it directly affects the quality of the cement product.
There is no difference in the X-ray fluorescence peak profile of the Ca Kα line between free lime and other calcium compounds; therefore, the concentration of free lime cannot be determined by X-ray fluoresce (XRF) analysis. Free lime is therefore generally analyzed by wet chemical analysis like titration or by an X-ray diffractometer (XRD). This paper introduces the Rigaku simultaneous wavelength dispersive X-ray fluorescence (WDXRF) system Simultix 15 equipped with a free lime diffraction channel, enabling both XRF analysis and free lime quantification by XRD. Quantitative analysis of free lime is also described.