CHARACTERIZING DIFFERENCES IN AGGREGATE BEHAVIOR FROM THE CLAY-SIZED FRACTIONS OF STABILITY FILLING MATERIAL USING A NOVEL COMBINATION OF X-RAY POWDER DIFFRACTION AND ATTENUATED TOTAL REFLECTION FOURIER TRANSFORM INFRARED SPECTROSCOPY

Aggregate fill material is a major mining product commonly used during construction activities to provide a ground surface that meets specifications for strength, stability, and drainage. Two sources of aggregate quarried from the Skippers Granite near Skippers, VA (sample A), and the Petersburg Granite near Richmond, VA (sample B) were used as engineered fill at a local project. Both engineered fill materials met the geotechnical grading requirements for grain size percentages and were classified as ‘granite’ by the vendors, but field observations showed that the two materials behaved differently when applied at the site. In this study we compare the mineralogy of the fine components of the aggregates to evaluate if this fraction could affect suitability of each fill material. Two sieved splits (<425 µm and <75 µm) of sample A and B were tested. X-ray diffraction (XRD) patterns were obtained using the Malvern PANalytical Empyrean X-ray powder diffractometer at James Madison University (JMU). Infrared spectra were obtained at 2 cm^-1 resolution using the ATR accessory and DTGS detector on the Thermo-Nicolet iS50 FTIR spectrometer at JMU. Illite, chlorite, montmorillonite, and kaolinite standards were analyzed for comparison to the aggregate splits. XRD confirms the sample A has a typical mineralogy of a granite but the sample B shows a more intense biotite peak consistent with it being a mixture of a granite and mica-schist xenoliths. The XRD patterns of the fine grain size fractions from both of the aggregates include a broad peak at 6° 2θ related to montmorillonite-chlorite interlaminated clay and a low-intensity peak observed at 10° 2θ in all XRD patterns assigned to actinolite. The FTIR-ATR spectra showed distinct differences in both the silicate structural and interlayer water regions, indicating a mix of illite, chlorite, and possibly minor montmorillonite in the sample B and illite with minor amounts of chlorite and possibly amphibole or mica in the sample A. These results show that differences in the clay-fraction mineralogy could explain differences in macroscopic properties between the two aggregates, and that combined XRD and ATR data can be applied to characterize engineering fill materials.