Precast Solutions

The Impossible Gleam

High-Reactivity Metakaolin Makes Glass-Aggregate Concrete Practical and Beautiful.

By Michael Chusid, RA, FCSI
Photos courtesy Wausau Tile

“Blues, reds, yellows, greens – vibrant colors that Mother Nature doesn’t make,” says Rodney Dombrowski, Terra Paving Division manager of Wausau Tile, Wausau, Wis. “Designers are very excited about the color and appearance of our new precast concrete products with recycled glass,” Dombrowski adds, “but honestly, Wausau Tile wasn’t even thinking of that when we got involved five years ago with the recycled glass concrete program at Columbia University.”

Dombrowski is speaking about his company’s new line of concrete products filled with brilliantly colored chips of recycled glass. Its story is the confluence of a precaster with a social conscience, a scientist with aesthetic vision, a supplementary cementitious material with all the right properties, and a design community searching for building materials that present fresh design options and greater sustainability.

For Wausau Tile, the key word was “recycled.” In tune with the gathering green revolution, the folks at Wausau Tile wanted to offer an environmentally responsible product. When they heard that a professor of engineering and his team at Columbia University in New York City, had conquered the age-old taboo against glass aggregate, they were interested.

Of all the commonly collected post-consumer materials, glass has been among the most difficult to recycle. Broken, mixed-color glass has proved virtually impossible to reuse cost-effectively. Mountains of it heap up without a buyer. New York City, for example, collects more than 150,000 tons of glass per year, much of which ends up in a landfill because it can’t be recycled.

Employing crushed glass as concrete aggregate would seem an obvious choice, but it has long been known that glass causes portland cement concrete to crumble due to alkali-silica reaction (ASR). ASR occurs when hydrated lime in concrete, released during portland cement hydration, reacts with siliceous materials like glass. The reaction forms a gel that expands and creates internal forces that can fracture concrete and destroy a structure.

For the most part, the concrete industry simply avoids aggregates that are known to be reactive in concrete. In many cities, however, the local sources of nonreactive aggregates are depleted, forcing the concrete industry to embrace alternative strategies for mitigating ASR. However, no one had solved the challenge of using the ultimate reactive aggregate: glass.

At least, no one did until about 12 years ago when Christian Meyer, Ph.D., professor of civil engineering at Columbia, was approached by Erik Vagle, owner of a roofing company in Brooklyn, who suggested finding a use for New York City’s waste glass. There had been extensive research on ASR, notes Meyer, “but no one had specifically addressed the issue of glass. Everyone just ‘knew’ that you don’t put glass into concrete.” Meyer, however, saw glass as the perfect way to study and quantify ASR, since the reactivity of glass is more consistent and predictable than any natural aggregate. “We went straight to the worst aggregate,” he says with a chuckle.

Meyer and his team’s thorough research made surprising discoveries. For instance, the severity of the ASR reaction varies with the color of glass. He also discovered that the alkali silica reaction of very finely ground glass particles did not create destructive forces within concrete; its alkali-silica reaction occurs so quickly – within hours after hydration begins – that it is completed before concrete hardens and therefore doesn’t fracture it.

While Meyer pursued the use of fine-ground glass as a partial replacement of fine aggregate in concrete block, he had more ambitious objectives in mind. Believing that the decorative value of glass could help support its recycling costs, Meyer sought a way to use it as coarse exposed aggregate. He reasoned that if he could include large glass chips, big enough for their beauty to be seen and used architecturally, it would create an important market for post-consumer recycled glass. After several years of research, he found the answer to this challenge.

Metakaolin
“We found that high reactivity metakaolin (HRM) is the most effective admixture you can use to suppress ASR,” declares Meyer. Metakaolin belongs to a class of supplementary cementitious materials known as pozzolans. Pozzolanic concrete additives react with lime to form additional cementitious materials, strengthening and densifying concrete (see sidebar Pozzolan Magic). Fly ash, the most commonly used pozzolan, is capable of controlling or eliminating ASR due to the use of reactive stone, but it can’t cut it with glass; metakaolin can.

Metakaolin was a perfect match for Meyer’s architectural ambitions, too. It is bright white, works excellently with white portland cement, and even brightens gray portland cement and integrally colored concrete. Set in a matrix of concrete brightened with metakaolin, the sparkle of glass is even more dazzling.

Wausau Tile saw the potential. “Building is going green,” explains Dombrowski, “and these recycled glass products fit the need. They all meet LEED requirements.” Wausau Tile licensed Meyer’s technology and, over a period of three years, perfected its glass concrete using MetaMax HRM and a proprietary mixture of other additives. MetaMax HRM is now produced by BASF Construction Chemicals since BASF’s recent acquisition of Engelhard Corp.

Using metakaolin, Wausau Tile has been able to make glass aggregate versions of all its precast products, including site furnishings such as benches, tables and planters; concrete pavers; and terrazzo tile. “The glass in our products is 100 percent post-consumer/post-industrial recycled content and accounts for approximately 33 percent of our product’s weight,” notes Dombrowski, adding that it helps designers qualify for the U.S. Green Building Council’s (USGBC) LEED credits promoting the use of recycled-content building materials.

MetaMax HRM is a good fit for Wausau Tile’s sustainability objective. It is a manufactured pozzolan that can be substituted for up to 20 percent of portland cement in a concrete mix.

Wausau’s glass concrete creations quickly found favor with architects and designers, not only for their recycled content but also for their striking appearance. “Our natural aggregates are rock and don’t offer the vivid colors or sparkle that can be obtained with glass,” says Dombrowski. “Designers who want to build with concrete now have new colors available to them. Probably 50 percent of our sales are because of the “green” nature of the product, but 50 percent are because of visual appeal.”

Wausau offers different mixes of glass color, set in both white and tinted matrix. The white matrix shows off the recycled material with a particularly gem-like sparkle. It’s made with white portland cement and metakoalin, a combination that has proved so bright and durable that it has also been approved for high-visibility precast concrete highway barriers used by the Port Authority of New York and New Jersey and other government agencies.

As a manufactured product, metakaolin’s brilliant white color is highly consistent, so batches of tile can be easily matched. By increasing the density of concrete, metakaolin helps to prevent concrete staining and weathering. Designers can be confident that the initial attractive appearance will not diminish.

For example, the stability and consistency of concrete made with metakaolin has been dramatically demonstrated in the ongoing restoration of Shepherd Hall, a 100-year-old building at the City College of New York. The building’s failing terra cotta ornamentation is being replaced by 40,000 precast units of glass fiber reinforced concrete (GFRC) made with MetaMax HRM. The replacement work began in 1991 and has involved several different precasters from across the country. The color uniformity has been maintained, and precast units installed more than 15 years ago blend seamlessly with new ones.

Wausau Tile’s most striking glass concrete product is probably the terrazzo tile, available in 12-inch, 16-inch and 24-inch squares, featuring glass chips ranging from 1/16-inch to 3/8-inch in size. Wausau Tile manufactures the only machine-pressed cementitious terrazzo tile made in North America. The richly hued tiles have been used in schools, libraries and even art museums.

Defying psychological associations of glass with fragility, Wausau’s tile is durable enough for high traffic uses. Ranging from 1/2-inch to 7/8-inch thick, they have compressive strength of 8,000 psi due in part to additional strength created by metakaolin. (It produces both high early strength and increased long-term strength, up to 18,000 psi.) “The tile itself is very strong,” says Dombrowski. “Bedding it using thin-set mortar over a concrete slab makes it pretty much indestructible.”

Dombrowski sees his glass concrete products as a way to make the recycling concept more accessible to the public. “With some products made from recycled materials, you wouldn’t know it,” he explains. “If it’s a stud in a wall, you never see it. With glass in terrazzo, it has a lot of aesthetic value.”

For more information, visit Wausau Tile’s Web site at www.wausautile.com and MetaMax HRM’s site at www.engelhard.com/metamax.

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Michael Chusid is an architect and a Fellow of the Construction Specifications Institute. He is principal of Chusid Associates and provides technical and marketing consulting services to support the development of innovative building materials. He can be reached at www.chusid.com.

Pozzolan Magic

According to Richard Zap, MetaMax product manager for BASF, pozzolans form additional cementitious material in concrete by chemically combining with hydrated lime (calcium hydroxide). The extra cementitious material increases the strength of concrete and plugs pores in concrete to make a structure less porous and more durable. The lime, on the other hand, is a product of inefficiency in portland cement hydration. It does not contribute to the strength of the concrete, but rather takes up space where cementitious material might otherwise be.

Lime can cause several problems affecting concrete’s appearance, integrity and porosity. The alkaline lime reacts with some siliceous aggregates to form expansive ASR that can fracture concrete. Lime can also leach out of concrete, forming unsightly powdery deposits on concrete surfaces (efflorescence) and leaving behind microscopic pores that make concrete more prone to water and chemical infiltration. Increased porosity also leaves concrete vulnerable to staining and to attack by external sulfates that may be dissolved in ground water. Pozzolans such as high-reactivity metakaolin help to prevent these problems by converting lime into cementitious compounds that create a stronger and denser concrete.

Pozzolans have been in use in concrete since the time of the ancient Romans who mixed pozzolanic volcanic ash with lime to build the Pantheon and the aqueducts that have withstood the test of time for more than two millennia. Some common pozzolans such as fly ash and silica fume are industrial byproducts that vary in color from batch to batch, posing color consistency problems for architectural and decorative products.

High-reactivity metakaolin, on the other hand, is specially produced to provide consistent coloration and performance in concrete. BASF produces metakaolin from kaolin clay. When purified and calcined under carefully controlled conditions, the mineral becomes a highly reactive pozzolan that has been demonstrated to increase concrete strength and durability, resist corrosion and minimize ASR, sulfate attack and porosity. It makes concrete easier to work. Concrete finishers often describe it as giving concrete a “creamy” or “buttery” feel. It is a consistently bright white in color and noticeably brightens both the gray and white portland cement concretes.

An AIA/CES-approved continuing education course on high-reactivity metakaolin is available online at www.engelhard.com/metamax. One hour of HSW credit is available upon passing the course.

MetaMax is a registered trademark of BASF Catalysts LLC

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