Mineralogical Investigations of High-Lime Fly Ashes

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Title: Mineralogical Investigations of High-Lime Fly Ashes

Author(s): M. Tokyay and F. H. Hubbard

Publication: Special Publication

Volume: 132

Issue:

Appears on pages(s): 65-78

Keywords: anhydrite; chemical analysis; clay minerals; ettringite; free lime; gypsum; hematite; hydration; lime fly ash; mineralogical analysis; pozzolanic reactions; quartz; x-ray diffraction; x-ray fluorescence; Materials Research

Date: 5/1/1992

Abstract:
High-lime fly ashes obtained from the combustion of lignites or subbituminous coals are common by-products of thermal power plants in many countries, including Turkey. Chemical analyses, mineralogical analyses, and the formation of hydration and pozzolanic reaction products at different ages of three Turkish high-lime fly ashes were carried out using X-ray diffraction (XRD) techniques. The relationships between the mineral phases in the fly ashes and the hydration and pozzolanic reaction products were investigated. Fly ash is formed at combustion temperatures of approximately 1000 C, at which the clay impurities decompose. These fly ashes contained highly reactive silica and alumina. The reaction of these oxides with the free lime and anhydrite present in two of the fly ashes led to the formation of C-S-H gel and ettringite starting with the beginning of hydration. The third fly ash, having anhydrite as the only major calcium-bearing compound, produced gypsum upon hydration. However, introduction of Ca(OH)2 into the system resulted in similar reaction products. At later ages, beside the previously mentioned products, C4ACH11 and C4AH13 were also observed in all three cases. Interpretation of the results indicated that although all three fly ashes were of high-lime type, two of them were hydraulic and autopozzolanic, whereas the third was pozzolanic only.