Usability of field X-ray fluorescence analyzers for determination of the chemistry of cement raw materials - using an example of Mokrá Quarry

Handheld X-ray spectrometers for field analysis have already become devices commonly used in environmental sciences to analyse a relatively broad spectrum of chemical elements, for example, in recent sediments and soils. The article describes the possibility of utilizing such instruments for the evaluation of cement raw materials, using as examples limestone and shale mined in Mokrá quarry in the southern part of the Moravian Karst. Measurement results using the "outdoor" XRF Analyser DELTA PREMIUM (manufactured by Olympus Innov-X Systems, Inc.) were compared with the data obtained from significantly more sensitive and accurate method ICP-OES/MS in Acme Analytical Laboratories Ltd (Vancouver, Canada). This comparison of data has shown that the use of XRF analyzer in "GEOCHEM" mode makes a relatively accurate quantitative determination of Al, Si, P, S, K, Ca, Ti, Mn and Fe possible in the studied material, using the following procedure. Measurements by XRF analyzer on essentially flat (fracture) surfaces and surfaces of the cut plates showed that although there are differences in the results of analyses carried out repeatedly under identical conditions on the same areas, the differences are not substantial (Fig. 1). On very rugged surfaces, the measurements are not comparable. The analyzer was used to determine content of the above elements in 126 samples of pulverized limestone and shale. Before the measurements, about three grams of "powder" were placed on the circular polyethylene pad (thin foil) and, with a cylinder of about 25 mm diameter, were compressed by hand to the "tablet", which was transferred to the measurement window of the analyzer together with the foil. In the case of 15 pulverized samples analyzed by ICP-OES/MS method in the Acme laboratories and by the XRF analyzer, differences were found in the element contents determined by both methods. In the case of aluminum, these differences are documented in Fig. 2. A part of the figure is the polynomial function that allows conversion of Al2O3 content found by the XRF analyzer to Al2O3 found in Acme. Analogous conversions were performed for the following elements: Al, Si, P, S, K, Ca, Ti, Mn, Fe, Cu, Zn, As, Rb, Sr, Zr, and Pb (coefficient of determination, R2, is always very high - 0.981 to 1.000). The contents of other elements determined in the "GEOCHEM" mode were mostly below the detection limit or just above. This is also true of magnesium, which was determined in a concentration of 0.58 to 3.25 wt. % of MgO in the group of samples analyzed in Acme. The recalculated results of 126 analyzed samples from Mokrá quarry are summarized in Tab. 1, which includes only the elements exceeding 1 wt. % at least in some samples, and also manganese and sulfur. Fundamental differences in the chemistry of limestone and shale are quite evident, as well as the differences between limestones and shales from different strata.