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Quantitative analysis of anhydrite


Anhydrite or anhydrous calcium sulfate (CaSO4) is an important naturally-occurring compound found in many limestone formations and karst topography. This material has recently gained more significance to the cement industry as a substitute for gypsum—calcium sulfate dihydrate (CaSO4·2H2O). Anhydrite shows great efficacy as a cement setting retardant. This allows valuable time for other phases to form which enhances the cement's tensile properties.


Implementation of the External Calibration Method for quantitative analysis on an anhydrite compound is simple using the MiniFlex. The measurement requires a single diffraction peak of the desired phase. Each of the overlaid peaks in Figure 1 took about two minutes to collect. As depicted in Figure 1, the weight percent of the unknown anhydrite pattern (magenta) was 4.2 %. A weight percent of 4.0 was expected. This test does not require any mixing of samples, digestions or pressing of pellets. Unlike elemental calibration curves, this method determines the exact compound; i.e. anhydrite, gypsum or bassanite (hemihydrate). Elemental analysis techniques such as ICP and XRF, only detect Ca and S, but cannot detect the correct form or phase of the compounds.


Figure 1: Using PDXL External Quantitative analysis of anhydrite (CaSO4), a weight percent of 4.2 was obtained.