WATER REPELLENCY EFFECT ON UNSATURATED PROPERTIES OF COMPACTED COAL COMBUSTION RESIDUALS

1 online resource (252 pages) : PDF

2019

University of North Carolina at Charlotte

The generation of electricity from coal-fired power plants continue to be a mainstream approach. It is estimated that about 107 million tons of coal combustion residuals (CCRs) were generated in 2017 in the US (ACAA 2018). This equal amount of CCRs requires safe and cost-effective management approach, including disposal and reuse. This dissertation focused on engineered water repellency of CCRs for unencapsulated beneficial reuse in civil engineering applications. Unencapsulated beneficial use of CCRs in geotechnical and geoenvironmental engineered systems such as structural fills/embankments, and liner and capping systems continue to see a downward decline with the recently implemented CCR final rule by the U. S. Environmental Protection Agency in 2015. Engineered water repellency has been identified as an alternative approach to control water infiltration and leachate generation in CCRs as it reduces water interaction with CCRs. The motivation of this study derives from the potential of engineered water repellent CCRs to contribute towards a more effective management of CCRs used in engineered systems while also serving as an infiltration control system. Specifically, this research investigates the use of organosilane (OS) chemicals towards achieving engineered water repellent compacted CCRs to be used in the aforementioned unencapsulated geotechnical and geoenvironmental applications. Additionally, the study focused on assessing any effects of the OS-based engineered water repellency on selected engineering properties of the OS-treated compacted CCRs. This dissertation experimentally evaluated the effect of OS-based engineered water repellency on select unsaturated engineering properties of compacted CCRs. The main objectives were to correlate applicable engineering properties of CCRs to unsaturated functions of compacted untreated CCRs, study the influence of soluble salts on the engineering performance of compacted select CCR; implement a treatment protocol by understanding OS treatment of CCRs by batch sorption approach while considering practical implementation of engineered water repellency; and assess the effect of engineered water repellency on unsaturated functions of compacted CCRs. A comprehensive experimental program was developed to address each of the research objectives. The experimental program involved five CCRs, including three Class F fly ash, one lignite fly ash, and one flue gas desulphurization (FGD) gypsum, all treated with an OS water repellent chemical. The selected OS chemical was based on results from a recent dissertation of our UNCC research group. The unsaturated functions of the compacted CCRs were determined by fitting closed-form equations to laboratory-measured water retention characteristics (WRC) of the compacted CCRs using pressure plate extractor (PPE), Tempe cell (TC), and the unsaturated flow apparatus (UFA) method, and dewpoint potentiometer (WP4C). The first three methods measure matric suction, and the WP4C measures total suction. Additionally, leaching program was performed to reduce the salt contents of a select CCR and evaluated the influence of salt contents on the total suction and related osmotic suction, and on the engineering performance of compacted CCR. The batch sorption approach was used to treat the CCRs for varying degrees of water repellency while changing variables, including CCR material, OS dosage, reaction time, and drying conditions. Two OS treatment protocols, namely pretreatment and precondition, were implemented for laboratory and practical evaluation of engineered water repellency effect on compacted CCRs, respectively. The compacted untreated CCRs were found to have high WRC defined by a wide range of suction. The estimated air entry values (AEVs) of the WRC curves were found to correlate well with the median particle sizes of the CCRs based on the pore size estimate for a cubic and tetrahedral grain packing. Osmotic suction as a function of salt content was assessed based on the independent measurement of matric and total suctions. Ionic concentration analysis performed on the CCR leachate indicates that salt-based additives from air emission control measures contribute to the osmotic activities. The pretreatment protocol was found to impact the hydrolytic stability of bond formation during the OS – CCR reaction. Further investigations indicated that the CCR chemical compositions, presence of certain soluble salts in CCRs and/or residual OS affect the hydrolytic stability. The preconditioned OS-treated CCR exhibited low surface energy resulting in less capillary and adsorptive forces resulting in a relatively lower WRC compared to the corresponding compacted untreated CCR. The unsaturated functions from the OS-treated CCR obtained indicated a wide range of suction and AEVs comparable to coarse-grained soil. However, the effect of varying degrees of water repellency was not clearly defined.

doctoral dissertations

Civil engineering

Ph.D.

Breakthrough Pressure
Coal Combustion Residuals
Contact Angle
Organosilane
Water Repellency
Water Retention Characteristics

Civil Engineering

Ogunro, Vincent
Pando, Miguel

Daniels, John
Hsiang, Simon
Tabarraei, Alireza

Thesis (Ph.D.)--University of North Carolina at Charlotte, 2019.

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