WHY USE FLY ASH?

As we have seen, there are many practical reasons to add some (if not a lot of) fly ash to concrete–not just in addition to portland cement, but in replacement of it. In Chapter 3, we looked at the performance-enhancing effects of fly ash on workability, pumpability, strength, shrinkage, and permeability. The effects are so many, and so positive, that senior figures in the world of concrete have recently stated that concrete without fly ash belongs in a museum.

This may come as shocking news to many engineers, who remain convinced that fly ash in some way “waters down” the quality of concrete. When, as a junior engineer, I first heard of fly ash in the early 1980s, my supervisor sneeringly referred to it as “hamburger helper” or “filler.” Those early impressions are sometimes hard to change, and they still linger in many parts of the concrete industry. In large part, those impressions are based on decades-old experience using ashes from earlier generations of power plants; those fly ashes were both coarser and higher in carbon residue than those commonly found today, and were thus much less effective as pozzolans. In another case, a 40% fly ash concrete was poured in 1981 for the Monterey Bay Aquarium using a superplasticizer. The project was ultimately successful, but incorrect dosages of the water-reducer caused premature gelling of the concrete; it started to set in the buckets before being emplaced, leaving in many people’s minds the false impression that the high volume of fly ash was to blame. As anyone in construction knows, you can screw up anything if you’re not paying attention, just as you can learn from mistakes (and are foolish if you don’t) if you take the time to study what went wrong. What is generally true in construction is particularly so for concrete: you get what you inspect, not what you expect.

The big picture
Many reasons for using fly ash are global, environmental, or societal in nature. The production of portland cement puts about a ton of carbon dioxide (CO2, a primary greenhouse gas) into the atmosphere for every ton of cement produced–roughly half a ton from the fuel used to cook the raw limestone, and half a ton from the calcination of the limestone. Worldwide, the production of portland cement alone accounts for 6-8% of human-generated CO2 (depending on whom you ask). So here, in a single industry, lies the opportunity to slow the very alarming trend toward global warming. According to one authority:

For every ton of fly ash used [to replace portland cement]–

Enough energy is saved to provide electricity to an average American home for 24 days.
The landfill space conserved equals 455 days of solid waste produced by the average American.
The reduction in CO2 emissions equals 2 months of emissions from an automobile.
The cement industry deserves great credit for recognizing this, and for taking many effective steps to reduce its local and global environmental impacts. But the fact remains that we have a readily available industrial waste product–fly ash–that happens to be a perfect replacement for half or more of the cement in almost any mix, and yields equal or better-quality concrete. This is why high usage of fly ash in concrete is now a component for global trading of so-called “carbon credits,” based on the Kyoto Accords and the Chicago Climate Exchange. Use of fly ash is also a means of making points in the increasingly important LEED™ (Leadership in Energy and Environmental Building) system of evaluating and rating buildings, developed by the US Green Building Council (USGBC). (For more detailed information, see the USGBC website at www.usgbc.org, and appendix C in this book for more elaboration and a sample calculation.)

Most of the fly ash produced today is currently being either landfilled, as in North America where lack of convenient rail spurs hinders bringing it to the marketplace, or simply flying freely out the smokestack of the coal-fired power plant from which it comes, as in China and India. Fly ash in the ground can pollute groundwater with heavy metals, while fly ash in the air constitutes particulate pollution–the bulk of the famous smog blanketing Beijing and many other cities that is a health hazard to everyone nearby. Fly ash trace metals and particulates cast into concrete, by contrast, are bound forever in a way that cannot hurt anyone.

Some in the green building movement question whether increasing the use of fly ash in concrete will effectively encourage coal-fired power production–itself a primary source of environmental degradation, pollution, and greenhouse gases. However, in light of the huge percentage of worldwide electricity generation already derived from coal, and the fact that so little of the ash currently being produced is stored safely in concrete, and the fact that both coal-fired power and concrete production are rapidly increasing along with the population, this seems like a weak argument at best. Current annual world production of cementitious and pozzolanic by-products of thermal power plants and metallurgical industries is about 650 million metric tons, of which only about 7% is being used by the cement and concrete industries. This is beyond wasteful; it is ridiculous. Simple arithmetic shows that coal and other fossil fuels have little or no place in the 100+ year plan for humanity. But for now and the next generation or two, we have the ash and we should use it intelligently. We can go on letting the existing output of fly ash be a landfill, pollution, and health problem, or we can use it to make better concrete.

There is also an enormous economic consideration in using fly ash. Portland cement-based concrete is the most ubiquitous construction material used in the United States and the world; currently more that 118 million tons are poured annually in the US, with fly ash now used to replace over 15 million tons of portland cement per year. Even so, in 2003 over 15 million tons of cement were imported into the US to make up for a shortage of native cement production. If domestic ash had replaced those imports, the result would have been an improvement in the US balance of trade of at least $1 billion. In a similar fashion, populous nations such as Brazil, India, and China are making or importing cement at great cost while grossly underutilizing their own native sources of ash and other industrial pozzolans. Things are, unfortunately, never quite so simple. For example, the cost of installing equipment to collect fly ash at the power plant is huge, and typically deemed uneconomical; what makes good business sense to the power plant owner is a disaster for society. Resolving a disparity like that–striking a balance between unfettered free-market capitalism and the need to protect the public welfare–can only be done in the societal and political arena.

Radically increasing the use of fly ash in concrete–whether blended at the readymix plant, or premixed and bagged at cement plants–is but one component of the broader effort to make concrete a more environmentally friendly building material without sacrificing quality or affordability. Other aspects in development include the use of other industrial by-products as cementitious materials or as components of cement manufacturing, the use of pervious concrete to absorb storm water, the use of light or white concrete to reduce the urban heat-island effect, and the reuse of demolished concrete as aggregate in new structures.

The widespread use of high fly ash concrete is an idea whose time has come. After decades of development in laboratories all over the world, and more recent use in demanding and varied projects, HFAC is now very much in the “real world” of concrete. In the preceding pages you saw how it works, how to use it, and pitfalls to avoid. In the references to follow you will find more technical detail. Here is your start, and some tools to help. Good luck with your projects.