Published on December 21st, 2008 | by Stephanie Evans2
Can Concrete Be Eco-Friendly?
Of course, any new material must answer the complicated question, “Is it really green?” In recent years, concrete and its use in flooring and countertops has gained popularity as a green building material, but not without an amount of controversy…
Traditional concrete is made from a mixture of cement, sand, gravel, and water—and therein lies the main problem: its heavy reliance on the use of cement. Cement is an environmentally hazardous material for the following reasons:
- In comparison to other industrial processes, cement production is extremely energy- and fossil fuel-intensive, making it the third ranking producer of carbon dioxide emissions (the primary greenhouse gas contributing to global warming).
- Cement production is increasing by approximately 5% a year, making it one of the most environmentally destructive materials.
- Aggregate materials like sand and stone that are mixed with cement are mined from quarries, further taxing our natural resources.
If we take into account how much energy is required to mill, heat, mix, and transport concrete, we can easily conclude that the use of traditional concrete in green building is not effective at decreasing the carbon footprint of its users.
Fortunately, viable cement substitutes and concrete alternatives are now available to assist in making concrete a truly green material, allowing end users to enjoy the benefits of concrete without sacrificing their commitment to the environment.
The mixture of cement with other aggregate materials is the primary reason that concrete cannot bear an eco-friendly label. Understanding cement and how it is made is therefore helpful when evaluating the greener alternatives:
Portland cement, invented in 1824, is the most common type of cement used throughout the world. It is created by heating ground limestone and a mixture of a second raw material, such as clay, sand, iron, ore, shale, or bauxite to a temperature of 1450 degrees Celsius. The heating process changes the chemical properties of the materials and creates a stronger cement.
The heating and mixing process requires massive amounts of energy and emits alarming amounts of carbon dioxide into the atmosphere. Looking to combat the amount of greenhouse gases released as a byproduct of the concrete industry, green innovators have invented cement substitutes and alternative production methods to create a more earth-friendly product.
Replacing energy-consuming Portland cement with recyclable materials and minerals offers two distinct benefits to the environment—it significantly reduces the amount of CO2 released into the atmosphere and it minimizes massive landfill disposal. Look for concretes made with some of these substitutes and ask your local concrete supplier about them:
A promising green concrete being heralded for sustainability is high-volume fly ash concrete. Fly ash is a by-product of coal-burning power plants, and in the past, almost 75% of fly ash produced made its way to landfills.
- Recycled fly ash, when mixed with lime and water, forms a compound similar to Portland cement and is extremely strong and durable.
- High-volume fly ash concrete displaces more than 25% of the cement used in traditional concrete, reducing the amount of emissions needed to make the concrete mix.
- Fly ash concrete was somewhat difficult to source in the past. Due to a significant increase in demand, there are more producers and distributors working to steadily increase the fly ash supply.
AshCrete is a concrete substitute that relies heavily on the use of recycled fly ash. Made up of 97% recycled materials, AshCrete is made from fly ash, bottom ash, borate, and a chemical from the chlorine family. (Note: The use of such a chemical is not environmentally friendly because chlorine, used in this way, is suspected to cause a number of environmental and human health problems. The inventor of AshCrete is currently seeking a natural substitute for this chemical.) As a building product AshCrete is known for its extreme strength, approximately twice the strength of Portland cement.
Similar to fly ash, blast furnace slag is another by-product that can be recycled and used as a cement substitute for concrete. It is produced from blast furnaces used to make iron and, like fly ash, creates a very strong cement when mixed with lime and water. Commonly referred to as slag, it can be easier to find than fly ash.
The newest cement substitute being introduced into green building is carbon concrete, a thermoplastic. To make this material, an oil refinery by-product (a heavy residual substance that is typically very difficult to dispose of) is turned into a binder material to replace the use of cement.
- Unlike fly ash and slag, carbon concrete cannot be used for tall buildings or towers because there is some “degree of creep” over time. This material is recommended primarily for use in flooring and paved roads because of its tendency to settle.
- Shell and The University of Delft have developed this technology and it is being promoted and distributed by a company called C-Fix. At this time it only is used in Europe, but C-Fix is looking to expand their operations very soon.
In addition to cement substitutes, there are other ways of making concrete more sustainable, based on two core environmental principles—recycle and reduce. These alternatives include:
- The use of recycled aggregate materials and preparations that results in reduced amounts of concrete needed to complete the job. Conventionally, cement was mixed with virgin materials, such as sand or gravel, to make durable, workable concrete. The use of recycled materials has gained credibility and momentum in the concrete industry and aggregate is now mined from various solid wastes, including: fiberglass waste materials, discarded glass, granulated plastics, wood products, old tires, and more.
- Papercrete or fibercrete/fibrous concrete that is made by using waste paper as an aggregate material. These concrete mixtures still rely on the use of cement, but the amount of cement used represents a fairly small percentage of the cured material by volume, so one can argue that it is a greener alternative than traditional concrete.
- Other alternatives, such as foam crete, ceramicrete, glass fiber reinforced concrete (GFRC), and grasscrete, which reduce the overall amount of concrete in use, resulting in decreased emissions and energy expenditures.
- Foam crete is a lighter, aerated, foam-based concrete that requires less energy to produce.
- Ceramicrete and glass fiber reinforced concrete are twice as strong as traditional concrete, so builders use less of it.
- Grasscrete is a method of laying concrete in a checkered, cellular pattern that allows grass to grow between the concrete blocks. The result? Less concrete used and improved drainage and storm run-off.
- Another green alternative is concrete produced in a dry-process kiln. These kilns are much more thermally efficient than wet-process kilns and drastically reduce energy consumption.
To find sources of sustainable alternatives to concrete, talk with local green builders and concrete suppliers and visit green building supply resources on the web. Once you’re ready to seal your eco-friendly concrete, or decorate it with paint, stain, or dye, choose water-based or bio-based products with low VOC emissions. There is a variety of non-toxic, biodegradable, and easy-to-use products such as concrete primer and sealer from soybeans.
The emergence of new cement substitutes and concrete alternatives has convinced many green builders that concrete really can be an eco-friendly product. The U.S. Green Building Council’s LEED program has published standards on the use of green concrete in high-performance eco homes with credits given for implementing some of the methods detailed here, including:
- replacing at least 30% of your foundation’s cement with a recycled cement substitute, and/or…
- …replacing at least 25% of your foundation’s concrete aggregates with a post-consumer recycled material (Credit MR 2.2).