People often argue about fly ash, and why we should or shouldn’t use it in concrete and other building products. (More about what fly ash actually is follows below.) It’s a bit of a hot topic in green building, and made for a lively but truncated group discussion at the recent Build Well 2010 Symposium near San Francisco. For that alone it makes a worthy topic with which to launch this Build Well Forum. As it turns out, it is also worth discussing because the issue is emblematic of two important, overarching green building issues that don’t get enough attention:
1) Industrial Ecology
If we’re going to make good use of all the stuff we’ve been throwing away—the plastic in oceans and landfills, the metals and tires in municipal dumps, the by-products of industry and agriculture—then we’re probably going to do it in the built environment. The construction industry uses more physical stuff, by an order of magnitude and by any measure, than any other industry. Rather than continue to scrape virgin resources out of ever more undisturbed landscapes, we will have to learn to use what others throw away. What if we as a society were to craft policy that truly encouraged beneficial, and careful, reuse of “waste” material in the built environment? What if (for example), in fostering better building, rather than award dinky green points for often dubious recycled content, we were to penalize product manufacturers for use of virgin resources?
No secrets! We want to know what’s in our food, what’s in our clothing, and what’s in our shelter. For several generations now we’ve been adding ingredients to everything we eat, wear and build with, giving little or no consideration to what effects those ingredients might have on our children, our bodies, and the planet that supports us. Increasingly, consumers want to know exactly what they’re buying, where it came from, and what trail it left. And yet . . . in some way we also have to protect intellectual property, and the keeping of trade secrets, because failure to do so puts a fast end to innovation. And innovation will surely be key to working our way out of the various jams we’re in.
All of which brings us back to fly ash, and its use in buildings.
I’ve been studying fly ash, and the industries around the issue, for 15 years now, and hope to here offer a few comments of worth. I wrote a book on fly ash concrete, "Making Better Concrete" five years ago, but am really no expert. Nor am I in the pay of the coal or ash industries; I’m just an engineer who hates waste and looks for elegant solutions to problems large and small (Disclosure: I am an unpaid advisor to CalStar Products, who make fly ash bricks and pavers). By no means is this post meant to declare some “answer”, but only to start a conversation that might unpack this very difficult subject. I do go on a bit, but that’s probably necessary: this isn’t a simple issue, and to oversimplify it—as is often done—is to miss larger, important considerations.
So, what is fly ash?
When you burn coal to make electricity, you get two things. You get a lot of thermal energy with which you then boil water which provides steam to drive turbines that generate the power that eventually shows up in your wall socket and enables you to read this. It’s a thrillingly inefficient way to power things, and is increasingly the method of choice in the world because what we got is a whole lot of coal, is what we got. There are no other large sources of energy on the horizon, with present technology, that would enable us to immediately ramp down coal usage. Like it or not, we’ll be burning a lot of coal for at least another generation. In North America, about half our electricity comes from burning coal.
The other thing you get from burning coal, as with a wood fire, is ash. Burn ten tons of coal, you’ll get about a ton of ash.
In North America the burning of coal for power generates about a half a cubic foot per person per year. That’s a bucket of ash in the name of every man, woman and child in America, every year. Whether we like it or not, we have to do something with it. You are producing the ash; what do you want to do with it?
Some of that ash falls to the bottom of the smokestack, and is thus called bottom ash. A lot of it—the finer, purer stuff— rises with the warm exhaust plume, and thus earns the name fly ash. There are basically three things that you can do with that flying ash. One: you can just let it fly away and scatter, where it will eventually settle onto the ground or be filtered by every air-breathing organism in the area. To see how well that plan works, visit any Chinese city, or just look at photos online; much of that dense, smoggy haze is fly ash floating in the air. (Fly ash particles are fine enough to remain airborne for a long time, but you do not want to breathe them.)
Or you can install collectors of one type or another on the smokestack and gather the ash at the source. Now you’ve got cleaner air and lots of fly ash-- but what to do with it? One solution is to dump it in artificial lined ponds. Big, big ponds. Ponds that sometimes burst their banks and flood downstream with unpleasant grey muck, as recently happened in Tennessee. The other thing to do with the ash—our third option—is to make concrete with it. Which is a pretty slick option, in that ash skillfully blended into concrete does several nice things at once:
- it makes economical use of a waste product,
- it makes better concrete, and in many ways,
- it chemically binds the heavy metals in the ash (yes, there are all sorts of metals in ash; which exact amount varies with the coal source and type of burning),
- and, best of all,
- its use means you don’t need as much Portland cement to act as the glue binding the concrete together. And that’s a solid win because the production of Portland cement (baked and crushed limestone with a bit of clay) gives off a lot of carbon dioxide. The production of Portland cement worldwide alone generates between five and eight percent (depending on who you ask and how you measure) of anthropogenic CO2 emissions.
So, what’s not to like? We pour lots of concrete, and have lots of ash that can be beneficially used to make better concrete. So let’s do it, right? Let’s gather as much of the ash as possible and pour it into concrete.
Increasingly, we are doing just that. But, to put it mildly, things are not so simple. Here are the main issues of particular concern:
Testing to date shows that the metals in fly ash chemically bind in a cement matrix in concrete; they are rendered harmless. Mercury, however, may be a different matter because it is unique among metals in being liquid (and potentially gaseous) at normal temperature and pressure. Many hold that just the possibility of mercury offgassing from fly ash concrete is reason enough to ban its use in schools, if not everywhere. Testing to date suggests that this is not an issue, but much more needs to be studied.
2) Other uses of ash and other coal by-products
From a physical point of view, fly ash is a very utilitarian material, and we’ve probably just begun to explore its value to industry and society. It is used as a stabilizer for clay soils, as a structural filler for some plastics, and as a component of sheetrock and other building products that do not also have cement. Thus, some wonder: what happens to all those heavy metals: are they in the dust? Are they getting into our bodies through air or direct contact? What will happen when it’s time to deconstruct a building with fly ash products: are they safe to handle or bury? We also use bottom ash for various purposes, and gypsum harvested from the flue gas to make sheetrock; what are the safety concerns there?
or the more simplified, overarching concerns that linger:
3) Isn’t fly ash just a hazardous material, period?
Even the guys who sell fly ash would tell you not to put this stuff on your pancakes; like most of the substances in the natural world, fly ash is not for consumption or respiration. Does that mean we (or rather, the EPA) should formally declare it a hazardous substance, thus ending any chance of beneficial reuse or recycling?
4) If we use a lot of ash in concrete, don’t we then encourage or justify the burning of coal for electricity?
Not even close. We use only a fraction of the ash produced in America or worldwide for any economical purpose, and the rest is a bit hard to access or use for a host of reasons.
Over the past several years I’ve had a great many conversations and arguments about ash with stakeholders from all over the spectrum: fly ash suppliers, concrete contractors, academics who study ash, and healthy building advocates who are very nervous about it, or flat out hate it. As is always the case with us crazy humans, opinions are perhaps sometimes based more in emotion than fact, but then some pretty important facts remain obscure because credible testing just hasn’t been done, we just don’t have the answers. Neither, however, do those who adamantly oppose the use of ash in buildings offer a viable alternative for disposal. “Hauling it off to a hazardous waste facility” usually ends up meaning “dump it on the next generation”—and into the soil and air of this generation.
A very complex matter, an urgent matter, and one with no easy resolution. We need to study and know so much more, but meanwhile that bucket of ash with “your name here” is being produced every year.