Building Even Better Concrete  

Concrete is the most widely manufactured material, with nearly three tons produced each year for each of the earth's inhabitants. Architects value concrete for its durability, compressive strength, versatility, and expressive possibilities. However, production of cement, the fundamental ingredient in concrete, accounts for 5 percent of man-made carbon dioxide (CO2) emissions globally, according to the cement-sustainability initiative, a group of international manufacturers.

The industry is making efforts to mitigate this impact. In the U.S., for example, where cement production accounts for less than 1.5 percent of U.S. CO2 emissions, according to the Portland Cement Association (PCA) estimates, manufacturers have adopted voluntary performance-improvement targets. Their goals include increasing energy efficiency 20 percent by 2020 when compared with a 1990 baseline, and on the same time line, to reducing CO2 emissions by 10 percent per ton of product.

On the first goal, the domestic cement industry has already made significant progress, with a 12 percent reduction in energy use, according to a survey of U.S. and Canadian cement plants conducted annually by PCA. The trade association is now analyzing data relating to its CO2 goals, but early assessments indicate that manufacturers are well on their way to meeting emissions targets, according to Andy O'Hare, the group's vice president of regulatory affairs.

But the kind of reductions necessary to counter rising demand from emerging economies like China and India may be hard to come by, simply because of the chemistry and physics involved in cement production, which is a multistep process. To make portland cement-the binder in most concrete, typically referred to simply as cement-manufacturers first collect limestone and other raw materials, such as sand, shale, iron ore, and clay, from quarries. These materials are analyzed, blended, and ground.

Domestic cement manufacturers are examining processes with an eye toward saving energy and reducing greenhouse-gas emissions. Photography: © Portland Cement Association

The next step expends the most energy and adds the most to greenhouse-gas production: The combined ingredients are heated at temperatures up to 3,400 degrees in a kiln where the components react and partially fuse. This chemical reaction, known as calcination, produces a marble-size substance called clinker, that is then cooled and ground with small amounts of gypsum and limestone to produce cement. Mixing the cement with sand, aggregates, and water produces concrete.

Calcination and the burning of fossil fuel required to maintain the kiln at the necessary high temperatures are responsible for about 95 percent of the greenhouse-gas emissions generated by cement manufacturing, according to O'Hare. Although manufacturers are tackling fossil-fuel consumption with strategies such as heat exchange and energy recovery from waste materials such as tires and biomass, the greenhouse gases associated with calcination will be harder to crack. "These emissions are almost impossible to reduce unless we find a substitute for limestone," says O'Hare.

Even concrete has DNA

And in fact, there are scientists working on identifying such a substitute. A team at the Massachusetts Institute of Technology is studying calcium-silicate-hydrate particles (C-S-H), the basic building blocks of concrete. The researchers are poking and prodding various hardened cement pastes with a nano-size needle and examining the indentations under an atomic-force microscope to better understand the material's properties. They have discovered the C-S-H has a unique "nanosignature," or genomic code, indicating that the strength of concrete does not rely on a specific mineral but on the organization of that mineral as packed nanoparticles, opening the door for identifying an alternative mineral to be used in cement.

Franz-Josef Ulm, a professor in the department of civil and environmental engineering at MIT, maintains that bone-or the apatite minerals that form the "ultrastructure" in bone-could provide a model. Like concrete, bone consists largely of calcium and "achieves a very similar packing density at the nanoscale, but is ‘manufactured' at body temperature with no appreciable release of CO2," he explains. Unfortunately, hydration and hardening of these minerals take about a month. "But if we can find a way to mimic the process and speed it up, we could replicate it to fashion a new building material."

Quarry Proportioning, blending and grinding Preheat tower Kiln Clinker cooler Bagging and shipping Dry Process Wet Process To make portland cement, manufacturers gather raw materials that include limestone, sand, shale, iron ore, and clay. The materials are blended and ground, typically using a dry process, though some older plants rely on a more energy-intensive wet process. The combined materials travel through a preheat tower (top left and right) and then to a kiln (middle) where temperatures reach 3,400 degrees. Inside the kiln, a chemical reaction known as calcination produces a marble-size substance called clinker. Manufacturers then cool and grind this intermediate material and add small amounts of gypsum and limestone to produce cement. Images: Courtesy Portland Cement Association

Another team, made up of scientists from the National Institute of Standards and Technology and Northwestern University, is also examining C-S-H particles at the nanoscale. The researchers are studying neutron beams as they pass through the particles to classify the location of water within the cement paste to help understand its chemical and physical structure. Though the methods and immediate interests of the two studies differ, the ultimate goals are essentially the same: "To create a stronger, more durable, and environmentally friendly material through a refined scientific understanding of C-S-H," says Georgios Constantinides, a postdoctoral researcher in materials science and engineering at MIT.

Mixing portland cement with gravel, sand, and water produces concrete. Image: © Portland Cement Association

The embedded energy in concrete can also be reduced by replacing some of the cement with so-called supplementary cementitious materials (SCMs). These materials-slag cement, fly ash, and silica fume-are waste products of other industrial processes. Slag is a by-product of steelmaking, while fly ash is created when coal is burned at power plants and other industrial facilities. Silica fume is a by-product of silicon and ferrosilicon metal production. The use of these materials reduces the amount of virgin material in concrete and also conserves landfill space. The strategy of replacing cement with SCMs is "dilution as a solution to pollution," says O'Hare.

Replacement of cement with SCMs can also impart physical benefits to concrete. For example, the addition of slag can create, generally speaking, concrete that is more dense and less permeable to water, and therefore more durable. One possible drawback is that mixes with high percentages of slag can take longer to gain strength. However, even though it is slower to set, slag cement can ultimately achieve higher compressive strengths than conventional concrete.

Although slag cement's longer set time could in theory mean slower construction, many designers and contractors say that they have not experienced delays related to use of high percentages of the supplementary material. Andrew Mueller-Lust, a principal at New York City−based structural engineer Severud Associates, says that even in sub-zero temperatures, contractors maintained a two-day pour cycle on the Helena, an apartment building on the West Side of Manhattan. The concrete mix for the 600-unit Helena, designed by FXFowle and completed in late 2005, contained 45 percent slag. Severud has so far used the mix on three New York City projects, including One Bryant Park, the recently topped out office tower designed by Cook+Fox. "We have not seen delays materialize," says Mueller-Lust.

Use of by-products from other industrial processes in cement mixes can help conserve virgin material and landfill space. Two New York City projects that include a high percentage of blast-furnace slag, which is a by-product of steelmaking, are the recently topped-out Cook+Fox-designed One Bryant Park (top left and bottom), and the Helena (top right), by FXFowle. Images: © dbox for Cook+Fox (top left); Jeff Goldberg/Esto (top right); Severud Associates (bottom)

Construction industry insiders say that concrete containing at least small quantities of SCMs is now more usual than that without. Use of slag and fly ash "is now as common as unleaded gas," says Mel Ruffini, senior vice president of Tishman Construction in New York City.

A few researchers are hoping to use these normally supplementary ingredients to completely replace cement. Civil engineers at Montana State University are working on using locally available high-calcium fly ash as the sole binder in structural-grade concrete. Since the summer of 2002, they have successfully completed a handful of field trials on small structures, including a residential foundation, using conventional mixing, transporting, and placement equipment.

The team members' first challenge was the mix's short set time, but they have since learned to control it by adding borax as a retarder. "By adjusting the admixture, we can dial in the set time, from 1 to 5 hours," says Jerry Stephens, a professor at the university's Western Transportation Institute. A huge unknown that remains, however, is durability, and they plan accelerated testing for such factors as freeze-thaw performance. "The material doesn't have the history of portland cement," concedes Stephens.

The whole building

Though cement production is energy-intensive and CO2-emitting, if architects focus exclusively on these aspects of the material, they run the risk of forgoing performance efficiencies that concrete can provide, especially when used as part of a whole-building approach to design. One example of a project that is the outcome of such a process is the recently completed U.S. Federal Building in San Francisco [record, August 2007, page 96]. Half of the cement in the 18-story tower's exposed reinforced-concrete structure was replaced by blast-furnace slag, preventing release of about 5,000 tons of CO2 into the atmosphere, according to estimates from Morphosis, the project's architect. But even more noteworthy is the building's reliance on natural ventilation to cool its upper 13 floors. The concrete structure and its thermal mass are key components of this strategy.

The tower has a building automation system (BAS) that controls operable components in its exterior window walls. The BAS opens these apertures to cool, or "charge," the concrete during the night, when warm weather is expected the next day. Once the structure's temperature has dropped sufficiently, the BAS closes the openings. Then, during the day, heat generated by occupants, computers, and lights is transferred to the slab by radiation.

The Morphosis-designed San Francisco Federal Building relies on natural ventilation and an exposed reinforced concrete structure to cool the upper floors of the 18-story tower (middle). The building's structure is "charged" at night when operable components in the exterior window walls are opened (bottom). The office areas (top) feature exposed slabs that have a wave profile in section. The configuration provides a large surface area and aids absorption of heat. Photography: © Nic Lehoux

The architects and engineers worked together to find the structural configuration that would provide maximum cooling. One decision made in order to enhance the performance of this system was to use normal-weight concrete despite the possible structural benefits of using lightweight concrete in the highly seismic region. The entrained air in the lighter material would have made the structure act like an insulator. Instead, a structure that quickly absorbed and released heat was required, explains Steve Ratchye, now an associate in the Los Angeles office Thornton Tomasetti. Ratchye was formerly project structural engineer for Arup, the m/e/p and structural consultant on the Federal Building.

Another feature of the structure that aids the natural ventilation strategy is its raised floor/upturned beam configuration. By avoiding perimeter beams, the designers enhanced the flow of air, the penetration of daylight, and access to views. The slab has a wave profile in section, providing a larger surface area than a flat slab, enhancing its ability to absorb heat. In the design of this element, the requirements of thermal mass, architecture, structure, and daylighting all coincide, says Ratchye.

In addition to more emphasis on an integrated approach to design like that employed for the Federal Building, new technology, such as ultra-high-performance concretes (UHPC), could help architects produce more environmentally benign buildings. One UHPC is Ductal, introduced several years ago and developed by chemical company Rhodia, the construction arm of Bouygues, and building products manufacturer Lafarge. It incorporates metallic or organic fibers and does not require reinforcement. Because its compressive strength is 6 to 8 times greater than conventional concrete, the material allows for smaller structural members and therefore has fewer associated greenhouse-gas emissions. According to a study commissioned by Lafarge comparing a bridge made of Ductal to one with a conventional concrete-and-steel structure, the Ductal-only solution required 50 percent less material by volume, which translated into a 50 percent reduction in CO2 emissions, explains Vic Perry, Ductal general manager, Lafarge North America. The study takes into account not only embedded energy, but also reduced maintenance needs over a projected 60-year life span, he explains.

Highway barriers by fieldoffice, made of photocatalytic cement, could protect adjacent areas from traffic-generated air, sound, and light pollution. Rendering: Courtesy Fieldoffice

Another recently introduced cement, TX Active, has environmental benefits discovered almost by accident. The material, which contains titanium dioxide, was first developed by Italian manufacturer Italcementi for Richard Meier's Jubilee Church, in Rome [record, February, 2004, page 101], to help to maintain the building's brilliant white appearance in perpetuity. But scientists later discovered that the product acts as a photocatalyst, using light to help break down airborne pollutants, such as particulate matter, volatile organic compounds, and nitrogen oxides. To some extent, conventional concrete also breaks down pollutants, explains Dan Schaffer, product manager for Essroc Cement, the U.S. distributor. "But the titanium dioxide accelerates the process," he says.

Since completion of the Meier building in 2003, the product has been used on several European projects, but has not yet been specified in the U.S. One prospect is a "superabsorber" developed by fieldoffice, a Clemson, South Carolina−based architectural practice. The firm first proposed using the material to create spongelike highway barriers that would dissipate the light, sound, and air pollution generated on heavily traveled roadways. But now several school districts are interested in deploying the porous panels to help improve air quality at urban sites, according to Doug Hecker, fieldoffice principal. Hecker is exploring both 3D printing and more conventional forming methods for producing the panels, which at least in his initial concept, were to be nonrepeating. But more practical concerns have taken over: "We are now investigating ways a limited number of panel types can be combined to produce different effects," he says.