Cement XRF application using a universal borate fusion methodology for ASTM C 114 & ISO/DIS 29581-2 qualification

Introduction

During the last 50 years, the cement industry has been relying on X-ray fluorescence spectrometry for the quantification of the elemental composition of cement industry-related materials. Sample preparation by pressed powders was commonly used during the first decades, but for the last 20 years borate fusion saw an important increase in popularity and both the pressed powders and the borate fusion are now accepted for those analysis^[1]. The 21st century saw a significant change in the management of the production of cement through the increase of production of cements with alternative raw materials and additives involving secondary fuels. This complication of the cement matrix and the use of spectrometer calibration reference materials from various sources in the world make the use of pressed powder more complicated due to the difficulty to matrix match the calibration standards and the production samples from the plant. In this new reality, the use of the borate fusion preparation allows for more accurate analysis and requires less calibration curves because this technique removes particle size and mineralogy effects^{[1, 2]}. For those reasons but also to facilitate the lab work, a single fusion method for the preparation of all cements, all process materials and a very wide range of raw materials is desirable, when combined with a wavelength-dispersive X-ray fluorescence (WDXRF), to allow compliance with the ASTM C 114 and ISO/DIS 29581-2 specifications of precision and accuracy. 

To reach these objectives, a robust analytical method using an automated fusion machine as sample preparation tool and a WDXRF spectrometer has been developed for the quantification of all elements of interest for the cement industry. This single method was used to prepare all cements, all process materials and a very large range of raw materials. Two sets of certified reference materials (CRMs), one from the National Institute of Standards and Technology (NIST) and the other from the Japan Cement Association (JCA) were used to verify that this fusion method allows a matrix match for cement from different origins. The evaluation of precision and accuracy was performed according to the instructions provided by two renowned international reference organizations, The American Society for Testing and Materials International (ASTM) and the International Organization for Standardization (ISO), through their respective standard methods for analysis of cement by X-ray fluorescence: ASTM C 114^[3] and ISO/DIS 29581-2^[4]. To further evaluate the robustness of the application, reference materials not included in the calibration were verified for precision and accuracy.

Experimental

Apparatus and instrumental conditions

A Claisse^® M4^TM propane fired automatic fluxer was used to generate all fusion disks. Its auto-regulating gas system and pre-set fusion programs allow for the most repeatable and reproducible fusion conditions while its oxidizing flames retain the volatile elements perfectly.

A Fisher Scientific Isotemp^® programmable muffle furnace was used for the LOI determinations and preparation of ignited samples. The LOI method used for all cement types and clinker included ignition at 950°C in a clean Platinum crucible for 60 minutes.

A sequencial WDXRF spectrometer with a rhodium end-window X-ray tube was used for data generation. A 28 mm collimator mask and vacuum were used for all measurements. Spectrometer analytical conditions, peak-line, background measurements, background position, pulse-height, counting time and others were selected and optimized by wavelength step-scanning of selected standard disks. The ISO validation test for repeatability of the spectrometer was used to verify proper spectrometer operation and to optimize the counting time for the peaks. Complete spectrometer and application settings are available in a previous publication^[5].

Global sample preparation method

During the development of this universal fusion method, many approaches were tested. Both ignited and non-ignited samples were assessed. Different flux and sample dispositions in the crucible and dry oxidation processes were also tried. In addition, different attempts were carried out to determine the best sample-to-flux ratio. Once this was found to be 1:10, different total amounts of sample and flux were tested. These attempts led to a final universal fusion protocol for the preparation of all cements and raw materials.

First 0.6000 g of ignited sample is weighed with ± 0.0001 g tolerance in a clean and dry Claisse^® Optimix^TM Pt/Au crucible. Then, 6.0000 g of Claisse® LiT/LiM/LiBr: 49.75/49.75/0.50, Pure Grade Flux is weighed with ± 0.0003 g tolerance on top of the sample. This particular flux was selected due to its more universal dissolving action and its higher homogeneity. A mini-vortex mixer is used to mix the sample with the flux. The mini-vortex mixer speed was controlled so as not to lose material, because variance from the ratio of flux to sample weight causes error in the results[6]. The maximum fusion temperature used for the fusion on the Claisse^® M4 fluxer is 1025 °C, because it is known that over the critical temperature of 1050 oC, flux begins to volatilize without consistency which changes the sample to flux ratio^[7]. Other compounds like SO3 begin to volatilize without consistency as well^[2]. Molten flux was poured in a 32 mm diameter, 1 mm thick mold.

Preparation for calibration, selection of control samples and preparation for validation

As discussed previously, one objective of this project was to calibrate the WDXRF with two sets of CRMs from different origins: NIST Standard Reference Material^® (SRM) Series 1880a, 1881a and 1884a to 1889a, and JCA Reference Materials for X-ray Fluorescence Analysis 601A Series XRF-01 to XRF-15. The second objective was to comply with the requirements of ASTM and ISO standard methods for analysis of cement. Those standard methods have two different philosophies. 

ASTM uses CRMs to verify precision and accuracy on two different days^[3]. ISO validates repeatability of the method using one or more CRMs, as control samples that are not included in the calibration over at least two weeks^[4]. An important thing to note is that for verification of ASTM requirements, results should include LOI, and for ISO, LOI free results are needed. Table 1 shows the element concentration range as oxide equivalent for the combination of the two sets. This table also shows the element concentration of the two control samples selected to evaluate the global borate fusion/XRF method with ISO standard method. Since two CRMs were used for the ISO validation, the selected samples had to cover both the high and low concentrations of all elements as prescribed in the standard method^[4].

For the calibration of the WDXRF instrument and for qualification of the Global Fusion/XRF method with ASTM Standard Test Method C 114, two sets of glass disks were prepared for every CRM, one on the first day and the second on the next day, not less than 24 hours apart. For the validation of the analytical method with ISO, 10 glass disks of each control samples (JCA XRF-03 and JCA XRF-14) were prepared over 15 days (not less than two weeks). The control sample glass disks were analyzed on the same day they were prepared.

Results and discussion

Robustness of the fusion method

It was determined that ignition of the sample is absolutely necessary in the analytical process for a global fusion method. This critical step allows fusion of the raw materials and some cements with additions, which are difficult or impossible to fuse in the non-ignited state with traditional fusion methods. A preparation with a sample to flux ratio of 1:10 with 6.6000 g of total mass takes a fusion program of 13 minutes heating at 1025 °C to prepare stable glass disks with high alumina and/or high silica samples. The cooling process is done with forced air for 5 minutes. 

To get an indicator of the robustness of this fusion methodology, more than 200 different samples from 20 different material types were fused with the global fusion method. The materials are listed in table 2. This list includes materials not commonly used as raw materials, but sometimes found in waste materials used as fuel, to test the limits of the global nature of the method.

The Global Fusion Method showed good efficiency to prepare homogenous and stable lithium borate glass disks with all of the materials except three: iron ores with high magnetite content, copper-rich iron ores and copper slags.

ASTM precision and accuracy

The ASTM precision test was applied as it is described in the method^[3]. The duplicates are the two disks prepared on two different days for every CRM. The results shown in table 3 are the largest absolute difference of the results of duplicate for all analyzed elements. The maximum difference for all elements is shown and compared to the ASTM precision limit. The maximum values obtained for all elements meet the specifications and well within the limits. 

The ASTM accuracy test was applied as it is described in the method^[3]. The results shown in table 4 are the largest absolute difference of the average of duplicates from the CRM certified values for all analyzed elements. The absolute maximum error for all elements is shown and compared to the ASTM accuracy limit. The maximum values obtained for all elements meet the specifications and well within the limits.

ISO precision and accuracy

It is important to note that the ISO limits for precision and accuracy are not a fixed limit as in ASTM C 114. The ISO limits are pending the concentration of the element in the samples analyzed. The ISO precision test was applied as described in the method^[4]. The absolute differences were calculated from the successive results of the control samples. The maximum absolute difference for all elements is shown in table 5 and compared to the ISO expert precision limit. The maximum values obtained for all elements meet the specified limits for both control samples.

The ISO accuracy test was applied as described in the method^[4], but without averaging the results of the different preparations. The accuracy values were calculated as difference of the results from the 10 preparations over 15 days against the certified values. The absolute maximum error for all elements is shown in table 6 and compared to the ISO expert accuracy limit. The maximum values obtained for all elements are in the requirement limits for both control samples.

^{A. Only the cements with known additions are listed here; the cement category probably included some cements with additions
B. The two slag samples that failed contained a higher level of copper
C. The iron ore samples that failed contained a higher level of copper}

^{A. Results of control samples are include in the calculation for Maximum Value}

^{A. Abs. Max. Error = Absolute Maximum Error
B. Results of control samples are include in the calculation for Abs. Max. Error}

Qualification of reference material not included in the calibration

The last step of the project was to use one set of CRMs for the calibration and then a different set of CRMs which were not included in the calibration, for the qualification of ASTM C 114. The same glass disks used for the calibration with both CRM series were used to investigate this point. The first test was to create a calibration including only NIST CRMs, then analyze JCA CRMs as unknowns and look for ASTM accuracy test results. The second test was to create a calibration including only JCA CRMs, and then analyze NIST CRMs as unknowns, looking to meet ASTM C 114 accuracy specifications. The outcome of this extra validation proved to be successful as all results are within the ASTM C 114 limits. Results are available in a previous publication^[5].

Conclusions

A cement XRF application using a universal borate fusion methodology for the characterization of cement industry materials was presented in this paper. Consisting in a single method of preparation by fusion, it allows fusing in a lithium borate glass disk the various cement types and all the raw materials normally found in a cement plant. Despite the versatility of the fusion method, the overall method still complies with the precision and accuracy requirements of the international standard methods for cement analysis (ISO/DIS 29581-2 and ASTM C 114). Moreover, qualification by ASTM C 114 of both complete series of reference materials (NIST SRMs and JCA CRMs) not included in the calibration was achieved, which is a step forward in quality control for chemical analysis in the cement industry. The complete scientific work and data set are available from a previous publication^[5]. That paper also presents an additional 6 elements that complement the traditional 8 elements analyzed in the cement industry.

References

1. ANZELMO, J. A., «The Role of XRF, Inter-Element Corrections, and Sample Preparation Effects in the 100-Year Evolution of ASTM Standard Test Method C114», Journal of ASTM International, Vol. 6, No. 2, Paper ID JAI101730, available online at www.astm.org, 2009, pp 1-10.
2. SPANGENBERG, J. and FONTBOTÉ, L., «X-Ray Fluorescence Analysis of Base Metal Sulphide and Iron-Manganese Oxide Ore Samples in Fused Glass Disk», X-Ray Spectrometry, Vol. 23, 1994, pp 83-90.
3. ASTM, Standard C114 - 08, “Standard Test Methods for Chemical Analysis of Hydraulic Cement”, Annual Book of ASTM Standards, Volume 04.01, ASTM International, West Conshohocken, PA, 2008, pp. 150–157.
4. DIN EN ISO 29581-2 (Draft standard, 2007-07), Methods of testing cement - Chemical analysis of cement - Part 2: Analysis by X-ray fluorescence (ISO/DIS 29581-2:2007), 30 pp.
5. BOUCHARD, M., ANZELMO, J. A., RIVARD, S., SEYFARTH, A., ARIAS, L., BEHRENS, K., DURALI-MÜLLER, S., “Global cement and raw materials fusion/XRF analytical solution”, Advances in X-ray analysis, Vol. 53, Proceedings of the 58th annual conference on applications of X-ray analysis (Denver X-ray conference), International Centre for Diffraction Data, ISSN 1097-0002, 2010, pp. 263-279.
6. BÉRUBÉ, L., RIVARD, S., ANZELMO, J. A., «XRF Fusion Precision with TheAnt», International Cement Review, March, 2008, 4 pp.
7. LOUBSER, M., STRYDOM, C., and POTGIETER, H., «A Thermogravimetric Analysis Study of Volatilization of Flux Mixtures Used in XRF Sample Preparation», X-Ray Spectrom. 2004; 33: 212–215, Published online 29 January 2004 in Wiley InterScience (https://onlinelibrary.wiley.com/). DOI: 10.1002/xrs.700