The data evaluation with Rietveld analysis software starts with a simulation of a diffraction pattern calculated from the crystal structures chosen in advance. By an automatic variation of predefined parameters the contribution of each phase is fitted to the measured pattern, until a best possible agreement between the calculated and the measured pattern is achieved. The quantitative phase composition is obtained as one of the refinement results, and is always normalised to 100 %. For the sake of the robustness of the evaluation, it could be advantageous to disregard accessory mineral phases, or to restrict the variation of structural parameters (e. g. crystallinity, lattice spacing) to avoid errors due to wrong overlay corrections. On the other hand, for some phases different crystal modifications or textures must be taken into account.
X-ray amorphous components like slags can be described using a pseudo-structure that fits the so-called “glass hump”, caused by the non-crystalline slag in the diffraction pattern background. Regular plant and cement type-specific definitions of the evaluation routine are necessary for reliable results.
For a comparative analysis, which is the common aim of production control, it is absolutely necessary to always use the identical evaluation routine. In this case, reproducibility scores as shown in Table 1 for a CEM I can be achieved.
Table 1: Reproducibility of the preparation, measurement and evaluation of XRD analyses on Portland cement.
Each automated evaluation routine has to be validated when the source of the raw material changes. With limestone or slag the pre-defined routines might also work with the new material, in the case of fly ash an adjustment of the evaluation is usually required. The same applies to changing the relative fineness of the cement main constituents. At constant preparation, measurement and evaluation conditions, an increase of e.g. the slag fineness of 200 cm2/g acc. Blaine could raise the determined slag content by 0.5 to 1.0 mass %.
Reliability in production
The suitability of automated QXRD is demonstrated by the application of pre-defined evaluation routines on production control data. In a research programme, data from extended production periods as well as from switchovers of cement types on a ball mill were analysed. In the latter case the point in time at which the requirements of the standard are met can be clearly defined.
QXrD in quality control
QXRD can accordingly be used in many cases for factory production control acc. EN 197-2 and could also be accepted by a third-party inspector. This does however pre-suppose a regular check-up of the processes on site by personnel experienced with the method and the performance of cross-validating tests with the standard reference method. Furthermore, agreement on and/or a factory-specific adaptation of the evaluation files for single cement types would be necessary in this case. Changes in the production processes generally cannot be accommodated. The use of different raw materials or production processes for one cement type may mean that third-party inspection via QXRD cannot be performed. As a general rule, the use of QXRD in third-party control requires close cooperation with the factory being inspected.
This is part 1 of a 2-part article that was originally published in Newsletter 3/2015 of the European Cement Research Academy and is reproduced by kind permission of ECRA.
Read the article online at: https://www.worldcement.com/special-reports/18092015/quantitative-xray-diffraction-analysis-cement-production-control-2/
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