Characterization of Pennsylvania Coal Combustion Products for Beneficial Use in Mine Land Reclamation

Open Access
Author:
Braun, Gregory David
Graduate Program:
Civil Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
May 29, 2012
Committee Members:
  • Angelica M Palomino, Thesis Advisor
Keywords:
  • CCPs
  • Coal Ash
  • Ettringite
  • Database
Abstract:
Over 130 million tons of coal combustion products (CCPs) are produced each year in the U.S. Less than half of these CCPs will be utilized in beneficial use projects, such as mine land reclamation, an important practice in Pennsylvania. The remainder will be landfilled. Yet, CCPs are an ideal fill material due to their abundance and their desirable engineering characteristics. The main disadvantage of utilizing CCPs is the variability in material properties seen between differing CCP sources and the change in material behavior over time. This variability makes predicting CCP behavior as an engineered fill difficult. One approach for minimizing the risk associated with CCP variability is to catalog all available data on CCPs, including formation processes, chemical properties, material characteristics, and mechanical behavior. Once this catalog of data has been developed, common trends in material characteristics and mechanical behavior between CCP sources may be identified. Therefore, the purposes of this study are to collect and organize all existing data on Pennsylvania CCPs into an electronic database as well as contribute to the database by obtaining properties of two additional CCPs. An attempt was made to use this data gathered in the database and the results from additional CCP testing to identify trends in material properties and behavior. The testing of the additional CCPs followed a CCP testing framework developed in a previous study. This work focuses on the creation and utilization of this database along with the findings of this additional CCP testing. The sources of the collected CCP data included published literature and results from years of the Pennsylvania Department of Environmental Protection (DEP) regulatory testing. The collected data was organized into a database designed in Filemaker Pro 11 and then launched as a website. For the additional CCP testing, two fly ash materials from two different power plants were selected for this study: a class F fly ash and an FBC fly ash. The testing methodology for the additional CCPs included general material characterization, such as compaction characteristics, particle size distribution, and specific surface area. Other tests focused on the mechanical behavior of these CCPs over time and included unconfined compression (UC) strength and hydraulic conductivity testing. The tests used to classify CCP mechanical behavior showed that the UC strength and hydraulic conductivity of the material may change, depending on the type of CCP tested. The results of quantitative x-ray diffraction and PHREEQCI modeling show that the formation of ettringite dictates CCP behavior. That is, as the amount of ettringite formed in FBC-PC increases, the strength of the CCP also increases. The lack of strength gain observed for the class F fly ash was due to the lack of formation of hydration products. In addition, ettringite formation over time reduced the void ratio of the FBC ash, and thus reduced the hydraulic conductivity. One important finding of this study is that the UC strength trend observed in FBC-PC is very similar to the UC strength trend for another FBC ash investigated in a previous study. Both FBC ashes experienced the most significant increase in UC strength in the first 7 to 14 days of curing. Another important finding of this study is the variability in the compaction characteristics of CCPs collected from the DEP. This variability prevents the identification of trends in maximum dry density and optimum moisture content based on CCP type. One critical finding of this study is that the hydraulic conductivity data collected by the DEP are not reliable for comparison purposes. The methods and procedures followed by different laboratories contracted for this testing were found to be inconsistent. The most significant inconsistency is the time that laboratories allow test specimens to cure before testing. It is possible that through eliminating these inconsistencies, the regulatory hydraulic conductivity testing can be standardized and the resulting data will be more meaningful. CCPs are a variable material and understanding how the mechanical behavior of the material changes, based on type and time, is essential for use in large volume applications such as mine land reclamation.