Determination of Limestone in Cement Using XRF and XRD

There is an increasing demand and interest in the cement industry for monitoring limestone concentration, of which calcium carbonate (CaCO₃) is the major constituent. Recent European regulations allow the addition of limestone as filler up in concentrations up to 30%, based on the type of cement required. Hence, it is economically imperative to be able to rapidly control calcium carbonate concentrations in cement for ensuring the quality and conformity of the final product. This is possible using X-Ray Fluorescence (XRF), among other methods. However, XRF analysis is not directly correlated to a phase (e.g. CaCO₃). It offers only the total carbon concentration.

XRF analysis of carbon (CKα) is also subject to some difficulties, which include the following:

• The fluorescence yield of light elements like carbon is poor and, due to matrix absorption, the carbon fluorescence only escapes from a highly thin layer at the surface of the sample (about 0.2 um). This means that the volume of sample effectively measured for carbon analysis by XRF is very small.
• Surface contamination and addition of binding/grinding agents (which are usually organic materials, e.g. stearic acid) can produce inconsistent XRF results because of their carbon content. Binding agents are utilized to enhance the pellet stability under vacuum. Hence, sample preparation and homogeneity become very important factors in obtaining a precise analysis of the carbon content using XRF.
• Measurement of carbon by XRF ensures that all errors are multiplied by a factor of 8 when converting to limestone concentrations. On the other hand, the X-Ray Diffraction (XRD) technique is capable of analyzing only a specific phase (CaCO₃ in this case).

Furthermore, XRD intensities are not impacted by the factors mentioned above, due to the following factors:

• The high energy of the incident radiation used enables the analysis of a larger (about 10 times) volume of the sample than with XRF; this makes the XRD analysis more representative.
• Surface contamination, organic binders or grinding aids do not contain the CaCO₃ phase and therefore do not alter the limestone analysis.

Instrumentation and Samples

The Thermo Scientific ARL 9900 Series (Fig. 1) comprises a spectrometer that can be fitted with several XRF monochromators for major oxides analysis and a diffraction (XRD) system that has the capability of measuring free lime (CaO) and calcite (CaCO₃) phases. In addition, an XRF goniometer can be installed for qualitative or semi-quantitative investigations and sequential analysis of any of 83 elements of the periodic table. Hence, this instrument performs XRF and XRD analysis on the same sample with the same hardware and software environment. The diffraction system can perform qualitative scans and also quantitative analysis. This is possible by using the proven technology of Thermo Fisher Scientific, namely the Moiré fringe positioning mechanisms. Since the peak positions and backgrounds in XRD are sensitive to different parameters (e.g. grain size, matrix effects), peak search and peak integration can be performed for accurate analysis. However, in the following case study, peak intensities have only been used since no significant peak shifts have been observed. A series of industrial cement samples classified as grey and white cements as well as finely ground clinkers were used as powders. All samples were pressed at 15 t for 40 s without a binder.

Figure 1. Thermo Scientific ARL 9900 Series.

Results and Discussion

Figure 2 shows the XRD scans of three white cement samples containing different concentrations of CaCO₃. Two distinct peaks can be identified in each of the scans. The diffraction peak at 2.495 Å is assigned to calcite while the peak at 2.447 Å is attributed to the C₃S phase. The two peaks are well separated enabling quantitative analysis without a correction for overlap.

Figure 2. XRD scans on three white cement pellets containing different concentrations of CaCO₃.

Figure 3 presents the calibration curve obtained using the CaCO₃ peak intensity in a set of 6 white cement and clinker standards.

Figure 3. Calibration curve obtained using 6 white cement and clinker standards. Note that CaCO₃ peak intensity is used as measured (no background correction).

The regression results are summarized in Table 1.

Table 1. Regression results on white cements.

A standard error of estimate (SEE) of 0.17 % enables an excellent correlation between the nominal concentrations (expressed as CO₂) and the XRD intensities. Figure 4 shows another calibration curve produced with a set of 8 grey cement standards with the relevant parameters in Table 2. Again an SEE of 0.08% shows the quality of the regression and hence that of the analysis on the Total Cement Analyzer. Short term and long term stability tests were carried out on sample Cement 3. An average of 21 analyses (each for 100 s) gave the excellent standard deviation of 0.024 % at a level of 7.17 % CO₂ (CaCO₃ expressed as CO₂).

Figure 4. Calibration curve obtained using 8 grey cement standards also with peak intensities.

Table 2. Regression results on grey cements.

Conclusion

These results show that, using the diffraction system integrated into the ARL 9900, CaCO₃ (limestone) can be quantified with:

• Good sensitivity.
• Consistency.
• Excellent stability of analysis in cements.

This along with the previous report on free lime analysis in clinkers clearly shows that the monitoring of two major phases needed for quality control in cement plants can be performed using the same integrated diffraction system. The combination of XRF and XRD in the same instrument can provide complete quality control of clinker and cement. Separate instruments or methods are no longer needed resulting in significant savings from increased operator efficiency and lower running costs.

This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific - Elemental Analyzers.

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