A new type of concrete as strong as steel. Sounds impressive. What would be the secret ingredient? How about the most abundant mineral in the earth's crust? Silica – silicon and oxygen (SiO2) – occurs in soil, rock, and similar granular forms throughout the world. In its many processed forms, silica sand is widely used in industrial and commercial applications. But it's already a common component in cement. So what's new?
"What's left over?" may be a better question. It's not exactly one of silica's processed forms that interests Tarun Naik, an associate professor of civil engineering and the director of the UWM Center for By-Products Utilization (CBU). He wants to make ultra high-strength concrete with the silica left over after it's processed.
The various processes of mining, grading, and packaging silica sand create a lot of dust. In the modern era of environmental regulations, such industrial by-products must be contained. Although it's pure silica, the dust is too fine for most industrial applications and is usually discarded after it's collected. But it could be just right as a component in the new concrete.
Naik has built a career out of finding ways to turn industrial waste into reusable resources – and disposal costs into profits.
"A waste is not a waste until you waste it," he is fond of saying. He has been widely recognized for improving the quality of concrete made with fly ash – a by-product of coal burning. Under Naik's leadership, the CBU has also found a wide variety of uses for by-products from pulp and paper mills, foundries, and other sources. "When I say these by-products are offering exciting opportunities for students and myself, it is really true: to make concrete as strong as steel," Naik says.
Using the fine dust, mostly from silica processing, limestone quarries, and foundries, would result in extremely dense, and thus very strong, concrete.
The standard steel for construction is A36, or strength of 36,000 pounds per square inch, which Naik hopes to match with the ultra high-strength concrete. "I'm pretty confident we'll reach 30. How close will we come to 36? I don't know. Even if we can reach 25 – which is what some people have been able to reach with virgin materials – if I can reach that with recycled material, that's a progress, because the cost will be tremendously reduced with recycled material."
While the ultra high-strength concrete could save silica dust from landfills, it could also someday help preserve the environment by reducing steel production, Naik says. Mining iron ore damages the land and the various processes of making steel from the ore pollutes the air.
Naik jokes that he'll have to be careful about who gets the recipe for the ultra high-strength concrete. "I'll make sure we don't sell it to guys like Saddam Hussein, because he will want to build a bunker a hundred feet below ground. But the real idea as I understand it, more seriously, did come from those types of needs."
Fly ash was Naik's first area of research, starting with his 1963 master's thesis at the University of Wisconsin in Madison. In power plants, coal is ground to about the fineness of talcum powder and fed to the fire. About 10 percent of inorganic material in coal does not burn off, and floats away. This is called fly ash. Some of it clumps together and falls down, and becomes bottom ash. Fly ash once filled industrial areas with black smoke, but environmental standards now require that fly ash be collected. About 100 million tons of coal ash is produced annually in the United States, about 1.7 million of that in Wisconsin, Naik says.
Fly ash, as well as bottom ash, have been found to have properties that can strengthen concrete. Concrete is made by gluing together sand and stone with a paste of water and portland cement. Portland cement gets its strong binding power from being a silica- and lime-based product. Fly ash is also silica-based, and can strengthen concrete when the silica reacts with a type of lime in portland cement.
In addition to its chemical properties, fly ash's physical properties also improve the performance of concrete. Fly ash is composed of smooth, spherical particles, which can improve the flow of fresh concrete. This more fluid mixture also requires less water, increasing the strength of hardened concrete.
In his worldly travels as a speaker, lecturer, and consultant on by-products utilization, Naik has found that the quality of fly ash varies from region to region, and even from boiler to boiler. Wisconsin fly ash is some of the best in the country for making concrete, he says, and the best he has worked with in the world comes from Barcelona, Spain.
Naik's most recent innovation with fly ash involves mixing two types together. The two major types of fly ash – from two different types of coal – are Class F, the type available when Naik first began his studies in the early '60s, and Class C, which appeared about 15 years ago. While Class C is much better for making concrete, Naik thought that certain properties of Class F could complement Class C and improve the overall quality of concrete. He found that at the right proportions, a C-F mixture added to cement yielded concrete with higher strength and durability than recipes using only Class C fly ash or only virgin materials.
Many Wisconsin utilities had set a goal of recycling 100% of their fly ash by the year 2000. Naik says stricter environmental standards scheduled to take effect in 2002 have complicated their efforts. The new EPA rules require more nitrogen and sulfur oxides – so-called greenhouse gases – to be captured during coal burning, which would give the resulting ash a strong ammonia smell. Concrete made from the new ash would still be high quality, "but that concrete will also stink," Naik says. Scientists in several countries and the United States, including Naik's research group, are currently trying to develop cost-effective ways to remove the ammonia.
Because the CBU is not publicly funded, Naik relies on industry and state agencies to support his research. As a result, he has broadened his focus from coal ash to include by-products from many industries. Major CBU sponsors include Wisconsin Electric Power Company in Milwaukee, Wisconsin Public Service Corporation in Green Bay, Madison Gas & Electric Company, and Manitowoc Public Utilities. Other supporters include pulp and paper mills, foundries, and other industries. Although the range of by-products has expanded, much of the CBU's work still relates to cement and concrete.
In much of his recent work with fly ash, Naik and the CBU have collaborated with Bruce Ramme, Wisconsin Electric Power Company's principal engineer and one of Naik's former students. "Dr. Naik is definitely an expert in the concrete field as well as the industrial by-products field," Ramme says.
"It's very helpful for us to have students that graduate from UW-Milwaukee with the knowledge they have on coal-combustion products," he continues. "When they go out into the work force, they think of using these (recycled) materials as an alternative…[T]hey know how to use them and the benefits of using them."
The CBU, located in UWM's Engineering and Mathematical Sciences building, includes three laboratories: Cement Laboratory, Concrete Laboratory, and the Construction Materials Laboratory. Lab workers include graduate and undergraduate students as well as faculty and staff. In the various labs, samples are prepared and subjected to simulated weather conditions, immersion in chemically treated water, and – especially important in Wisconsin – salt exposure.
Foundries are another large industry in Wisconsin – and another major source of by-products. In foundries, the molds into which metals are cast are made primarily of sand. Also, the iron that foundries collect and melt down contain impurities such as rust, which form a scum called slag on top of the molten iron. The CBU estimates that 900,000 tons of used foundry sand and slag are discarded every year in Wisconsin. The CBU has developed recipes for regular- and high-strength concrete using foundry sand, and adds slag to various types of concrete. Naik's foundry sand research has been sponsored by many Wisconsin foundries, the Wisconsin Department of Natural Resources, and other organizations.
Using by-products from paper recycling may also improve the quality of concrete.
When used paper is recycled, it goes through what Naik calls a paper "laundry." First the ink and coating are removed. Next to be removed is the filler, made of calcium carbonate or similar materials. What's left are wood fibers. Some of these wood fibers are too small or damaged to be recycled back into new paper. The non-recyclable wood fibers, filler, coating, and ink – about one third of the original paper – are collectively called de-inked solids, and Naik has found that its wood content would make it useful in concrete. He has worked with the Wisconsin Recycling Market Development Board, as well as forest-products company Weyerhaeuser, based in Tacoma, Washington, to use the sludge from de-inking facilities in concrete.
Concrete is strong under compression load – when it's under pressure or supporting weight. But under tension load – when it's being pulled apart – wood is stronger than concrete.
"The fiber can provide certain value to the concrete," says Naik, who teaches wood as well as concrete engineering courses. "What the fibers do is basically stitch together the micro-cracks in concrete in order to improve the tensile strength. Wood is actually strong in tension and not as strong in compression. So I said, 'A-ha! Why not?' If you put the two and two together, some material that is strong in tension and some that is weak in tension, we can get an improvement out of it.
"Instead of de-inked solids, you can use virgin materials, but that costs you money to buy them. De-inked solids come basically for free. Or, in effect, de-inked solids is costing the industry 30, 40, 50 dollars a ton to throw away."
The process of making pulp, used for paper production, yields more by-products that Naik sees as useful. The CBU reports that pulp and paper mills produce over 3.7 million tons of primary effluent treatment solids, commonly called "sludge," every year. Forty-five percent of that is placed in landfills, which costs the industry about $70 million annually.
Timber is turned into pulp by various chemical and mechanical processes that require a lot of water. A typical Wisconsin pulp mill uses millions of gallons of water a day from nearby rivers, Naik says. Mills are required to clean the water they return to the rivers, so it's often cleaner than before. A critical component for both the chemical processes in making pulp and the cleaning of the water is lime, which is made from limestone. A by-product of these various chemical processes is lime mud. From this mud come grits and dregs. Grits are non-lime materials that are dug up with the limestone. Dregs contain limestone that was not converted to lime.
Like portland cement, Grits and dregs are mostly lime- and silica-based, so Naik thinks they could be used to make cement-based materials such as concrete or concrete products. While not nearly as effective as portland cement, grits and dregs could still be used in concrete indoors or in climates more stable than the volatile Midwest.
The National Council of the Paper Industry for Air and Stream Improvement (NCASI), a Kalamazoo, Michigan-based environmental research group supported by forest products companies, had approved preliminary funding for the CBU to develop concrete using grits and dregs for bricks, blocks, and paving stone. For this project, Naik's recipe also includes coal ash and another by-product the CBU has begun studying – materials dredged from harbors.
In ports in Wisconsin and elsewhere, agricultural and forest runoff and natural wave actions cause silt and sand to accumulate at the bottom of the harbors. When the bottom is dredged, the silt and sand are usually put in a landfill or placed in a confined disposal facility nearby. But with landfill space shrinking, Naik envisions more productive uses for the dredged materials. This could be especially valuable in Wisconsin, where Naik says the tonnage of cargo through harbors equals the tonnage of truck cargo. Dredged materials were among the by-products Naik identified in 1988 when he founded the CBU, but he has only recently received funding to work with them.
Naik says the more organic dredged material is "beautiful stuff for agriculture." In a project with the U.S. Army Corps of Engineers, the CBU is working on combining dredged materials with wood ash and other materials to make fertilizer and topsoil that could be used by nurseries, Christmas tree farms, and forests planted by paper mills.
One problem with dredged materials is that they contain polycyclic-aromatic hydrocarbons. "Basically that means that it is a whole bunch of things that stink," Naik jokes. PAHs come from inadequate combustion of fuels from cars, wood stoves, barbecues, and other sources. "Any of those hydrocarbons that we burn as an energy source eventually end up in the rivers, in the lakes."
By making something solid from dredged materials, such as bricks, the odor can be contained. In the case of fertilizer or topsoil, wood ash can help absorb the odor.
Another by-product of the paper and timber industries, wood ash comes from the burning of twigs, leaves, bark, roots, and areas with knots, which are unsuitable for making paper or lumber. Wood ash is an effective odor absorber because it contains activated carbon. Activated carbon, which also exists in coal, coconut shells, and peat, is often used in municipal and industrial water supplies to reduce undesirable tastes, odors and dissolved organic chemicals. The wood ash can also disinfect and dehydrate the dredged materials.
The CBU is combining many industrial by-products into its recipe for flowable fill, also known as flowable slurry or, as Naik calls it, manufactured dirt. The material can be used in construction work to pour around new building foundations, re-fill trenches and other excavations, or pour into abandoned underground pipes and tunnels. Often made from cement, water, and sand, the fill begins as a very thick, pea soup-like mixture that is transported and poured much like ready-mix concrete. Flowable fill can take the shape of the space and fill it more tightly than traditional backfill materials. The fill hardens into a material similar to very low-strength concrete that can be re-dug if necessary. It is often specified by municipalities, state highway departments, and engineers for many applications.
Much of the CBU's work with flowable fill is in collaboration with Wisconsin Electric Power's Bruce Ramme. Together they conduct frequent flowable fill seminars, which attract people from all areas of engineering and industry, Naik says.
When Naik wrote his first paper on flowable fill in 1988, "there was hardly a handful of similar ideas in technical literature available," he says. "Nowadays the flowable slurry has become so popular that it would not shock me if you found 500 (articles) published worldwide in the last 10 or 15 years. There's a huge amount of interest." Naik's recipe for the fill includes industrial by-products such as coal ash, used foundry sand, dredged materials, crushed glass, or wood ash, along with water, a small amount of portland cement, and sometimes regular concrete sand. It can be made with a strength of up to 1200 psi.
In another project with the U.S. Army Corps of Engineers, Naik has traveled to West Virginia to develop a fill recipe using local materials for a project to rebuild a lock on the Kanawha River near the city of Marmet.
"The new idea that we came up with is to use the old material that they dig up, to use that soil material itself, into making this manufactured dirt," Naik says. "Normally they would haul it away and throw it away, and then bring in clean soil or sand. Instead, we'll take the old soil, not haul it away, mix it up with some cement and coal ash, and a lot of water, and basically pour it in there like flowing mud."
Reusing the old dirt will also save valuable space. Originally, the Corps had planned to create huge mounds from the dirt and leave them near the riverbanks, which would have required the federal government to buy out adjacent property that was developed since the old lock was built some 100 years ago. By reusing the dirt, fewer residents and businesses will have to be relocated.
Naik has also worked with foundries to develop flowable fill with used foundry sand, and with Weyerhaeuser Company to develop a recipe using wood ash. Naik worked with Manfred Buder, Weyerhaeuser's solid waste minimization project manager in Tacoma. Buder says he has been impressed with Naik's perspective on reusing by-products. "He brought some great new ideas into the fold – things that we as a forest products company don't normally work on," he says. "He has taken us in a new direction that we need to go." Buder also credits Naik for being an effective arbiter, which allows ideas to be exchanged among different companies. "It enriches the whole industrial community."
Lori Pennock, generation services coordinator at Wisconsin Public Service Corporation, a utility company serving northeastern and central Wisconsin and part of upper Michigan, says Naik has been important for her company and its customers. "One of the ways we use Tarun is to increase awareness … that the CBU can benefit our industrial and commercial customers in analyzing their waste streams and trying to find ways for them to beneficially reuse their product, in addition to helping us with our own particular ash research needs."
Naik told Research Profile in 1994 that the abundance of industrial by-products in Wisconsin could lead to small cement mills throughout the state. While that hasn't happened yet, there has been activity – at the CBU and elsewhere – to formulate new cement recipes.
The CBU has developed several new recipes, but has not made it public yet, as Naik explains with an analogy. "We want to have the idea that will sell some people the syrup and they can do the bottling of the Pepsi or the Coke. We have the recipe for the syrup part."
So far, no one has taken the leap to using their cement recipe, but Naik says Wisconsin Electric and National Minerals Corporation in Minneapolis – which both started their by-product-utilization efforts as CBU customers – are working on their own recipes. "Not exactly the way I would have made it," he says, but he's encouraged by the interest.
When Naik founded the CBU in 1988, his vision for recycling pre-dated that of the state government, which did not pass recycling legislation until 1990. He first envisioned the profitability of recycling industrial waste as a master's student in Madison in the early '60s, where he learned about using coal ash in concrete.
"I knew we had a lot of fly ash in the city where I came from," he recalls, referring to his hometown in India. "And that power plant back then, and to a point even today, made a big mess in the neighborhood. I said, ‘I'm going to be a rich guy when I go back. I'm going to get this ash and sell it.’ "
While he never returned long enough to get rich from fly ash, Naik's early years in India have been a lifelong influence on his career. "There, people recycle everything," he said in 1994. "If you buy juice in a plastic jug, afterwards you wash out the jug and return it to the store for refills. I can't remember my mother throwing away any garbage or anything, ever." Even in the 1950's, recycling vendors in India bought back glass, plastics, steel cans, and newspapers.
Today, Naik says his backyard compost pile is probably the only one in his Wauwatosa neighborhood, where his household creates only about a quarter of the waste produced by others.
"It is an abhorrence to waste that motivates me."
His motivation has given him a reputation as an energetic and productive researcher. "He has a very good working relationship with local and national industries," says Al Ghorbanpoor, chairman of UWM's Civil Engineering and Mechanics Department. "He is one of the more active faculty members in the College of Engineering and Applied Science."
Even with his worldwide recognition for engineering research, Naik considers himself an educator first – and it shows. Despite his reputation as a challenging and demanding instructor, Naik's students consistently give him positive evaluations. In addition to teaching and service awards from UWM's College of Engineering and Applied Science, Naik has been honored by professional organizations including the American Society of Civil Engineers, the Wisconsin Society of Professional Engineers, the American Concrete Institute, and the Mexican Cement and Concrete Institute.