ENHANCING THE BENEFICIAL USE OF FLY ASH IN CONCRETE AND IN MINE RECLAMATION: MEASUREMENT OF ORGANIC CARBON AND PREDICTING THE LONG TERM LEACHING OF HARMFUL ELEMENTS

Open Access
Author:
Mohebbi, Mina
Graduate Program:
Civil Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
June 14, 2017
Committee Members:
  • Farshad Rajabipour, Dissertation Advisor
  • Farshad Rajabipour, Committee Chair
  • Li Li, Committee Member
  • Ming Xiao, Committee Member
  • Aly Marei Said, Outside Member
Keywords:
  • Fly ash
  • mine reclamation
  • concrete
  • organic carbon
  • long-term leaching
  • reactive transport modeling
Abstract:
Combustion of coal to generate electricity produces huge volume of coal combustion products (CCPs) annually in the United States. Due to their promising physical and chemical characteristics, these byproducts can be beneficially used as supplementary cementitious materials (SCM) in portland cement concrete, mining applications, structural fills, and soil and waste stabilization. However, the efficiency and environmental impacts of such beneficial uses need to be further evaluated and enhanced. The unburned carbon (UC) content of fly ash impacts the performance (e.g., air entrainment and rheology) of concrete mixtures. The conventional loss on ignition (LOI) test to measure the UC may be overestimating as the weight change upon igniting fly ash could be the result of other physical and chemical reactions (e.g., calcination of carbonates, removal of bound water, and iron and sulfur oxidation) in addition to organic carbon burning. Moreover, reclamation of mine sites using coal ash has been shown to potentially alleviate the negative effects of mining activities such as neutralizing the acid mine drainage. However, during coal combustion process, trace elements are concentrated onto fly ash particles, and the long-term leaching of harmful elements from coal ash to subsurface aquifers is an environmental concern. This research studies focuses on evaluating and enhancing the beneficial uses of fly ash (as the predominant coal combustion byproduct) in two areas: 1- in portland cement concrete through measuring the UC content, and 2- in mine site reclamation through evaluating the leaching behavior of fly ash deposits to water resources in short and long terms. In the first part of this research study, a two-atmosphere thermogravimetric analysis (2A-TGA) coupled with mass spectrometry was performed to evaluate the chemical reactions that occur upon heating of fly ash and to measure the true UC content. 2A-TGA was performed under two distinct atmospheres: (i) in non-oxidizing helium gas, to measure weight loss due to decomposition of carbonates and loss of bound water, and (ii) in oxidizing air, to measure weight loss due to conversion of UC to CO2. The results were also compared with the total carbon (TC) measured using infrared spectroscopy. It was found that there is no one-to-one relationship between the LOI and the TC or UC contents of fly ash. LOI overestimated TC by up to 2.5 and overestimated UC by up to 6.4. Based on the results of this study, a practical alternative to 2A-TGA could be to heat fly ash in a non-oxidizing atmosphere (e.g., vacuum, He- or N2-purged furnace) up to 750 oC, followed by a conventional LOI test. In the second part of this study, leaching behavior of fly ash deposits was evaluated through (i) defining the host phases for environmentally important elements and (ii) developing a reactive transport model to predict the long-term leaching behavior. Determining the host phases was achieved through micro-characterizing the coal ash and flow-through column leaching tests. It was found that amorphous aluminosilicate is the main host phase for Si, Al, Fe, and, Mg. Alkalis such as Na, K, and trace elements including As and Se are also distributed in the bulk Al-Si glass in low concentrations. The Initially high concentrations of Ca and S in the leachate were mainly due to the dissolution of gypsum. Surface associated salts (e.g., sulfate and borate salts) dissolve Na, K, S, and B ions at early stages of leaching. Iron was found both as ferromagnetic particles containing magnetite and hematite, and also included in the amorphous phase in lower amounts. The host phases were then considered as input data for a quantitative reactive transport 1D model using CrunchFlow code. The calibrated model was used to predict the concentration of major elements (Ca, S, Si, Al, Fe, Na, K, Mg), and trace elements (As, Mo, Se, B) along 10 years of leaching. The leachate composition at early ages of leaching might exceed the environmental limits for S, B, Mo, and Al. However, in long-term the overall composition meets the leaching limits except for aluminum content. The porosity of compacted fly ash starts to increase from the top layers, and in long-term (e.g., 30 years of weathering) it reached from 28% to 45%, which can significantly affect the stability and transport properties.