Concrete Minus Carbon

New technologies can reduce the environmental footprint of the most-used construction material.

By Meg Calkins, FASLA 

It is easier to achieve bright colors with carbon cement, which is whiter than ordinary portland cement. Image courtesy of Solidia Technologies.

Concrete in the 21st century promises to be a more sustainable material, and given the nine billion metric tons used globally each year, it must be. Portland cement, the binding agent in ordinary concrete, has a very high carbon footprint, resulting in just under one ton of carbon dioxide (CO2) released for every ton of cement produced. With 4.2 billion metric tons of the binder used each year worldwide, cement production is responsible for nearly 8 percent of total global carbon emissions. The high lime content of ordinary portland cement contributes about two-thirds of cement’s CO2 impact through the process of limestone calcination. The other one-third of CO2 released is from combustion of fossil fuels.

Technologies to improve the carbon footprint of concrete are currently in the early stages of development, but some, including carbon sequestration in concrete and substantial reductions of cement using energetically modified cement, are now commercially available. Concrete surface products for paving and walls to scrub air pollution, as well as new self-healing concrete products, are also worth investigating. We have heard about some of these innovations for a decade or more in the research community, but many are finally being brought to market—some more quickly than others. Europe is ahead of the United States in the adoption of these technologies, largely because of more rigorous clean air and carbon reduction initiatives.

New technologies in any field can take a long time to move from the laboratory to the marketplace, but
recent sustainable concrete technologies have experienced challenges in scaling up to the global concrete trade, and some companies with promising technologies have gone bankrupt before the market could embrace their products. Although specific products are discussed in this article, it is not an endorsement of these products over others. Few products in these areas are on the market, and the products that are discussed are those that provide the most online information to consumers.

Solidia Technologies, a company that produces a low-lime cement and uses CO2 instead of water to cure concrete, has marketed its products as better-performing and cheaper than ordinary portland cement concrete. Bo Boylan, Solidia’s chief commercial officer, said in a phone interview that Solidia products use the same production equipment, methods, and supply chains as ordinary portland cement concrete. This has helped the company gain a foothold with the Federal Highway Administration and some state transportation departments in a market that can be slow to change.

Sequestering carbon in concrete

Solidia is one of a handful of companies using concrete to sequester carbon. It has developed a nonhydraulic cement that is low lime, containing primarily calcium silicates. Carbon dioxide emissions from production of Solidia cement are reduced by about 30 percent, owing to its low lime content and an 18 percent lower firing temperature. Solidia cement cures with a carbonation process, not through the typical hydration activation process used with ordinary portland cement. Waste CO2 used for curing is injected into the concrete mix in amounts equal to about 5 percent of the mix weight. Combined, Solidia cement and concrete can reduce the carbon factor of concrete up to 70 percent, or about 550 kilograms per metric ton. To further “green” the Solidia products, ground fly ash or slag can be used as a replacement for Solidia cement in amounts up to 40 percent, depending on the concrete precast producer’s practices.

Because waste CO2 is more easily brought to the controlled factory environment of a precast plant, Solidia’s offerings are best used for producing precast concrete products such as pavers, blocks, and wall panels. Poured-in-place concrete applications of Solidia concrete are challenged by the difficulty of moving waste CO2 gas into the field. Boylan estimates that poured-in-place Solidia concrete is still two years away from the market. Solidia precast concrete is expected to be available within months, at least in the Mid-Atlantic and Northeast regions, where several precast producers have been licensed to use the technologies.

Aggregates comprise about 67 percent of a concrete mix, and limestone is the most common stone used for aggregates. Carbon 8 is a British company that uses accelerated carbonation technology (ACT) to produce carbon-negative lightweight aggregates. ACT uses carbon gas to treat thermal wastes, resulting in the production of stable carbonates that are blended with binders and fillers, then pelletized to form artificial limestone.

Blue Planet in California has developed another carbon capture and mineralization technology that produces aggregates and sack concrete. Its biomimetic technology uses osmotic pressure between fresh- and saltwater to create a proprietary alkaline solution, which is then combined with CO2 from flue gas to form carbonates. This process is inspired by naturally occurring marine biomineralization. Blue Planet offers CO2 capture as an emissions control service, and the company is currently interviewing candidates for project demonstration. Its website states that potential sites include fossil fuel-powered electricity generating facilities, refineries, and cement plants.

Sequestering carbon in concrete or aggregate can be helpful environmentally, but the production of ordinary portland cement has huge carbon impacts of its own. When specifying these products, designers should be aware of their carbon footprint and make sure that the benefit of the carbon sequestration product outweighs the other impacts of using concrete. For example, a company that uses CO2 to cure concrete masonry units (CMUs) estimates that 100,000 CMUs absorb 3,000 pounds of CO2—the same amount sequestered by 67 full-grown trees. It sounds pretty good; however, life cycle data from the University of Bath shows 100,000 CMUs to have a carbon footprint of about 165,000 pounds of CO2, so carbon absorption of 3,000 pounds is not a sufficient offsetting strategy. If concrete block producers can reduce the ordinary portland cement used by substituting fly ash, ground granulated blast furnace slag (GGBFS), or carbon cement, then this particular carbon sequestration technology may make more sense.

Energetically modified cement

Perhaps a more productive way to lower the environmental impact of concrete is to reduce the use of ordinary portland cement through high-volume substitution of waste materials such as fly ash (from coal combustion in power plants) and GGBFS (from iron blast furnaces) or natural pozzolans such as volcanic ash or calcined clays. Concrete companies regularly substituted these materials for low percentages of portland cement as a cost-saving strategy, but a recent technology, energetically modified cement (EMC), has allowed for substitution in much higher percentages. EMC activation is a process that modifies the surface of hydraulic materials such as fly ash, natural pozzolans (such as silica sands and metakaolin), and blast furnace slag. This process increases the surface area of the particles, rendering microcracks and dislocations of crystal structures at the nano scale. This results in greater reactivity with no significant increase in powder fineness, which allows for much higher substitutions for ordinary portland cement.

A Swedish company, EMC Cement, offers a product called CemPozzFA that uses this process with fly ash to achieve a product that is 70 percent fly ash and 30 percent portland cement. The company claims an approximate carbon footprint reduction of concrete made with CemPozzFA of up to 80 percent. Additionally, this concrete has been found to exhibit reduced cracking, improved long-term strength, and increased durability. A company white paper on the product in paving indicates that the Texas Department of Transportation has been using it for more than five years with success. For western states where fly ash can be scarce, EMC Cement offers CemPozzNP, containing natural pozzolans such as volcanic ash and calcined clays. The company claims up to a 60 percent reduced carbon footprint with this product.

Human and environmental health concerns about heavy metals in fly ash have been raised in recent years. Fly ash can contain trace amounts of mercury, cadmium, arsenic, and lead, depending upon the coal source. The green building community and the U.S. Environmental Protection Agency (EPA) have raised concerns about the use of fly ash in concrete and other building products. After a review of research literature, these parties continue to support use of fly ash as a cement substitute. Environmental Building News offers the criteria that fly ash use should be supported as long as it reduces greenhouse gas emissions in the materials stream and the fly ash is chemically or physically locked up so the risk of leaching is low.

Given that fly ash and GGBFS are by-products of coal combustion in power plants and iron processing, respectively, their supplies are not always abundant. As utilities and industry move toward cleaner energy, coal combustion is being reduced and new iron processing technologies are reducing the quantity of blast furnace slag. Natural pozzolans such as volcanic ash are still abundant and continue to be a reliable cement substitute, particularly for energetically modified cement.

Self-healing concrete

The most sustainable concrete structure is a durable one that lasts beyond its full design life. Unfortunately, studies indicate the actual service life of concrete pavements averages just 25 years, even though they are designed for longer use. Self-healing concrete technologies, in the research phase for several years, hold promise to extend the life of concrete structures.

Many different self-healing concrete technologies are being tested using chemical, biological, and mineral constituents. Scientists in Belgium and the Netherlands are working on one such product that is close to commercialization. It uses limestone-producing bacteria and calcium lactate encapsulated in clay pellets and mixed directly into the uncured concrete. When a fissure opens in the concrete, the pellets crack open and release the bacteria. Moisture in the air triggers the spores to germinate. The bacteria feed on the calcium lactate and form limestone, sealing the cracks (up to 0.8 millimeters wide) within three weeks.

Self-healing with mineral constituents is an unexpected benefit of EMC concrete using CemPozzNP. Although all concrete is somewhat self-healing, as some re-cementing happens when moisture reaches the portland cement in cracks, the reaction is too slow to prevent the cracks from opening too wide before healing takes place. Constituents of CemPozzNP, silicon dioxide and calcium oxide, react more quickly to re-cementing in the presence of water, filling voids and cracks of widths up to 0.2 millimeters within two to three months.

The titanium dioxide in concrete pavers with TX Active cement acts as a photocatalyst, converting nitrogen oxides and other urban pollutants into neutralized compounds. Rain can wash the compounds away. Image courtesy of Heidelberg Cement.

Photocatalytic concrete

In the United States, numerous metropolitan areas have air pollution levels higher than what the EPA deems safe for human health. There are many contributors to air pollution, but one of the main culprits is fossil fuel combustion by vehicles being driven on urban streets. So it seems fitting that streets, sidewalks, and parking areas should contribute to reduction of the pollution in some way.

Photocatalytic concrete, as its name suggests, contains a catalyst, titanium dioxide (TiO2), that interacts with sunlight to abate organic and inorganic air pollution through an oxidation process that converts noxious compounds to harmless ones. The TiO2 catalyst can be applied on a structure’s surface as a coating, or it can be mixed into cement as a constituent of concrete. Owing to the cost of photocatalytic cement, it is usually used in a 3/8-inch concrete topping layer rather than through the entire section of a concrete structure.

Photocatalytic concrete is capable of reducing air pollutants such as nitrogen oxides (NOx) and volatile organic compounds up to two and a half meters from the surface. It has been in use on building exteriors for self-cleaning since 1996, though the technology has been slow to catch on as a pollution-reducing technique because of varying conditions that may limit its effectiveness.

Photocatalytic surfaces on pavements and sound walls may someday be widespread enough to reduce some ground-level air pollution. Research, primarily in Europe and Japan, has shown concrete with a TiO2 surface layer to be a promising technology for reducing air pollutants, with reductions of NOx ranging from 15 percent to 70 percent. This wide range of results is owing to variables of wind, humidity, orientation, and urban configuration.

If humidity is too high, the photocatalytic reaction may not work or may even exacerbate pollutant intensities. An ideal setting for maximum pollutant removal by photocatalytic paving or walls is one with low average humidity, low wind, and high ultraviolet intensity. Concrete is an ideal material for photocatalytic surfaces, as the reaction products can be adsorbed at the surface then washed away with rain. To date, photocatalytic site construction applications using TiO2 mixed into cement have primarily been in the top 3/8 inch of concrete pavers, as using TiO2 in a full depth, cast-in-place concrete structure would be quite expensive.

Other applications of photocatalytic paving use a coating agent that is applied after the concrete cures. Lab tests show that these products may wear off over time, particularly with vehicular traffic, losing their effectiveness. Application on porous pavements can protect some of the coating from wear, as it is slightly below the surface.

TX Active, a product sold in the United States by Lehigh Hanson, Inc., is a photocatalytic cement that integrates TiO2 into a portland cement mix. The company recommends that TX Active cement be used according to the same standards as portland, but that mix designs be verified with its technical staff. Unilock offers a custom paver that uses TX Active cement in its top centimeter.

Adoption of paving and sound walls with photocatalytic surfaces has been slow for several reasons. First, nonmunicipal clients have no financial incentive to use this technology—and there is an added cost. Brad Swanson, ASLA, the director of commercial sales with Unilock, estimates that it adds about $3 per square foot to the pavers’ cost. Municipalities have some incentive to take steps toward pollution reduction because of Clean Air Act regulations. The City of Chicago Department of Transportation, working with Site Design Group, has used photocatalytic pavers from Unilock and poured concrete benches with TX Active on its Cermak and Blue Island Streetscape projects.

Photocatalytic surfaces also have been slow to be adopted because of the challenges new technologies have getting from the lab to the marketplace. Very little research exists to show how photocatalytic pavers and other surfaces actually perform with respect to pollution removal. It is difficult to replicate lab conditions in the field, so it is still not certain that benefits will equal lab projections in actual applications.

Lastly, there are some concerns about nanoscale TiO2 particles and their impacts on human health. The Healthy Building Network points to studies that have found TiO2 particles to be carcinogenic at the nano scale. This could be a concern for workers in manufacturing and construction and even in the use phase, when the particles may run off into water.

Meg Calkins, FASLA, is a professor in the Department of Landscape Architecture at Ball State University. She is the author of Materials for Sustainable Sites and the editor of The Sustainable Sites Handbook.

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