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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:
Furthermore, XRD intensities are not impacted by the factors mentioned above, due to the following factors:
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.
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.
These results show that, using the diffraction system integrated into the ARL 9900, CaCO3 (limestone) can be quantified with:
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.
For more information on this source, please visit Thermo Fisher Scientific - Elemental Analyzers.
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