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6In 2009, 63 million tons of pulverized coal combustion fly ash, or coal fly ash, was produced in the United States, of which approximately 39% was beneficially utilized (American Coal Ash Assoc., 2011) and the remainder represents an unsustainable solid waste burden to society. The single larg- est beneficial utilization of CFA is in the production of port- land cement concrete (PCC) and concrete products (both as a partial cement replacement and as a constituent in blended cements), which accounts for approximately 50% of the total CFA beneficial use. CFA has been used in PCC since the 1930s, with the first publication reporting this use appearing in 1937 (Davis et al., 1937). CFA can improve some concrete properties, such as reducing permeability, increasing strength, and mitigating ASR. However, CFA can sometimes result in a reduction in desirable concrete properties, depending upon its physical and chemical nature. Therefore, tests and specifications need to be developed to accurately determine the properties of CFA and help its use to produce concrete with acceptable structural performance and durability. AASHTO M 295-11, Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use in Con- crete, is the current AASHTO specification for CFA used in concrete. This specification categorizes fly ash produced from coal combustion into two classesâClass F and Class Câand places limits on a number of chemical and physical param- eters for both ash classes. The only distinction between the two classes is the requirement for a minimum cumula- tive weight percentage of the silicon, aluminum, and iron oxides (i.e., sum of the oxides) of 70% for Class F and 50% for Class C. These limits on the âsum of the oxidesâ gener- ally result in a distinction based on the CFAâs pozzolanic and hydraulic reactivity. Class F fly ash is considered to be primarily pozzolanic whereas Class C fly ash, mostly due to the presence of calcium phases, may have cementitious properties in addition to being pozzolanic. In spite of this classification method, Class C ash is generally considered to be a high-calcium-content ash, while Class F is considered to be a low-calcium-content ash (calcium is expressed as percentage of calcium oxide by weight). Existing CFA speci- fications (e.g., AASHTO M 295 and ASTM C618) have been noted as not being sufficiently specific in classifying CFA (Diamond, 1981) or not being performance oriented (Manz, 1986; Mehta, 1986). CFA Properties Coal fly ash is the airborne residue from pulverized coal combustion processes and is typically collected as part of pol- lution control systems by a variety of means including fabric filters and electrostatic precipitators. These combustion units typically burn pulverized coal as a fuel and, with stable operat- ing conditions and fuel sources, produce CFA with a reason- ably consistent quality. CFA consists primarily of spherical aluminosilicate or calcium aluminosilicate glass particles derived from the mineral matter in both the coal and the extraneous material mined along with coal. The source of the combusted coal is a major factor in determining the composition of the result- ing fly ash. Anthracite and bituminous coals are preferred from an energy point of view, but sub-bituminous and lig- nite (western) coals are increasingly being used because of their abundance, ready access, and lower sulfur content. The principal elemental constituents of CFA are silicon, aluminum, calcium, and iron, all present as oxides (i.e., SiO2, Al2O3, CaO, and Fe2O3). Combusting anthracite and bituminous coals will produce fly ash typically containing over 70% by weight SiO2, Al2O3, and Fe2O3. Sub-bituminous and lignite coals typically contain a much higher content of calcium-bearing mineral matter and will therefore gener- ally produce a CFA containing significantly higher percent- ages of CaO (i.e., lime) and a lower cumulative percentage of SiO2, Al2O3, and Fe2O3. The lime occurs either as separate C H A P T E R 2 Literature Review
7 crystalline compounds or incorporated into the glass matrix (Hemmings, 1988). Another important characteristic of CFA is the presence of various forms of carbon intermixed with the inorganic particles, with the carbon occurring typically as discrete particles but also as discrete phases included within in- organic particles. The different forms of carbon may be classified as (1) char particles that are typically 5 to 50 µm or (2) soot and carbon black particles that are typically a micrometer or less in diameter. The char particles can have a wide range of morphologies depending on the coal mac- eral from which they originated. The texture, porosity, and specific surface area of these chars vary with changes in particle morphology. The general concern with carbon in CFA is the adsorption of organic chemicals onto the car- bon, which significantly influences the function of AEAs in concrete mixtures. Coal Fly Ash in Concrete CFA has been used in concrete for many years, but it was not until late in the 1940s that its use was widely accepted (Schlorholtz, 2006). Benefits from the use of CFA include improved workability, decreased heat of hydration, reduced concrete cost, potential increased sulfate resistance and ASR mitigation, increased late strength, and decreased shrinkage and permeability (Schlorholtz, 2006). However, use of CFA in concrete can reduce early strengths, reduce AEA effectiveness because of adsorption by carbon in the CFA, and accentuate ASR at some usage levels. Each of these potential problems can be addressed by tests and specifications that accurately predict and specify CFA properties that affect performance of the PCC mixture. Fly Ash Specifications and Tests A large portion of the literature pertaining to CFA is focused on concrete performance when CFA is included as a constitu- ent. Most performance concerns are typically associated with the applicable specifications, which are often described as not adequately identifying the characteristics of CFA that affect performance in a concrete mixture. These concerns focus on the chemical and physical classification of the CFA with regard to the inorganic and organic fractions of the ash. In review - ing the literature, it was clear the purpose of existing specifi- cations and tests for CFA are not adequately understood. For example, AASHTO M 295 is not intended to provide a means for predicting performance of a concrete mixture containing CFA but to provide a means of quality control of the CFA proposed for use in the concrete mixture. CFA specifications have not been adequately revised to reflect the changing practices in the use of CFA in highway concrete. For example, both the air content test and the shrinkage test treat fly ash as fine aggregate, not as a replacement of portland cement as it has often been considered in mixture design. This research performed a critical analysis of these test meth- ods and specification limits to assess their applicability to fly ash commonly used in highway concrete. Survey of CFA Users and Producers As part of this research, an on-line survey of state highway agencies (SHAs) was conducted to obtain information on fly ash sources used in pavement concrete and any concerns on existing fly ash specifications. The responses indicated a need for specifications and tests that better predicted performance. Attachment C contains the complete responses to the survey and a summary of its findings.
