Quantification of Gypsum in Soils via Portable X-ray Fluorescence Spectrometry

Abstract

Abstract This study explores the potential of X-ray fluorescence (XRF) as a rapid, nondestructive, and cost-effective technique for in situ sulfate quantification. Gypsum, the main source of sulfate in soils, reacts with calcium-based stabilizers to form expansive minerals, which reduces the long-term strength of the treated soil. Therefore, accurate detection of sulfate content prior to employing calcium-based chemical stabilization is important to mitigate the possibility of expansive mineral formation and ensure acceptable engineering behavior of the stabilized soil. Portable handheld XRF (PXRF) has shown the ability to estimate gypsum content accurately, utilizing calcium as a proxy. However, detecting sulfur, in the form of sulfite or sulfate remains challenging due to device sensitivity limitations. This research aims to address this limitation and develop a method for direct sulfur detection, enhancing the utility of PXRF for in situ sulfate quantification. Laboratory standards were created with known amounts of gypsum and portions were sent to a commercial laboratory for whole rock analysis. The remainder of the reference standards were used to calibrate several soil library standards within the PXRF. The calibrated PXRF was able to accurately detect the anhydrous form of gypsum, anhydrite (CaSO4), in these reference standards for contents ranging from 0 to 8 %. The proposed XRF-based approach offers the potential to revolutionize sulfate detection in soils, providing a rapid and reliable tool for assessing soil stability and optimizing chemical stabilization efforts. By enabling real-time, on-site analysis, this method holds promise for improving construction practices and reducing the risk of structural damage associated with soils containing sulfate-bearing minerals.