FROM GREY TO GREENISH …
If you consider yourself a green person, you may think using fly ash, a by-product of burning coal, in your concrete mix rather sacro-saint! After all, coal plants are some of the biggest polluters and CO2 emitters. In recent decades, in an attempt to minimize its environmental impact, the EPA forced coal-powered plants to trap particulate matter before it is released in the smokestacks, to properly capture and dispose coal ash, a.k.a. as fly ash, the residue found after smokestacks are cleaned. Disposal of fly ash is usually done by mixing it with water and placing it into open impoundments (that could be damaged by storms), storage ponds or buried in landfills that have been known to occasionally breach or leak as in the case of the December 2008 collapse at the Tennessee Valley Authority’s Kingston Fossil Plant, releasing 5.4 million cubic yards of coal fly ash into the Emory River! So how much are we reusing? Approximately 43-45% of all fly ash is being recycled today.
With 50% of all electricity in the US being generated by coal plants, one couldn’t begin to imagine the huge amount of fly ash produced by coal-powered plants daily. Capturing it and reusing it in a safe way would in fact prevent the inevitable spread of some of the heavy metals it contains (depending on the coal bed makeup: arsenic, lead, chromium, mercury, dioxins etc.) in the atmosphere we breathe, the water we drink… So when the first attempts at substituting fly ash to Portland cement resulted in a concrete with greater strength and durability, what was once a worrisome problem looked like it could be turned into a solution for the environment and the construction industry. Recycling fly ash into concrete would not only reduce CO2 emissions but also neutralize the harmful effects of the known carcinogens it contains and the need to mine for new raw materials. Also called geopolymer concrete, this “green” concrete is cheaper to produce and is being widely adopted for all types of concrete applications.
2 TYPES OF FLY ASH
There are 2 classes and both can be used as a replacement for Portland cement or hydrated lime. They act as a filler providing contact points between larger aggregate particles in asphalt concrete mixes.
Class F: is made from burning anthracite and/or bituminous coal
Class C: is made from lignite or subbituminous coal
BENEFITS OF USING FLY ASH IN CONCRETE
The process of making cement is quite energy-intensive and could account for 5-8% of greenhouse gas emissions around the world. Factor in steady population growth and the number will continue to increase unless a massive adoption of greener alternatives happens. Fly ash concrete is one of them because of its abundant availability, cheap cost and the fact that it could reduce as much as 90% of CO2 emissions when considering the end to end process.
Green builders can gain points towards their LEED goals (Leadership in Energy and Environmental Design) if they can replace at least 40% of Portland cement with it. Bricks that are made with fly ash can even store CO2 from the atmosphere.
ALL AROUND BETTER CONCRETE
Fly ash is known to improve concrete’s workability, pumpability, chemical resistance, finish, strength, and durability. Fly ash particles are similar in size with Portland cement and as a result requires less water during the mixing stage. This means better surface finish, sharper edges in precast concrete. Its fine particles reduce bleeding and segregation and improve overall workability. This is especially helpful in extending working time in hot weather.
Fly ash produces a concrete that is less porous than Portland cement. The pozzolanic effect of fly ash creates a denser product due to smaller pore sizes. It also reduces bleed channels and permeability in concrete.
Widely used in road projects, fly ash increases the stiffness of the asphalt, improving rutting resistance and mix durability.
EXAMPLES OF FLY ASH REUSE
In addition to replacing Portland cement, fly ash is being used in the following areas and the list keeps expanding:
- Road construction: ideal self-compacting backfill material (as a replacement for compacted earth) for backfill, embankments, road sub base construction, mineral filler in asphaltic concrete, as a loose application on roads for ice control, in highway sound barriers…
- Agriculture: fertilizer, soil amendment and stabilization.
- Aggregate material substitute: for brick production replacing clay.
- Dam construction: in roller compacted concrete dams.
- Waste management: waste solidification and stabilization, conversion of sewage sludge into organic fertilizer or bio fuel.
- Roofing material: tiles, granules.
- Marine: pilings, artificial reefs
- Binding agent: paints, undercoating.
And much more
Only time will tell whether or not small amounts of heavy metals could leak from the concrete over time and if this new found durability is truly long lasting. Recent spills have prompted environmental groups to call for tougher regulations, so whether or not the EPA decide to classify it as hazardous material remains to be seen. But until then and until coal powered plants are being phased out for greener alternatives, and the huge reserves of fly ash remains an environmental issue, the safe use of fly ash in concrete makes sense and should continue.