Free lime determination in clinker

Significance of the Topic

Free lime (CaO) content in cement clinker is a critical quality parameter influencing dimensional stability, setting behavior, and compressive strength of cement. Precise monitoring of free lime levels enables optimization of kiln operations, energy savings through reduced over-grinding, and consistent product quality across production cycles.

Objectives and Study Overview

This study presents the development and validation of an integrated XRF–XRD analytical system capable of determining free CaO content in clinker. The main goals were to combine chemical and mineralogical analysis in a single instrument, achieve high sensitivity and repeatability for free lime quantification, and streamline laboratory workflows.

Methodology

Clinker samples were milled with 10 % binder and pressed into pellets at 10 t for 40 s. Five reference clinkers with known free lime levels (0.10 % to 2.78 % CaO) were analyzed. Diffraction data were collected at the d = 2.395 Å free lime peak, while simultaneous WDXRF measurements provided oxide compositions. Counting times ranged from 20 s to 60 s for XRD and 10 s–20 s per XRF channel.

Instrumentation

The ARL X900 Series spectrometer integrates a compact XRD system into a WDXRF goniometer. A single 2,500 W X-ray tube serves both XRF and XRD functions. Key features include a Moiré fringe positioning mechanism for precise angular repeatability and a crystal discriminator optimized for phase sensitivity.

Main Results and Discussion

• Diffraction patterns resolved free lime and alite (C₃S) peaks without fluorescence interference, enabling clear phase identification.
• Calibration produced a linear response (R² = 0.996) with a sensitivity of 774 cps/%, a standard error of estimate of 0.081 %, and a detection limit of 0.033 % CaO in 20 s.
• Ten successive analyses of a clinker sample yielded free lime repeatability with standard deviation of 0.018 % at 30 s counting time, outperforming traditional wet chemistry by an order of magnitude.
• Short-term stability tests demonstrated precisions of 0.007 % (60 s) and 0.012 % (20 s) for free CaO, confirming robust performance even under reduced measurement times.
• Simultaneous XRF data (SiO₂, Al₂O₃, Fe₂O₃, etc.) showed excellent repeatability (RSD < 0.02 % for major oxides), indicating that integrated XRD does not impair routine elemental analysis.

Benefits and Practical Applications

• Single-instrument workflow reduces capital and maintenance costs compared to separate XRF and XRD systems.
• Integrated analysis provides both elemental composition and phase quantification under identical conditions.
• Rapid measurement times support real-time process control and kiln optimization.
• Data integration in common software streamlines reporting and quality assurance.

Future Trends and Applications

• Extension to quantification of other clinker phases (e.g., C₂S, C₃A, C₄AF) and raw material minerals (quartz, limestone additions).
• Integration with automated sampling and process analytics for closed-loop control in cement plants.
• Application of advanced Rietveld refinement for comprehensive mineralogical profiling.
• Adoption in related industries such as ceramics, mining, and metallurgy for combined chemical-phase analysis.

Conclusion

The ARL X900 Series spectrometer demonstrates that integrated XRF-XRD analysis provides high-sensitivity, high-precision free lime measurements in cement clinker, while maintaining robust elemental analysis capabilities. This innovation simplifies laboratory operations, improves data reliability, and offers significant cost and energy savings for the cement industry.