Determination of limestone addition in cement manufacture

Importance of Topic

The accurate determination of limestone (calcium carbonate, CaCO₃) content in cement is critical for compliance with modern regulations that allow up to 30 % limestone addition in Portland limestone cement. Rapid and reliable measurement of the CaCO₃ phase helps cement producers control product quality, optimize raw material usage and reduce production costs.

Study Objectives and Overview

This application note demonstrates how an integrated XRF/XRD instrument (Thermo Scientific ARL 9900 Series) can be used to quantify limestone addition in cement and clinker samples. The goals are to establish calibration models, evaluate analytical performance and compare the combined technique with standalone XRF analysis of carbon.

Methodology and Instrumentation

The ARL 9900 Series combines an XRF spectrometer with interchangeable monochromators for major oxide analysis and an XRD system for phase-specific measurements. Key features include:

• Simultaneous XRF and XRD analysis on the same pelletized sample (15 t for 40 s, no binder)
• XRD detection of calcite (2.495 Å) and C₃S phase (2.447 Å) without peak overlap
• Goniometer option for qualitative/semi-quantitative XRF screening of up to 83 elements

Sample sets included eight grey Portland cement standards and six white cement or clinker standards covering a range of CaCO₃ levels. Peak intensities of the calcite reflection were used for quantitative calibration without background correction.

Main Results and Discussion

Regression of XRD peak intensity versus nominal CaCO₃ concentration yielded:

• Grey cements: standard error of estimate (SEE) 0.08 %, sensitivity 162 cps/%, detection limit 505 ppm.
• White cements and clinkers: SEE 0.17 %, sensitivity 131 cps/%, detection limit 645 ppm.

Short- and long-term repeatability for a 7.17 % CO₂ level showed a standard deviation of 0.024 % over 21 analyses, demonstrating excellent stability.

The note also highlights limitations of direct XRF carbon analysis:

• Poor fluorescence yield and shallow penetration (≈0.2 µm)
• Interference from surface binders and organic contaminants
• Error magnification when converting carbon to limestone equivalent

Benefits and Practical Applications

Using a single integrated ARL 9900 system for both phase-specific XRD and elemental XRF analysis streamlines quality control workflows in cement plants. Advantages include:

• Comprehensive monitoring of limestone content and free lime in clinkers
• Reduced sample handling and instrumentation costs
• Improved operator efficiency and lower total running costs

Future Trends and Potential Uses

Integration of XRF and XRD is likely to expand into other industries requiring phase-specific quantification (e.g., minerals, ceramics). Advances in detector technology and software algorithms may further enhance sensitivity, lower detection limits and enable real-time process monitoring.

Conclusion

The Thermo Scientific ARL 9900 Series XRF/XRD system provides a robust, sensitive and reliable solution for quantifying limestone addition in cement. Its combined capabilities eliminate the need for separate instruments, delivering significant time and cost savings while maintaining high analytical performance.

Reference

Thermo Fisher Scientific. Application Note AN41302: Determination of limestone addition in cement manufacture. 2022.