What do highways, bridges and skyscrapers have in common? Yes, budgets and construction delays, but if you look closer you'll see that it is concrete that holds them together. It's the most consumed substance after water - the world's most sought after building material!
Concrete is everywhere and serves as the backbone of our infrastructure (Portland Cement Association, 2008). Its production and distribution however, is becoming expensive due to the rising fuel cost.
As the price of diesel fuel continues to climb, now more than ever the ready mix industry aims to cut costs while reducing its carbon footprint. Promoting the use of fly ash in concrete is a great way to do just that, though some types are better than others.
Beneficiated Class F fly ash is an unparalleled, high quality, and consistent product that provides tremendous value to the ready mix industry.
Portland cement as we know it has proved immensely useful throughout history, and modern technology has created an ideal complementary material in fly ash.
To make concrete, you combine water, sand, gravel, Portland cement and, in many cases, fly ash. Despite representing only a small fraction of concrete's principal ingredients, Portland cement produces 1 ton of CO2 per ton of cement during its manufacture.
One ton of fly ash can replace one ton of Portland cement; thus emitting one less ton of CO2 into the atmosphere.
Each ton replaced has a monetary component as fly ash has cost advantages compared to Portland cement. Thus, the use of fly ash, a cementitious material, increases sustainability and allows for a significant reduction in the producer's material costs.
Many studies have shown that fly ash can provide beneficial properties to concrete. Because of the shape and particle size of fly ash it leads to an increase in workability and reduces water demand and bleeding in fresh concrete.
Moreover, concrete made with fly ash will gain higher ultimate strength, permeability and corrosion protection, as well as greater resistance to freezing and thawing (ACI,232.2R-12). So what, exactly, is fly ash? Is all fly ash the same?
Fly ash is a coal combustion product which comes from boilers used in the electrical generation process.
The coal source is an important determinant of chemical composition, which categorises fly ash as Class F or Class C according to ASTM C 618 regulations (American Concrete Institute, 232.2R-2).
Class F ashes are normally produced from coals with 'higher heat energy such as bituminous and anthracite (ACI, 232.2R-3)'. Class C ashes show different performance characteristics and are usually 'not as effective as Class F in mitigating alkali-silica reactivity (ASR)'. Thus, Class F fly ash is often considered a better concrete ingredient than Class C fly ash because of its versatility, particularly in sulfate environments.
Due to its ASR mitigation and resistance to sulfate attack, Class F is optimal for concrete use while beneficiated or processed Class F fly ash further enhances concrete performance.
Beneficiation allows for consistently low LOIs (loss on ignition) which means the carbon content has been controlled to a consistent level. This helps eliminate the air entraining problems associated with variable LOIs.
It's important to remember that landfill avoidance plays a key role in the need to beneficiate fly ash.
The utilisation of a recycled industrial by-product is certainly a point of emphasis, but the story doesn't end there. At Separation Technologies, LLC, the unburned, high carbon particles are returned to the power plant.
As noted by Dr James D Bittner: "Recovery of the residual energy contained in the high-carbon fly ash increases the value of the beneficiation process directly to the power plant operation in the form of reduced fuel costs."
This occurs after two, distinct particle sets have travelled to opposite ends of the fly ash processing equipment.
Thus, Separation Technologies helps to minimise land-filling and CO2 production while adding value to concrete products and electric coal burning power plants.
Fly ash intended to be marketed and sold must comply with ASTM C 618 and, in the case of new fly ash sources, should include a quality history of at least six months of testing. Concrete users benefit when fly ash marketers carry out a comprehensive quality-control programme (ACI, 232.2R-21).
Processed fly ash offers many quality control benefits which include the ability to bid jobs requiring fly ash, better control of concrete mixtures, fewer batch plant or job site adjustments of air content, and the reduction in rejection of loads due to out-of-spec air contents.
At Separation Technologies, LLC, the costs typically associated with installing fly ash beneficiation systems are more than offset by proven technology and a consistent supply of top quality, Class F fly ash to the concrete industry.
Generally speaking, the more fly ash included in a concrete mix the more beneficial properties to the concrete.
High volume fly ash (HVFA) concrete offers even more improvements in workability and pumping, higher ultimate strength, reduced permeability and an increased resistance to sulfate attack and alkali-silica reactivity (ASR). HVFA concrete is typically defined as having a fly ash content of 50 per cent or greater by mass of cementitious materials; however, this number is often debated.
HVFA concrete can be charcaterised as concrete having more fly ash in its mix than normal for the intended application (ACI, 232.2R-24) and is simply more durable than conventional concrete.
Performance charcteristics
It's no surprise that fly ash is more widely available than ever before. The use of processed fly ash in concrete continues to positively impact performance characteristics.
In 2006, fly ash accounted for 20 per cent of coal combustion products worldwide as more than 15 million tons were used in the production of concrete (American Coal Ash Association, 2007).
Despite its increasing popularity in the ready mix industry, not all fly ash is created equal. Ultimately, it's up to concrete users to ensure their products are manufactured with consistent, high quality fly ash that can stand the test of time.
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Thomas Cerullo is a Vice President Business Development, Separation Technologies, Daleville, VA, USA. www.proash.com
WORKS CITED:
American Coal Ash Association, 2007, www.acaa-usa.org;
Committee 232. American Concrete Institute 232.2R. March, 2004;
Farmington Hills, Michigan; www.fhwa.dot.gov/infrastructure/materialsgrp/flyash.htm;
Portland Cement Association, 2008. www.cement.org;
www.netl.doe.gov/publications/proceedings/05/UBC/pdf/Bittner_paper.pd.
