Building Small

With its soaring concrete sails reaching high into the sky, the Dio Padre Misericordioso Church, just east of central Rome, beckons religious and architectural devotees alike. The structure is also something of a temple to nanoscience—for it retains its bright white hue thanks to the presence of nanostructured titanium dioxide particles embedded within the cement binder that was used to make its concrete walls.

Completed in 2003, the church, also known as the Jubilee Church, is a flagship when it comes to the use of nanotechnology in construction. But there are more humble examples, too. Whether it’s in steel, concrete, or windows, nanotechnology is finding a growing number of applications in the construction industry, where it promises to make structures that last for centuries and look as clean as the day they were built.

One only has to look at the Jubilee Church to see why it is the foremost example of what nanotechnology has to offer the construction industry. It was designed by Richard Meier, an American architect with a reputation for creating bright white structures that he wants to stay that way. So far, the concrete shows no signs of darkening. Italcementi, the company that supplied the material for the church, checks it each year for signs that its white color is still as bright as the day it was cast.

Nanostructured TiO₂ particles theoretically will keep the concrete white forever, even in smoggy Rome, says Luigi Cassar, one of the material’s inventors. Titanium dioxide, known for its snowy white hue, is used as a pigment in paint and food coloring. But it has self-cleaning properties as well. When ultraviolet light strikes the anatase form of TiO₂, it excites the material so that it becomes a catalyst for oxidizing organic grime.

And the concrete doesn’t just resist smog, it eats smog. The same photocatalytic chemistry that keeps the church clean also cleans the air around it, gobbling up NO_x, SO_x, carbon monoxide, aromatics, ammonia, and aldehydes. Italcementi estimates that if it covered 15% of the visible surfaces of a large urban area, such as Milan, with its current product containing the smog-eating nanostructured substance, TX Active, it could cut the city’s air pollution in half.

“The work started from a marketing request to make a white concrete stabilized with respect to pollutants,” says Cassar, a research and development consultant who served as Italcementi’s corporate R&D director until 2006. Cassar and his research group began working on the project in 1994 and filed two patents in 1996 and 1997 for the photoactive cement, which is used as a binder to make the white concrete. It was first used in a construction project in 2000, for the Cité de la Musique building in Chambéry, France. Subsequently, the material was chosen for the Jubilee Church, as well as other projects around the world.

“I am an old chemist. I worked in several industries before coming to Italcementi,” Cassar says. “I never worked on cement before, but the mechanism of TiO₂’s photodecomposition was known in other materials, such as polymers.” Even so, he adds, his group at Italcementi was surprised to see that TiO₂ particles nanostructured to have a high surface area were so active when used in concrete. “We observed a synergistic effect between the photocatalyst and the concrete because concrete is a porous material which absorbs pollutants, helping them to de compose.”

Although the photocatalytic cleaning effect is observed in most forms of anatase TiO₂, Cassar says it’s important that the material be nanostructured to have a high surface area in the cement application. The material used in Italcementi’s cement has more than 200 m² of surface area per gram.

“Improvement of the material is ongoing,” Cassar tells C&EN. The TX Active material currently sold by Italcementi is more efficient than the cement that went into the Jubilee Church, he adds.

They’ve also improved the price. When the Jubilee Church was built, Cassar estimates, the special cement cost 10 times as much as standard cement. Now, depending upon the application, its cost can be as low as three times that of conventional material.

Even with such high-profile buildings, nanotechnology’s progress has been somewhat slow in the construction industry, where tight bottom lines don’t always leave room for technological advances. “Construction is rather different from other industrial sectors,” says Peter J. M. Bartos, former head of the Scottish Centre for Nanotechnology in Construction at the University of the West of Scotland. “Unlike other industries, for example, microelectronics, aerospace, or even the automotive industry, the level of investment in construction research is very low.”

Also, Bartos says, “the construction industry is dominated by small and medium-sized companies. There are no big players in construction like there are in the chemical industry, where there are massive R&D efforts. Most of construction is, in fact, done by small companies that just employ a few people.”

Surendra P. Shah, a civil engineering professor at Northwestern University and the former director of the school’s Center for Advanced Cement-Based Materials, has spent more than a decade studying how nanotechnology can improve cement and concrete. He agrees with Bartos that there’s been growing interest in bringing nanotechnology into the construction industry, but, he says, “as far as commercial applications are concerned, it’s still in the beginning stage.” Shah thinks there are promising applications, and he notes that the growing presence of nanotechnology in construction materials prompted the National Academies’ Transportation Research Board to hold a symposium on the use of nanotechnology in concrete and cement last year.

One area in which Shah’s group has been conducting research is using carbon nanotubes and nanofibers to reinforce cement and concrete. “When you see cement and concrete, you see cracks because it’s a brittle material,” he explains. “These cracks start at the nanoscale. We have shown that once you know how to disperse them, very small amounts of carbon nanotubes, such as 0.05%, can substantially increase the cracking resistance.”

Well-dispersed carbon nanotubes or carbon nanofibers alter cement’s nanostructure, providing reinforcement, Shah notes. “That’s exciting because that’s what nanotechnology is about, altering materials at the nanoscale,” he says.

Nanotechnology can also make concrete a more sustainable material, Shah says. For example, fly ash, a by-product of coal production that would otherwise be dumped in a landfill, can be used in place of cement to make concrete. The problem, Shah explains, is that the concrete-making hydration chemistry takes longer with fly ash.

Seeding fly-ash concrete with small amounts of nanoparticulate silica can accelerate hydration, Shah says, so you could use fly-ash concrete and have the same speed of construction that you would when using conventional concrete. Furthermore, he adds, the nanoparticles change the nanostructure of hydrated cement, thereby improving durability.

Chemical giant BASF has already taken this seeding idea and created a commercial product for speeding up the hardening process for conventional concrete. Known as X-Seed, the product contains nanocrystals of calcium silicate hydrate. The extremely small size of the X-Seed crystals creates many sites for nucleation, accelerating the speed at which the concrete hardens.

Practically, that means that it’s possible to make precast concrete structures, such as bridge girders, sewer pipes, and staircases, faster than it would take without X-Seed, says Bruce Christensen, BASF’s vice president of global technology and innovation management for construction chemicals. The company estimates that X-Seed can cut hardening time for precast concrete structures from 12 hours to six hours at ambient temperature.

“It’s not something that’s a completely new idea,” Christensen says of X-Seed. According to the patent literature and publications, using nanoparticles to accelerate hydration in concrete was proposed decades ago. “Our research group has found some creative ways to make the particles in such a way to realize the seeding effect,” he says. Specifically, the researchers developed some polymer technology to keep the nanocrystals from fusing together when dispersed in solution.

As far as cement-related products go, BASF also makes a whole line of additives bearing the nano moniker, including Nanocrete, Nanoflott, Nanolight, and Nanosilent. These products don’t contain nanoparticles, Christensen explains; rather, they form nanostructures as they’re used. “Nanostructures have been around for centuries in that regard, but we are understanding better and better how these nanostructures form and in which way this process can be influenced to enhance the properties of the material,” he says.

BASF’s use of nanotechnology in construction materials isn’t limited to cementitious products. They also make Col.9, a dispersion of organic plastic polymer particles and nanoscale particles of silica, which is used as a binder to produce façade paints. According to the company, this combination of elastic organic material and hard mineral resists cracking in cold temperatures but doesn’t become tacky when it’s hot outside.

The nanoparticles in Col.9 are also hydrophilic, spreading rainwater across the surface of the coated area. During heavy rain, this property helps the coated surface wash away dirt, and after the rain, it disperses any remaining water into a thin film that dries quickly, thereby preventing mold.

Christensen expects nanotechnology’s presence in construction chemicals to grow. “The nanotechnology ideas finding their way into construction in a practical way are probably now starting to gain momentum” because the first commercial products have finally hit the shelves, he says.

Another nanostructured building product that’s on the market is a type of steel, known as MMFX 2, developed by MMFX Technologies. Its inventor, University of California, Berkeley, materials science professor Gareth Thomas, first used electron microscopy to peer into steel’s nanostructure in the 1980s. Twenty years later, Thomas led MMFX in developing a series of key patents for making nanostructured steel.

“Making steel was always a black art or a black science,” says MMFX founder and former chief executive officer David C. Pollack. “They used to heat, beat, and hope. They kind of understood what was going on, but with electron microscopy they could actually see what was happening at the nanoscale,” he says. “This gave us a whole new understanding.

“Conventional steel, when it cools, goes through a transformation where it loses its affinity for binding carbon. What happens then is carbon precipitates and that precipitation forms carbides at the grain boundaries,” Pollack explains. “These carbides are very hard, but they’re also very brittle and they’re dissimilar to the rest of the steel microstructure. They’re the Achilles’ heel of the steel.”

In a moist environment, Pollack continues, the carbides form a microgalvanic cell with the steel’s ferrites, which begins to corrode the steel from within. But MMFX 2 steel is different. It’s made of alternating nanoscale layers of austenite and martensite—two crystal forms of steel—and is virtually carbide-free at the grain boundaries.

Without the carbides at the grain boundaries, the material is ductile, rather than brittle, and resists the corrosion seen in conventional steel. The nanolayered structure also makes the material strong, Pollack says, because it’s composed of both hard and soft layers of material that can bend without breaking.

MMFX 2 steel is made with conventional steel-making equipment. Pollack says that when talking about nanotechnology, people often marvel at materials made by the gram. “In the case of MMFX, we can make nanotechnology at 100 tons an hour,” he says.

The material has been used in buildings, highways, and bridges and has an expected service life of 200 years. And because it’s twice as strong as conventional steel, Pollack notes, structures require less steel to do the same job. So although the steel itself is more expensive than conventional material, labor costs are reduced.

A sector of the construction industry where nanotechnology has been making a clear difference in products is in window glass. By adding a nanoscale coating of TiO₂ to glass, companies make low-maintenance windows that can clean themselves.

It’s the same chemistry that keeps the Jubilee Church in Rome clean: UV light activates TiO₂ so that it oxidizes organic grime, both directly and by converting water vapor into hydroxyl radicals that can convert organic compounds into CO₂.

The practice of using titanium dioxide to make surfaces self-cleaning is fairly old, says Chris Barry, director of technical services at the glass-making company Pilkington. But TiO₂ is typically white, so it was mainly used in paints. “What is new is the ability to make the coating in a thin enough layer to put it on window glass so that you can see through it and it can be applied uniformly enough so that it doesn’t make streaks or variations in the appearance of the glass,” Barry explains. “We make four invisible coatings. One of the biggest problems we have is to make sure that the glazier knows how to install the glass properly so it’s not put in backward.”

In addition to TiO₂’s photocatalytic properties, the material also becomes hydrophilic when exposed to UV light. “It’s an invisible squeegee effect,” Barry says. “Normally, when rain falls onto glass, it tends to bead up and run down in rivulets, but if the glass is hydrophilic and attracts water, then the water will run down as a sheet and it has a flushing action that’s quite effective in removing specks of inorganic dirt, such as silica sand.

“The chemistry of it is very elegant and beautiful,” Barry says of the self-cleaning glass, but he cautions that windows with such coatings won’t always be squeaky clean. “The coating works at the molecular level, and dirt tends to be at the macro level: bird droppings, a lump of spiderweb, resin from a tree. You’re asking a two-dimensional coating to break down a three-dimensional mountain of material. It doesn’t happen instantly. And if you’ve got inorganic dust on a window, it won’t be clean until you get some rain.”

To create a coating that’s just 50-nm thick for its self-cleaning Activ line of windows, Pilkington uses chemical vapor deposition to apply the material to freshly formed glass while it’s still under a nitrogen atmosphere. “We pass the glass under beams that expose the top surface to TiO₂ vapors,” Barry explains. The coating “fuses perfectly with the ultraclean surface of the glass.”

The company uses the same technique to apply nanoscale coatings of other chemicals. Silica and silicon coatings, known as solar-control coatings, help regulate the amount of heat from sunlight that comes through the window, thereby cutting down on air-conditioning use. Low-emissivity coatings, made from fluorine-doped tin oxide, prevent infrared heat from escaping a building, reducing overall heating costs.

Although nanotechnology has matured in certain sectors of the construction industry, most people working in the field expect many more advances in the future. George Elvin, an architecture professor at Ball State University and director of Green Technology Forum, an information hub that focuses on emerging green technologies in architecture, has been studying the intersection of the two disciplines for a decade. He teaches a course in which students examine nanotechnology advances that have been proven in the lab and envision how they could be used in architectural works should they ever be commercialized.

“For example, if you look at the strength-to-weight ratio of carbon nanotubes, they are many times stronger than steel and yet lighter. They can be transparent; they can conduct electricity,” Elvin says. “If you could have large-scale sheets of carbon-nanotube-reinforced transparent material, then you could have a building structure that basically looked like a glass window. You really wouldn’t have the traditional components of columns and beams or concrete and steel that we have now.”

The area of sensors also sparks a lot of interest in the construction industry, says Pedro J. J. Alvarez, chair of the civil and environmental engineering department at Rice University, who wrote a review about nanotechnology in construction last year (ACS Nano, DOI: 10.1021/nn100866w). Such nanoscale sensors would be embedded within a structure’s foundation and could “give you early warnings if you need to do something about a bridge or a building,” Alvarez says.

Both Elvin and Alvarez note, however, that despite the enormous potential of nanotechnology in construction, no one knows for certain whether there will be adverse consequences. “Unfortunately, we haven’t always had the best track record in how we have used technology, and we have found out after the fact that certain applications, in some cases, were quite harmful, such as asbestos,” Elvin says. He notes a recent study that showed how TiO₂ nanoparticles may disrupt the nitrogen cycle in aquatic ecosystems (Environ. Sci. Technol., DOI: 10.1021/es101658p).

But Alvarez cautions that current studies on nanomaterials’ adverse effects might not paint a realistic picture of exposure to humans. “We are using test animals or cells or bacteria that are exposed to exaggeratedly high concentrations to elicit a response,” he says. “The concentrations that are harmful, at least for acute exposure, tend to be unrealistically high, much higher than a person would likely be exposed to.”

Furthermore, Alvarez says, studies often look at nanomaterials in their virgin form, when in reality what people will be exposed to are nanoparticles that have been embedded in some sort of matrix or that have been weathered in the environment. Such particles are expected to undergo transformations that reduce the materials’ bioavailability and toxicity. “As the result of that, we are really looking at the worst possible scenarios that are not really realistic,” he notes.

For now, Alvarez says, the key is to minimize exposure. For the construction industry that means making sure nanomaterials won’t leach out of structures easily. More important, Alvarez says, is to make sure that the workers who handle these materials as they’re created wear proper respiratory protective equipment. “Exposure control is extremely important, and that is clearly within our means because we protect ourselves from things that are much nastier,” he says.

Ultimately, Alvarez thinks that nanotechnology will revolutionize the construction industry. “But we’re at the infancy right now,” he says. “Before we move too fast, let’s make sure that the risk assessment and the eco-responsible use, design, and disposal don’t fall too far behind. We want to use nanotechnology as a tool for sustainability, and we want to make sure that we’re not creating a future environmental or public health liability.”