Myneni – Probing the Amorphous and Nanocrystalline Aluminum Silicate Hydrate Phases in Alkaline Systems
Satish C. B. Myneni
M 51, Guyot Hall, Department of Geosciences, Princeton University, Princeton, NJ 08544
Amorphous and poorly crystalline phases are common in most natural samples, and these act as metastable phases before they convert to more stable crystalline forms. The amorphous phases form rapidly in systems where precipitation rates are high, and in solutions containing polyvalent ions, such as Al3+, Fe3+, and Si4+. Their particle sizes also tend to be smaller (mostly into nanoscale), and their X-ray diffraction patterns are either weak or absent. The stabilities of these phases also vary significantly as a function of chemical composition and hydration state, and temperature. These features make the characterization of amorphous hydrated phases challenging.
The amorphous phases of Al and Si are expected to be much more common in cement hydrates because of the common presence of oversaturated solutions (e.g. for Al3+, Ca2+), and the strange geochemical behavior of Si and Al in alkaline systems. While Si and Al exhibit tetrahedral and octahedral coordination, respectively, in most ambient pH conditions, these elements show the contrasting character in alkaline systems. In addition, they interact with each other strongly and form gels or amorphous phases with other cations and anions, such as Ca2+ and SO42-. Because of the poorly diffracting nature of these materials, their characterization, especially with respect to the chemical and structural heterogeneity and chemical evolution, is tricky. This limits the geochemical modeling of alkaline systems. However, recent developments in synchrotron based soft and hard X-ray spectroscopy, spectromicroscopy and high-resolution imaging capabilities can help in understanding the nature of these amorphous materials in alkaline systems. My presentation will focus on the nature of Al and Si in alkaline systems, formation of amorphous phases, and how new X-ray imaging and spectroscopy tools can be used to study these phases.