If Jeffery Volz has his way, millions of tons of fly ash will be diverted away from ponds and landfills and into the nation’s infrastructure. Volz, assistant professor of civil, architectural and environmental engineering, plans to use the stuff — the fine particles that rise with flue gases during combustion — as an additive in concrete.
Currently the nation’s power plants generate about 130 million tons of fly ash and bottom ash during the coal combustion process. Particles are captured through filtration to reduce air pollution and are often stored at coal power plants or placed in landfills.
Adding fly ash to concrete isn’t a new concept. For more than 70 years, the waste product has been a component of concrete used to build the nation’s bridges, roads, dams and overall infrastructure. It increases concrete’s durability, extending the service life of these structures.
“Traditional specifications limit the amount of fly ash to 35 or 40 percent cement replacement,” says Volz. “Recent studies have shown that higher cement replacement percentages — even up to 75 percent — can result in excellent concrete in terms of both strength and durability.”
Concrete typically has three key components: portland cement, water and aggregates like gravel and sand. During the manufacture of cement, limestone and other materials are heated to extreme temperatures, releasing tons of CO2 from both chemical reactions and the heating process. If fly ash could replace cement, it would not only reduce the amount of fly ash that ends up in ponds and landfills but CO2 emissions as well, says Volz.
High-volume fly ash is significantly more sustainable, but also can be unpredictable. The physical and chemical characteristics of the material can vary, which can change how it reacts to additives.
“At all replacement rates, fly ash generally slows down the setting time and hardening rates of concrete at early ages, especially under cold weather conditions, and when less reactive fly ashes are used,” Volz says.
Volz is working with the Missouri Department of Transportation to develop guidelines for the proper application of high-volume fly ash concrete in bridges, roadways, culverts, retaining walls and other transportation-related infrastructure components.