Multitechnique Approach to Understanding the Microstructure of Cement-Based Systems
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*Gill Chair of Analytical Chemistry, Lamar University, Beaumont, TX 77710 "**VisitingProfessor, Department of Chemistry, Dhaka University, Bangladesh
ABSTRACT
The chemistry of cement, its hydration and the development of microstructure in cementbased systems is extremely complex, and it becomes even more complex in the presence of additives. The elucidation of the mechanisms of these processes is a challenging problem and requires the applications of multiple techniques including the latest microscopic methods. The applications of molecular spectroscopies, surface spectroscopies and microscopies have helped develop models and mechanisms for the retardation of cement setting by Zn, Cd and Pb, the chemical and structural effects of superplasticizers, and the interaction of hydrating cement with aggregates, selective sorbents and fillers. The results of these studies indicated that the inhibition of hydration is controlled by dispersion of various charges present in hyperalkaline solution in cement paste. According to this charge dispersal model, the Ca2+ions from initial hydration form a tightly-bound bilayer with the negatively charged C-S-H surface. Consequent to this intrinsic process, the metalhydroxy or superplasticizer anions immediately surround the bilayer to constitute a trilayer which inhibits further hydration. INTRODUCTION
The hydration of ordinary Portland cement (OPC) and subsequent development of microstructure involve a series of competing chemical reactions. The mechanisms of these reactions are complex and not yet fully understood. Some of the characterization techniques that have been used for studying the chemistry of cement-based systems include [1]: X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), ion scattering spectroscopy (ISS), X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), energy dispersive spectroscopy (EDS), Fourier transform infrared
spectroscopy (FTIR), Raman spectroscopy and
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Si solid state NMR [2-4] employing cross-
polarization and magic-angle-spinning techniques. In the present article we are presenting results
from our laboratory concerning the application of FTLR, XRD, XPS and SEM/EDS techniques to characterize cement-based systems involving metals, fly ash, and superplasticizer. CHARACTERIZATION TECHNIQUES X-ray Photoelectron Spectroscopy (XPS) The XPS technique is recognized as one of the most important surface sensitive tools. This technique probes the surface core-level electronic states of atoms in the near-surface region (sampling depth -50 A) and can provide qualitative chemical state and semi-quantitative information about these surface and near-surface regions [3-5]. In this method, the usual practice is to measure the energy, known as the binding energy (BE), by which the photoelectron had been bound to its parent atom. The binding energies of the electrons are then compared with those of the pure elements or compounds and thus the changes in their chemical environment are
reflected in BE s
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