Analysis of Cement According to ASTM C114

Cement analysis in a production environment is one of the most vital applications of X-ray fluorescence (XRF) spectrometry. Generally, wavelength dispersive instruments are preferred because of their accuracy, speed, precision, and long-term stability.

In recent times, an increasingly number of Energy-Dispersive XRF (EDXRF) instruments have also been installed. In fact, in large cement industries, EDXRF is usually set up as a backup of Wavelength-Dispersive XRF (WDXRF), while smaller industries may use EDXRF as their only XRF for production control.

ASTM C-114 Requirements

Undoubtedly, ASTM C-114 is the most commonly used standard test method for assessing the performance of XRF instruments (and other analysis techniques) for cement analysis. This standard defines a very stringent protocol for evaluating both the precision and accuracy of the method while setting rigorous performance requirements.

The protocol requires proper calibration of an XRF instrument after which it is used for analyzing a set of at least seven reference cements — preferably NIST certified reference materials (CRMs).

On different days, two rounds of analyses are to be completed by repeating all the sample preparation steps. It is also necessary to calculate the differences between values and averages of the values from the two rounds.

When seven CRMs are used —– as is the case in this article — at least six of the seven differences acquired for any single analyte should not surpass the limits indicated in column 3 of Table 1 and the remaining differences by no more than twice that value.

Table 1. Maximum permissible variations in results according to ASTM C-114

Likewise, at least six of the seven averages for each analyte should not differ from the certified concentrations above the value indicated in column 4 of Table 1, and the remaining average by more than twice that value.

Instrumentation

The ARL QUANT’X™ XRF spectrometer from Thermo Scientific™ is an EDXRF system that offers an economical and rapid analytical capability. It is equipped with an air-cooled Rh end-window tube with thin Be window of 0.05 mm thick and has the highest power of 50 W. An electrically cooled silicon drift detector (SDD) with an area of 30 mm² is equipped in the ARL QUANT’X spectrometer. In total, nine primary beam filters are provided in the instrument guaranteeing that an optimal excitation condition is always found. For unattended analysis, an optional 10 position sample changer is provided.

Excitation Conditions

To cover all the analytes listed in Table 1, two excitation conditions are used. One condition at 4 kV without a filter to activate all light elements from sodium up to sulfur and a second condition at 16 kV with a thin Pd filter for all the remaining analytes. A total live time of 300 seconds was used to complete a single sample analysis. All measurements were carried out in vacuum. An outline of the conditions is given in Table 2.

Table 2. Excitation conditions

Sample Preparation

Seven cement CRMs of the 18xx series (NIST SRMs 1880a, 1881a, 1884a, 1885a, 1887a, 1888a, and 1889a) were prepared in the form of pressed powders. Using a puck and ring mill, Cement CRMs are ground for 90 seconds and pressed onto a boric acid backing at 20 tons to create a pellet of 32 mm Ø.

Results

An outline of the results obtained for the different cement CRMs is given in Table 3a and 3b. Table 3a compares the variation between duplicates with the maximum permissible value, while Table 3b compares the variation between the average of duplicates. The results obtained for most of the analytes fulfill the ASTM C-114 requirements. Table 3b shows one value for SiO₂ which varies more than the maximum difference listed in Table 1. While this value was obtained for NIST SRM 1887a, it is still within the rounding error and less than twice the permissible value. Therefore, requirements according to the ASTM C-114 test method are still met for this analyte.

Table 3a. Difference between values obtained on day 1 and 2

Table 3b. Difference between averages of values and certified values

Detection Limits

Shown in Table 2 are the detection limits that can be obtained with the ARL QUANT’X spectrometer for the different analytes in a cement matrix. The limits are reported for excitation conditions and measurement times as used in this article to meet the ASTM C-114 standard (Table 2).

Table 4. Minimum detection limits

Conclusions

The results demonstrated in this article show that a compact EDXRF instrument such as the ARL QUANT’X spectrometer is capable of meeting the ASTM C-114 requirements for cement analysis. They also demonstrate that analysis times can remain short without affecting performance.

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.