Evaluating the Use of Fluidized Bed Combustion Fly Ash as Concrete Pozzolan
Open Access
- Author:
- Zahedi, Mona
- Graduate Program:
- Civil Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- July 19, 2019
- Committee Members:
- Farshad Rajabipour, Dissertation Advisor/Co-Advisor
Farshad Rajabipour, Committee Chair/Co-Chair
Shelley Marie Stoffels, Committee Member
Tong Qiu, Committee Member
Aly Marei Said, Outside Member
Patrick Joseph Fox, Program Head/Chair - Keywords:
- Fluidized Bed Combustion
Fly ash
Anthracite coal
Bituminous coal
ASTM C618
Mechanical properties
Durability
Concrete
Hydration mechanism
Beneficiation - Abstract:
- Supplementary cementitious materials (SCM) are a key ingredient of today’s concrete and can vastly improve the durability and sustainability of concrete mixtures. While the demand for fly ash and other suitable pozzolans continues to escalate, the supply of high-quality and economically available fly ash has been shrinking. To maintain and expand the market share of concrete products, it is critical that high-quality, long-lasting and cost-competitive concrete is widely available; this requires a stable and abundant supply of cheap fly ash. Fluidized bed combustion (FBC) fly ash is an alternative source of fly ash that is widely available, but currently not used in concrete. This is due to the lack of guidelines and protocols to evaluate the quality/performance of FBC fly ash and identify necessary beneficiation procedures before it can be incorporated into concrete mixtures. The FBC process is a cheaper and more efficient way of burning waste coal compared to conventional pulverized coal combustion (PCC). In this technology, sulfur-absorbing minerals (e.g., limestones) are added as kiln feed and turbulence is increased, to enable combustion at lower temperatures (750-900°C), which in turn reduces NOx emissions. Although FBC fly ash may be a great SCM source, its performance as concrete pozzolan is not yet well understood, and there is a significant need for research to develop guidelines that distinguish usable sources of FBC ash. To address these knowledge gaps, the purpose of this research is to evaluate the feasibility, performance, hydration, and beneficiation of FBC fly ash and determine if and how this alternative fly ash can be used as a viable pozzolan for concrete. In this study, circulating fluidized bed combustion (CFBC) fly ashes were collected from two sources in Pennsylvania (products of anthracite and bituminous waste coal combustion) and characterized for their physical properties, unburned carbon content, bulk chemistry, mineralogy, and reactivity. Results were compared against the requirements of ASTM C618-19 and areas of non-compliance were identified. Further, the characteristics of CFBC fly ashes were linked to the fresh and hardened properties of concrete and mortar mixtures. The fly ashes were used to substitute 20% of Portland cement in concrete mixtures, and their effect on the slump, fresh air content, hardened air properties, compressive strength, chloride ion permeability, and water absorption rate of concrete was evaluated. Equivalent mortar mixtures were also prepared and tested for their drying shrinkage, autogenous shrinkage, expansion in water, resistance to sulfate attack and alkali-silica reaction. The fly ashes met the chemical and physical requirements of ASTM C618-19, except for elevated LOI (in both fly ashes) and elevated SO3 (in bituminous fly ash). Despite this, concrete with proper slump, air content, and strength development was produced by adequate dosing of chemical admixtures. The high SO3 content in bituminous fly ash did not produce deleterious expansion during autogenous shrinkage testing and the value of 14-day expansion in water was close to the ASTM threshold. Use of CFBC fly ash was found to be most effective in mitigating chloride ion penetration and alkali-silica reaction. This was mainly due to the contribution of CFBC fly ash in lowering the alkalinity of the pore solution, increasing its aluminum and silicon ion concentration, as well as refining the pore structure. Both fly ashes did not have a significant effect on drying shrinkage. Samples containing anthracite fly ash were able to withstand severe sulfate attack, but the use of bituminous fly ash led to premature failure. Understanding the pozzolanic mechanism of CFBC fly ashes in concrete was one of the main components of this research. For this purpose, X-ray diffraction (XRD), scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS), and thermogravimetric analysis (TGA) was performed on binary paste mixtures containing 20% CFBC fly ash. Hydration of reactive phases (calcined clay, aluminosilica glass, anhydrite, free lime) in CFBC fly ash which made up close to 75 % of its mass, resulted in the generation of ettringite, CO3-AFm (i.e., hemicarboaluminate, monocarboaluminate), and C-A-S-H phases. Bituminous fly ash contributed to greater ettringite and secondary C-A-S-H gel formation, while anthracite fly ash was responsible for greater AFm phase production. Use of CFBC fly ash led to greater Si/Ca and Al/Ca values for the C-A-S-H phase, compared to the 100% OPC mixture. Finally, carbon neutralization/reduction techniques were investigated for their efficiency in reducing the interference of CFBC fly ash with air-entraining admixtures (AEA) performance in concrete. Coating of unburned carbon with sacrificial surfactants improved the slump and compressive strength of concrete, but slightly reduced its fresh and hardened air content. Combustion of fly ash at 500oC for 2h was very effective in reducing the AEA uptake by CFBC fly ashes, and increased combustion temperatures did not yield any significant improvements.