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The use of fly ash reduces CO₂ in two ways: by lowering the demand for Portland cement and by absorbing more CO₂ from the atmosphere during curing.

One drawback of fly ash is that it slows the curing process somewhat, which can be a problem if scheduling requires applying structural loads soon after pouring. "We can only use fly ash in the summer because it slows the curing time," says Arkansas concrete contractor Pete Cockram. "Most specs I deal with call for a maximum of 20 percent fly ash," he adds. Some areas allow 25 percent or more in the mix.

Transportation costs are often higher for fly ash than for Portland cement because U.S. coal plants producing adequate material are spread out unevenly around the country. This factor is offset by the fact that most coal plants are served by rail, a much cheaper mode of distributing raw materials than trucking.

On balance, the net cost for fly ash is lower and the embodied energy is dramatically less for fly ash than for Portland cement.

New Concrete Admixtures

Another promising by-product admixture is blast-furnace slag. Slag consists of limestone and other impurities gleaned from the processing of iron ore in steel production. When used as an aggregate in concrete, slag improves the strength-to-weight ratio significantly. It can also be ground into a powder and used as a partial substitute for Portland cement.

An unlikely but surprisingly effective admixture is rice hull ash, trademarked under the name of Agrosilica. When power plants that burn rice hulls control combustion temperatures carefully, the resulting ash can be used to significantly increase the strength — and reduce the permeability — of concrete.

The major drawback is that rice hull ash currently costs twice as much as Portland cement. But when mixed with fly ash, which is much cheaper, the result is stronger, less permeable concrete at a cost comparable to purely Portland cement-based concrete. AIM Associates Architects recently used a mixture of fly ash and rice hull ash in the green showcase Sonoma State Environmental Technology Building.

The chemistry behind these admixtures can get complicated, so designers should do a bit of research — at BuildingGreen.com, for instance — to determine which admixture is best for a particular application. It's also important to find out what materials are available locally and whether there's a concrete contractor willing and able to use them. Fly ash is becoming common enough that many concrete plants now stock it as a standard admixture.

Masonry Units

Autoclaved aerated concrete (AAC) has been used in Europe since the 1930s. Its production is somewhat similar to that of making bagels: it is "leavened" with tiny air bubbles and "cooked" with steam. Aluminum powder is added to a concrete mix; the cement has a chemical reaction with the aluminum, causing tiny air bubbles to form while the mixture is autoclaved (steamed).

Because AAC is entrained with air bubbles, it provides significantly higher insulation (R-values up to 1.25 per inch, or 8.66 mK/W) than conventional concrete. AAC is generally formed into blocks that can be readily cut and drilled with standard carpentry tools. With only 20 percent of the density of standard concrete, it is much lighter, making it easier to handle and allowing for larger units. However, AAC also has only about 10 percent of concrete's compressive strength and is limited to low-rise construction or else must be reinforced with steel.

In order to make AAC more environmentally friendly, fly ash can be used as a portion of the cement, and substitutes for the aluminum powder are being explored. The product Flexcrete incorporates fly ash into aerated concrete but cures the block at low temperatures, eliminating the high-pressure, energy-intensive autoclave system.

The Headwaters Fort Mandan Visitor's Center in North Dakota used Flexcrete for the exterior walls. Fly ash was used in the exterior stucco, in a synthetic gypsum board, and even in the carpet. Exterior walls and the building's fireplace are covered with artificial stone containing coal fly ash.

The company E-crete is using tailings from a nearby copper mine for the silica content of AAC blocks. The tailings account for 25 percent of a block's weight. The company claims an R-value of 31 (5.5 m²K/W) for their 10-inch (25-centimeter) block wall assembly.

Alternatives to Formwork

There are several varieties of stay-in-place forms that generally come in large lightweight interlocking blocks. The blocks are usually dry-stacked and reinforced with steel. The interior shape of the units form a matrix of cavities into which a slurry of concrete is poured, forming a reinforced grid. Like AAC, these alternatives are good barriers to heat transfer, fire, and sound.

The oldest of this genre is Durisol, first produced in Switzerland in 1945. The process uses mineralized wood chips which are mixed with cement and formed into blocks. Durisol is used extensively in Canada, and a similar product, Faswall, is available in the United States in standard sizes.

There are also several products that use ground-up, recycled, expanded polystyrene foam mixed with cement to form blocks. Techblock boasts 100 percent recycled polystyrene content and the company claims it uses about a quarter of the structural concrete filler as other types. These products have average insulation values of around R-20 (3.52 m²K/W) for a 10-inch (25-centimeter) thickness; some manufacturers claim values as high as R-35 (6.1 m²K/W).

All of these block wall systems are easy to cut and rout with standard tools, making assembly fast and facilitating the installation of plumbing and electrical services.

There have also been some interesting recent innovations in how standard concrete is formed. The company Fab-Form has come up with a continuous footing form made from fabric, eliminating the need for wood forms.

With the "Fastfoot" system, a fabric form hangs on wood rails that act as the top screed level. The company also makes "Fastbag" — steel is inserted into flat fabric bags which are then filled concrete. This system appears to dramatically simplify forming large pads on uneven surfaces.

Architects and builders have been exploring and expanding the design potential of concrete ever since the ancient Romans cooked up the first batch of cement. Ironically, the original recipe that was used for the dome of the Pantheon in Rome is closer to the fly ash formula now being explored than to contemporary Portland cement-based concrete. We keep learning to look back as we look forward.

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Michael Cockram is an adjunct assistant professor of architecture at the University of Oregon. He is the director of the Italy Field School Program.

SUBSCRIPTION SAMPLE Aerated autoclaved concrete blocks can be used like standard masonry units to make arches. Photo: Rudy Berg AAC is easy to cut and drill with common tools. Photo: Rudy Berg For the Headwaters Fort Mandan Visitor Center in North Dakota, the exterior walls are covered with artificial stone containing coal fly ash. Photo: Headwaters Corp. A new fabric footing replaces wood forms in the product "Fastfoot." Photo:Fab-Form The fabric sack, "Fastbag," simplifies pouring a concrete pad footing. Photo:Fab-Form   Click on thumbnail images to view full-size pictures.