Mineralogical characterization of coal fly ashes via Raman imaging

Witte, Andrew Christopher

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https://hdl.handle.net/2142/122270 Copy

Description

Title

Mineralogical characterization of coal fly ashes via Raman imaging

Author(s)

Witte, Andrew Christopher

Issue Date

2023-12-06

Director of Research (if dissertation) or Advisor (if thesis)

Garg, Nishant

Department of Study

Civil & Environmental Eng

Discipline

Civil Engineering

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

M.S.

Degree Level

Thesis

Keyword(s)

• Coal Fly Ash
• Raman Imaging
• Raman Spectroscopy

Abstract

Coal fly ashes have always been an integral component of cementitious systems due to their myriad benefits in enhancing concrete’s workability, strength, and durability. However, these ashes are quite complex, consisting of a mixture of crystalline and amorphous phases, leading to unpredictable behavior. The ability of a pozzolan to release silica into the pore solution to react with calcium hydroxide is the primary way of measuring pozzolanic effectiveness, and this ability is itself governed by glassy dissolution. Thus, a comprehensive mineralogical understanding of fly ashes and the siliceous pozzolanic glasses they consist of is invaluable in evaluating performance and understanding fly ash reactivity. This thesis aims to apply Raman spectroscopy and imaging to the mineralogical study of coal fly ash by 1) identifying the Raman spectra associated with coal fly ash glasses, and 2) quantifying and spatially mapping the glass present in various coal fly ashes. Regarding the first goal, individual particles from a set of 18 unique fly ashes were analyzed at the individual particle level via Raman spectroscopy and Scanning Electron Microscopy with Energy Dispersive X-Ray Spectroscopy (SEM-EDS). Firstly, there is a distinct correlation between particle shape (roundness/circularity) and degree of crystallinity (FWHM of Raman peaks) where jagged particles happen to be almost always crystalline. Secondly, the position of the symmetric stretching Raman band of the silicate tetrahedra (between 700-1100 cm -1) is an indicator of the degree of polymerization of the glassy phase in any given particle. These results highlight the value of understanding these complex systems at the individual particle level when comparing techniques. Regarding the second goal, Raman image scans were performed: two large area epoxy scans and nine large area powder scans on ten distinct coal fly ashes. A python protocol was developed to analyze the large quantity of spectral data and directly categorize glassy spectra based on the Raman band location. This python imaging protocol, specifically meant to accommodate glassy phases, was compared to proprietary WITec Project FIVE software. Based on the result, it was found that glassy phases, which shift wavenumbers positions gradually/in a gradient like fashion, are better identified with the python SNR protocol developed as opposed to a basis analysis. Furthermore, after generating glassy band location histograms, it was observed that the ratio of Q0 glass to the total Raman active glass content is not directly related to the calcium content of an ash, but can vary for ashes with similar calcium contents. The reduction in total glass content after the dissolution of one class C ash was also observed. Finally, the Raman imaging python protocol was applied to a large area scan of a coal bottom ash. The results showed high agreement with the proprietary software for crystalline phases as well as epoxy. In imaging glass, however, the python protocol differed significantly from the proprietary software, likely still due to the limitations of basis analysis as it is related to imaging amorphous Raman phases. Raman spectroscopy was validated to be a promising tool for a more comprehensive understanding of silicate glasses in cementitious pozzolans. The author expects that it will not only be used to evaluate existing pozzolans but also to better understand new and upcoming materials.

Graduation Semester

2023-12

Type of Resource

Thesis

Copyright and License Information

Copyright 2023 Andrew Witte

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