Concrete is a strong, durable material with high heat storage capacity that can be used to moderate building temperature swings, it is also a good product from an indoor air quality standpoint. Concrete does, however, have some environmental drawbacks. Ongoing developments in concrete production can reduce environmental impacts by substituting other materials for cement.
Concrete is a strong, durable material with high heat storage capacity that can be used to moderate building temperature swings. Because traditional concrete is one of the most inert building materials, it is also a good product from an indoor air quality standpoint, even for chemically sensitive people. (This is not completely true for high-tech concretes that contain chemical agents for workability and air-entraining.)
Concrete does, however, have some environmental drawbacks. It can cause water pollution if wash-out water from equipment at concrete plants or on job sites finds its way to local waterways. The pH of washout water is so high, it is toxic to aquatic life. Another concern is the production of cement, the binding agent used in concrete, which accounts for about 10%–15% of concrete’s mass, but 92% percent of its embodied energy. Cement manufacture is a major contributor to atmospheric greenhouse gases due to both its production and process emissions. According to a U.S. EPA report, in producing a total of 90 million metric tons of cement in 2001, the U.S. cement industry emitted 77 million metric tons of CO2. About 46% of these emissions are attributed to combusting fuel, predominantly coal and coke, to fire cement kilns to temperatures up to 3,400 degrees Fahrenheit; the remaining 54% of the emissions result from the chemical process of making cement, which involves converting limestone to calcium oxide and CO2. Cement production accounts for 1.5% of all U.S. CO2 emissions (according to the Portland Cement Association); worldwide, however, cement production causes over 8% of the total CO2 emissions attributed to human activity. To its credit, the cement industry has made great strides in recent decades to increase energy efficiency and reduce emissions. For instance, the chemical process in cement production that releases CO2 can be used to capture other combustion emissions such as sulfur dioxide and nitrogen oxide.
Furthermore, ongoing developments in concrete production can reduce environmental impacts by substituting other materials for cement. Up to 60% of the cement content used in traditional concrete may be replaceable with “supplementary cementitious material” (SCM) salvaged from industrial waste (or derived from natural soil or rock), depending on the concrete application, the type and quality of the substitute, and the results of batch testing. Industrial by-product SCMs include fly ash, a waste product from coal-fired power plants, blast furnace slag, a waste product from steel production, silica fume, a waste product from the silicon metal industry, and rice hull (or husk) ash, which is generated when agricultural rice waste is burned to produce power.
Replacing a percentage of the cement in concrete with an SCM reduces energy consumption and CO2 production, reduces solid waste, and can improve concrete strength, performance, and durability. Because power plants are common in most cities, fly ash can usually be obtained locally. Global implications of using rice hull waste are tremendous. Rice is the world’s main staple crop, generating 100 million tons of hulls annually, which is traditionally burned along with the straw in the fields, causing pollution and health problems. Burning the crop waste in small power plants could generate electricity, dispose of the waste, and provide a high-quality cement substitute.
To minimize the environmental problems with concrete, the following measures should be taken:
- Reduce concrete waste by recycling crushed concrete for fill material or road base, or grinding it up for aggregate. (Currently only 5% of concrete is recycled. By weight, it represents up to 67% of construction and demolition waste.)
- Carefully estimate the amount of concrete required to avoid ordering excess amounts that become waste.
- Consider less material-intensive alternatives to poured-in-place concrete, such as insulation-form walls and autoclaved cellular concrete block. Precast concrete is factory-made to order, which, due to controlled production processes, also reduces concrete waste.
- Use insulated shallow foundations in northern climates; consider pier-and-beam foundations instead of slabs on grade.
- Protect aquatic ecosystems by washing forms and equipment where runoff will not contaminate waterways.
- Use the maximum amount of fly ash or other SCM appropriate to the construction application, location, and material quality.
Adapted from Green Building: Project Planning & Cost Estimating, 3rd Edition, available through RSMeans.