Using XRF Techniques for Quantitative Analysis of Clinker Phases

The cement industry faces a very interesting challenge in terms of quantitative analysis of clinker phases. Aside from traditional chemical analysis with XRF techniques, the role of XRD has been highly solicited for analyzing phases or minerals.

When it comes to controlling the kiln process and the quality of the end product, analyzing phases such as free lime in clinkers and limestone additions in cement by XRD, respectively, has been very useful.

Over the past decade, XRD has substituted traditional wet chemical or other methods in many cement laboratories. To address clinker phase analysis, a study was performed to introduce cost-effective, reliable, and rapid methods in real-time kiln conditions and examine the potential use of XRD as a substitute for microscopy or other indirect methods of calculation.

Instrument and Sample Preparation

For this study, a Thermo Scientific ARL 9900 Series was used. The instrument features a series of XRF fixed channels (monochromators), an XRF goniometer, and an XRD goniometer. This latter goniometer is integrated so that with the same tube conditions and under a vacuum, both XRD and XRF measurements can be done on the same sample.

A series of 30 clinker samples were collected over the production time. To obtain quantitative data for C₃S, C₂S, C₃A and C₄AF phases, microscopy measurements were done in parallel on the 30 samples.

A set of samples were chosen so that a reasonable dynamic range (working range) for concentrations of the four clinker phases could be obtained. Before being measured by the ARL 9900 Series Total Cement Analyzer, these clinker granules were ground and pressed into pellets.

During the normal microscopic observation of clinkers, the presence of a vitreous interstitial phase, which makes it difficult to differentiate the ferrite and the aluminum oxide, is typically the biggest challenge to overcome. This does not happen with the study samples since the clinker samples are cooled slowly, allowing for C₃A crystallization.

On the other hand, the microscopically determined interstitial phase (alumina and ferrite) is always smaller than that which is calculated in Bogue formulæ, thanks to the assumption that all the Al₂O₃ and Fe₂O₃ contribute to the formation of C₃A and C4AF, without consideration of the possibility that part of those oxides can form solid solutions with silicates.

It takes less than 4 minutes to complete the total analysis using the compact integrated XRD system for free lime and the four clinker phases. Peak intensities are used directly in this process without corrections.

Results & Discussion

Table 1 demonstrates the results of chemical analysis (total oxide concentration) obtained by the XRF aspect of the instrument. Using Bogue formulæ, these concentrations are used to calculate the equivalent concentrations of clinker phases:

%C₃S = (%CaO - %CaO free) x 4.07 - (%SiO₂ x 7.6 + %Fe₂O₃ x 1.43 + %Al₂O₃ x 6.72)

%C₂S = %SiO₂ x 2.87 - C₃S x 0.75

%C₄AF = %Fe₂O₃ x 3.04

%C₃A = %Al₂O₃ x 2.65 - %Fe₂O₃ x 1.69 

Table 1 displays the results of the clinker phases gathered using the compact XRD system combined in the ARL 9900 compared against the values obtained from microscopy and the Bogue calculations.

Table 1. XRF analysis of clinkers. Source: Thermo Fisher Scientific - Elemental Analyzers and Phase Analyzers

Figure 1. Calibration curve for alite (C₃S) in a series of industrial clinker samples using the Integrated XRD system. Image Credit: Thermo Fisher Scientific - Elemental Analyzers and Phase Analyzers

Figure 2. Calibration curve for belite (C₂S) in a series of industrial clinker samples using the integrated XRD system. Image Credit: Thermo Fisher Scientific - Elemental Analyzers and Phase Analyzers

Figure 3. Calibration curve for aluminate (C₃A) in a series of industrial clinker samples using the Integrated XRD system. Image Credit: Thermo Fisher Scientific - Elemental Analyzers and Phase Analyzers

Microscopy values and XRD results obtained with the ARL 9900 TCA have a strong correlation, while the Bogue data can create a number of errors in comparison to microscopy, as is shown in Figure 4. The very poor correlation between microscopy as a reference and Bogue calculation for C₃S obtained using the data in Table 2 is displayed in Figure 5.

Figure 4. Microscopy vs Compact XRD for C₃S: good correlation. Image Credit: Thermo Fisher Scientific - Elemental Analyzers and Phase Analyzers

Figure 5. Microscopy vs Bogue XRD for C₃S: no correlation. Image Credit: Thermo Fisher Scientific - Elemental Analyzers and Phase Analyzers

Table 2. Quantitative XRD analysis of free lime and clinker phases in comparison with microscopy and Bogue calculations. Source: Thermo Fisher Scientific - Elemental Analyzers and Phase Analyzers

Conclusion

Within this case study, quantitative results were compared and obtained under three methods, including Bogue formulæ, microscopy and compact Integrated XRD using an ARL 9900 Series Total Cement Analyzer from a series of clinker samples.

It must be noted that the results from Bogue are not as anticipated. It is shown by the comparison between microscopy and quantitative XRD data that it is possible, in real process conditions for clinker phases analysis, to exploit the unique integrated compact XRD system of ARL 9900 Series.

Such XRD analysis can help predict the 3-day resistance to compression of the future cement obtained from these clinkers, as C₃S phase provides resistance to cement, while other factors can influence 7-day resistance and 28-day resistance like fineness and particle size distribution, for instance.

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

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