Date of Award

5-2012

Document Type

Thesis

Degree Name

Master of Science (MS)

Legacy Department

Civil Engineering

Advisor

Rangaraju, Prasad R

Committee Member

Klotz , Liedy E

Committee Member

Pang , Wei C

Abstract

As biomass power production becomes more main stream, problems with waste disposal arise. Coal fly ash, a by-product of coal power production, has beneficial strength and durability properties when used as a cement replacement material. This study examined strength and durability characteristics of mortar and concrete samples containing biomass fly ash obtained from Craven County Wood Energy in North Carolina and examined methods of processing the material to enhance its pozzolanic characteristics.
The chemical and mineralogical compositions of the ash were determined using X-ray fluorescence (XRF), X-ray diffraction (XRD), and energy dispersive X-ray spectroscopy (EDS). The compositions met the requirements outlined by the ASTM C618 specification, the Standard Specification for Coal Fly Ash and Raw of Calcined Natural Pozzolan Use in Concrete, with the silica being the most abundant compound. The measured loss on ignition was greater than that allowed by the ASTM C618 specifications. The microstructure of the ash was observed using a scanning electron microscope (SEM). As-received, the biomass fly ash had a high carbon content and required further processing. To determine the optimal method of processing the ash, mortars were created using different ash replacement levels, varying the temperatures at which the ash was fired, and varying the grinding regime. Compressive strength tests and thermogravimetric analyses (TGA) were used to compare the samples. It was found that a 20 percent replacement level and a 30 minute grinding time produced the most favorable results. Further studies are required to determine the optimal firing temperature. The TGA showed only a slight reduction in Ca(OH)2 indicating minimal pozzolanic reactivity. This means the compressive strength results could be the result of the filler effect rather than pozzolanic reactivity. Reducing the firing time may improve the pozzolanic reactivity of the biomass fly ash, although further studies would be required to confirm this hypothesis.
Concrete properties were measured using compressive strength tests and rapid chloride permeability tests. Concrete cylinders were cast using a biomass fly ash replacement level of 20 percent, grinding the ash at 250 rpm for 30 minutes and firing the ash at 950 oC for one hour. The samples containing biomass fly ash produced a compressive strength 77 percent that of the control and biomass fly ash did not improve the permeability.
This study found that additional processing was required to remove excess carbon from the ash. This trend may not be true of all biomass fly ash samples. Tests should be performed on biomass fly ashes from many different power plants to determine the impacts of different maximum temperatures, cooling rates, fuel sources, and burning efficiencies. One of the first tests on new ash samples should be an XRD to determine if a glassy form of silica is present. The XRD results from the Craven County biomass fly ash used in this study showed no glassy silica, and no amount of processing could make the quartz reactive enough to be useful as a supplementary cementitous material.

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