Reference Spectra for Environmentally Important Secondary Minerals: Ettringite (Ca6Al2(SO4)3(OH)12⋅26H2O) by XPS

Abstract

Ettringite, found in metamorphosed limestone and also as an important secondary mineral in concrete and certain combustion waste environments, plays an important role in concretion strength development, and in the control of pH and immobilization of trace elements in alkaline hazardous wastes. Consequently, the ability to identify ettringite via surface-sensitive spectroscopies is important for understanding cementitious mechanisms and the environmental behavior of wastes. We have used x-ray photoelectron spectroscopy (XPS) to study the binding energy of principal photoelectrons from an air-exposed external crystal surface of a well-formed needle-shaped ettringite sample from a bituminous limestone deposit in Asad, Israel. General survey and high resolution spectra were collected under standard operating conditions using a Perkin Elmer Physical Electronics 5200C spectrometer. Adventitious carbon was used as the energy referencing method. Charge-corrected binding energies were identified for the Ca 2p3/2, Ca 2p1/2, Al 2p, S 2p3/2, S 2p1/2, and O 1s photoelectrons. The mineral contains three oxygen species. Probable loss of waters of hydration resulted in the determination of binding energies for only the hydroxyl and sulfate oxygens. Nominal binding energies for all three species are also provided. These spectra are useful in using XPS to identify ettringite in leaching environments of combustion residues where the secondary mineral precipitated at ash particle surfaces.