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Fly Ash Facts for Highway Engineers
Several new and important technologies are being commercialized in the areas of fly ash benefaction and utilization to bring fly ash into conformance with current AASHTO and ASTM specifications for use in concrete. As the fly ash utilization industry has matured, quality control, quality assurance, and improved product performance have increasingly become important. Technologies have been commercialized to improve and assure fly ash quality for conventional concrete applications. Also, fly ash utilization technologies have been developed to produce high performance concrete products.
Changes in boiler operations or alteration of air emissions control systems at power plants will alter the quality of fly ash produced. Factors that may impact ash quality in this way include:
Particle Size Control TOP
Screening. In mineral processing, it is common practice to use screens to remove coarse particles from powdered products. In general, dry screening of powders is not economically feasible at below 45 µm (325 screen mesh size). Typical fly ash has a large proportion of the particles (typically more than 50 percent) finer than 45 µm. The use of coarse screens (100 or 80 mesh), might be effective for the removal of most of the coarse particles, many of which comprise UBC. The ability to remove carbon by this method depends on the degree to which the carbon-rich particles are discrete (liberated), and the size and shape of the carbon particles. As such, screening may be effective as part of a general ash processing scheme to reduce coarse particle content, reduce carbon content, reduce variability, and improve concrete workability.
Air classification. Classification systems that use air to separate particles by size and weight are also used to retain the finer ash fraction. Air classification may be performed on ash for the removal of coarse particles or the selective concentration of fine particles. In some instances, the products differ not only in particle fineness, but also in carbon content. The type of carbon removed by air classification is comprised of the coarse unburned coal particles.
Carbon Reduction Technologies TOP
Carbon removal by combustion. A simple approach to remove carbon is to re-burn it, producing a pozzolan quality fly ash. Carbon removal by combustion is commercially available.
Electrostatic separation. Electrostatic separation exploits the forces acting upon charged particles in an electrical field. This technology basically involves passing fly ash through a high voltage electric field, thus inducing opposite charges on the mineral fly ash particles and residual carbon. The fly ash is separated into a low carbon (less than three percent) pozzolan fraction and a carbon rich reject. Electrostatic separation is commercially available.
Treatment Processes TOP
Chemical treatment/carbon modification. Residual carbon in fly ash, as represented by the LOI value, is not the only factor that may affect the performance of AEAs in concrete. Adsorption of AEA due to the level and type of unburned carbon and conductance due to the interaction of soluble ions from the trace mineral components of the fly ash are also factors that interfere with the performance of AEAs. Fly ash can be treated using a chemical reagent to passivate the carbon adsorptive properties. In this technology, carbon is not removed, but its effect on air entrainment is minimized. The chemical treatment of fly ash for carbon passivation is commercially available.
Ammonia removal processes. Certain air emissions control systems may deposit excess ammonia on the fly ash. Technologies under development to deal with ammoniated fly ash include: heat treatment, wet washing/stripping, and chemical treatment. Low concentrations of ammonia have no impact on concrete properties, however, a strong ammonia odor may be emitted.
Other Developments in Technology TOP
Ultra fine fly ash. As compared to typical fly ash, with a mean particle diameter ranging from 20-30 micrometers, ultra fine fly ash can be produced with a mean particle diameter of 1-5 microns. The reduced particle size means that the pozzolanic reaction, which is normally a slow process is accelerated. Further, the finer particles may more completely react than the coarser particles of fly ash. So, the durability and strength benefits that one observes with a typical fly ash at a late age (more than one year) can be attained at a much earlier age (less than 90 days) and with a smaller dosage of an ultra fine fly ash. Table 10-1 shows typical mix designs containing ultra fine fly ash.
Typically, ultra fine fly ash is used at a replacement rate of 5 to 15 percent of the cement weight. At these dosage levels, it has been demonstrated that ultra fine fly ashes contribute more to concrete strength gain and permeability reduction than common AASHTO M 295 (ASTM C 618) fly ash, and will perform comparable to highly reactive pozzolans such as silica fume. Concrete durability properties such as resistance to alkali-silica reaction, sulfate attack, and corrosion are also enhanced by ultra fine fly ash.
Fly ash blends (Class C and Class F). Both AASHTO M 295 (ASTM C 618) Class C and Class F fly ashes have their own specific advantages when used as a cementitious material in concrete. Table 10-2 summarizes some general properties of concrete made with Class C and Class F fly ashes. It may be ideal to have a fly ash with a low to moderate LOI and that can be used to prepare a concrete that is very effective in resisting ASR, sulfate attack, and at the same time have high early strengths. One way to achieve that is to blend Class C and Class F fly ashes. The exact ratio of the blend will depend upon the specific fly ashes and their desired behavior in concrete. An example of fly ash blend mix design is shown in Table 10-3. Blended fly ashes are being marketed on a small scale in the United States.
High volume fly ash concrete (HVFAC). HVFAC refers to concrete where the fly ash comprises more than 30 percent of the total cementitious materials. HVFAC has a lower cost and is more durable than conventional concrete, and affords improved resistance to ASR and sulfate attack. Several successful field applications have been completed. Adequate early strengths and set times are obtained by using high range water reducers to achieve a very low water/cement ratio. Allowable cement substitution rates are currently limited by state transportation department specifications. An example of an HVFAC mix design is shown in Table 10-4.