Unconventional Fly Ashes and their Impact on Concrete Performance
Fly ash, which is the material left over after coal is burned to produce electricity, is the second largest source of waste in the U.S., after municipal waste (general trash). The largest consumer of fly ash is the construction industry, where it is used both for concrete, as well as ground stabilization applications. Fly ash and concrete have a symbiotic relationship – concrete consumes the waste and encapsulates all the bad parts of the material (in particular the toxic heavy metals), while fly ash reacts within concrete to increase its strength and durability through the pozzolanic reaction. Fly ash also increases the sustainability of concrete, allowing us to reduce the proportion of portland cement, which is a very CO2 intensive material, and replace it with the fly ash.
However, despite these benefits, recent changes in coal power regulations, as well as declines in power generation from coal due to the availability of inexpensive natural gas, have resulted in shortages of available fly ash for the construction industry. This is despite the fact that we have millions, perhaps billions, of tons of fly ash available in wet or dry storage facilities (ponds or landfills) scattered throughout the country. This fly ash is not necessarily bad, its properties are simply unknown.
At Ohio State we are now investigating the effect of these ‘unconventional’ fly ashes on concrete performance, as well as understanding the role of fly ash properties on adsorption of air-entraining agents with the goal of enabling greater usage and consumption of impounded fly ashes, as well as mitigating the issues of fly ash shortfalls for the construction industry.
Effects of Retarding Admixtures on Calcium Sulfoaluminate Cement Hydration and Microstructure
One pathway to reducing the environmental impact of concrete infrastructure is through use of alternative cements comprised of lower calcium content, and requiring lower clinkering temperatures, such as calcium sulfoaluminate cement. However, in contrast to portland cement systems, many of the basic mechanisms controlling hydration and property development are poorly understood for alternative cement systems. Our research group is working to bridge the gap between current understanding and what is needed to properly design and use alternative cement systems.
Our first focus has been on understanding the effect of retarders on hydration and property development in calcium sulfoaluminate systems.
Pervious Concrete for Acid Mine Drainage Remediation
As a result of America’s extensive history of mining, acid mine drainage (AMD) is a significant problem in many states, with over 10,000 miles of impacted streams in the U.S. When iron sulphide minerals (especially pyrite) interact with moisture and oxygen, typically as a result of mining operations, acid mine drainage develops. The oxidation of sulphide minerals releases hydrogen ions into the water, lowering its pH. This, in turn, causes dissolution of heavy metal complexes and leaches elevated concentrations of these ions into the AMD, causing acute and chronic impacts to nearby surface waters.
Abatement and treatment of AMD is difficult, expensive, and complicated, since mineral reactions which lead to AMD problems continue to occur for decades to centuries after mine closure. AMD issues affect more than Ohio, with treatment costs exceeding $200 million/yr in the US and $40 billion globally. One aspect contributing to the difficulty of remediation is the considerable costs of AMD control methods: the Ohio Division of Natural Resources reports that two of the most common AMD treatment systems it uses, lime dosing and vertical flow ponding/lime or slag leach beds, amount to as much as $825,000 per site, for a treatment period of 20 years.
Pervious concrete is a type of concrete produced using a single size aggregate and a coating of cement which allows the aggregates to stick together. The large holes (void spaces) which then occur throughout the concrete’s structure allow water to flow freely through.
Although concrete looks solid and fairly smooth to the human eye, at the microscale calcium-silicate-hydrate (C-S-H), the primary binding, and strength-giving component of hydrated cement, has extremely high surface area. This high surface area allows ions to bond through van der Waals forces, as they near the cement surface.
Additionally, as a result of alkali cations (Ca2+, Na+, K+) present in the cement, water within concrete (pore solution) is very alkaline – having a pH as high as 13.5-14.
These two components together make concrete the perfect material to use as a filter for acid mine drainage – lowering solution pH and capturing heavy metal ions as they make contact with the cement surface. We are currently working to develop mixture designs to allow for control of flow rate of water through pervious concrete, so that we can mirror the flow requirements of streams in which the pervious concrete will be placed, and to understand the mechanisms controlling pH reduction and contaminant removal by pervious concrete.
In September 2019, Dr. Burris and Dr. Ryan Winston, AMD Project collaborator, presented on this research to the Women & Philanthropy organization, a group of philanthropic women from very diverse backgrounds. Before the research presentation, W&P event attendees were able to interact with students, learn about Dr. Winston’s work with pervious concrete to control water infiltration and make their own sample of pervious concrete, with the help of Dr. Burris’ students.
In June 2020 our group was generously awarded funding from the Women & Philanthropy group in order to pursue additional research on pervious filters. Over the summer of 2020 several students, including Finn Haugh, Alec Grimm, and Jake Bertemes, cast and tested the durability, clogging potential, and capacity of the pervious filters for pollution removal. Thanks to their hard work a publication on our findings is forthcoming!
Ultra-lightweight Concrete for Floating Concrete Vessels
Nutrient overloads in our watersheds, from agricultural runoff, have led to the development of dangerous algal blooms in lakes throughout Ohio, most famously, Lake Erie. We are working with Jake Boswell, Assistant Professor of Landscape Architecture in the Knowlton School, Nan Hu, Assistant Professor of Civil Engineering, and Rachel Gabor, Assistant Professor in Watershed Hydrology in the School of Environment and Natural Resources, to design and fabricate beautiful floating islands, which will also facilitate removal of nutrient overloads through their facilitation of growth of plant life.
Using Bacteria to Increase Concrete Durability
One potential method of densifying concrete microstructure and preventing cracking due to freeze-thaw events could be through the introduction of bacteria into concrete. When the bacteria is exposed to air, as a result of a crack, the bacteria precipitates a filler mineral, typically a form of calcite, into the concrete. This bio-mineral fills the cracks, and additionally, has been shown to densify concrete microstructure, mitigating the negative impacts of concrete cracking, and leading to increased (tensile and compressive) strengths, reduced permeability and diffusivity, and through these mechanisms, increased concrete service life. If this method is successful and cost-effective, this could provide local public agencies with an opportunity to reduce maintenance and repair activities and the associated costs.
Topics related to Upscaling the Usage of Alternative Cements
One of the most pressing issues to understand relative to upscaling usage of alternative cements, is their resistance to degradation from environmental factors. In this study we investigated the performance of alternative binders in sulfate environments, to understand whether ACMs might outperform, or be problematic in these types of conditions. We found that, in general, ACMs show less expansion than portland cement, when subject to high sulfate environments.
Novel Alternative Cementitious Materials for Development of the Next Generation of Sustainable Transportation Infrastructure
Increasing the Reactivity of Natural Zeolites through Pretreatment Methods
The supply high-quality fly ash meeting ASTM C 618 requirements has decreased significantly over the past decade with the shift in energy to natural gas, as well as ever-increasing rigor of environmental regulations. Natural zeolite may be a viable alternative when supplies of other supplementary cementitious materials are unavailable or unreliable. The following studies explored the effect of treating natural zeolites through milling and acid-washing, in an attempt to increase their reactivity.
Milling as a Pretreatment Method for Increasing the Reactivity of Natural Zeolites Used as Supplementary Cementitious Materials
The Effect of Acid Treatment on the Reactivity of Natural Zeolites Used as Supplementary Cementitious Materials