Concrete and the Environment

Concrete holds our lives together. The majority of our buildings, bridges, roads, and dams are constructed and held together by concrete. There are also many things that you probably didn’t know about this wonder material:

1. Concrete is one of the primary materials in building construction, which is the nation’s largest manufacturing activity, accounting for 60% of the raw materials consumed in the United States.¹
2. Every year humans use seven billion cubic meters of concrete; “that’s one cubic meter for every person on Earth.”²
3. When the building stock turns over, the materials used in demolished buildings become waste. The demolition waste stream in the U.S. is “130 million tons per year, or about 25% of all of the solid waste that is discarded in the United States.”³

So, what’s behind these large numbers? Let’s explore the history, environmental impact, and future of concrete.

A Brief History of Concrete
It is difficult to pinpoint when concrete was first invented, because the date will depend on how you define “concrete.” Concrete is made of a “hard, chemically inert particulate substance, known as an aggregate (usually made from different types of sand and gravel), that is bonded together by cement and water.”⁴ Both the aggregate and the cement portions of concrete mixture have varied over time. Numerous ancient civilizations made crude cements, for example:

• The Assyrians and Babylonians used clay as cement.⁵
• Around 3000 BC, the Egyptians used lime and gypsum mortar to build the Pyramids.⁶
• The Chinese used cementitious materials to hold bamboo together in their boats and in the Great Wall.⁷
• Around 300 BC, Romans used both naturally reactive volcanic sand called harena fossicia, and slaked lime, a sandy volcanic ash called pozzuolana.⁸

Modern Cement and its Environmental Impacts
After the fall of the Roman Empire around 400 AD, the art of concrete was lost until the year 1756, when British engineer John Smeaton discovered hydraulic cement after conducting many experiments mixing powdered brick and pebbles in fresh and salt water.¹⁰ In 1824, the English inventor Joseph Aspdin invented Portland cement, which through today remains the dominant type of cement used in concrete production. Portland cement is created by burning ground limestone and clay together at temperatures between 1,300° F and 1,500° F. The burning process changes the chemical properties of the materials to create stronger cement.¹¹ Unfortunately, the burning process also creates carbon dioxide —the lime in cement is obtained by heating calcium carbonate until it breaks down into lime and carbon dioxide (CO₂).¹² The seven billion cubic meters of concrete we generate every year produce up to eight percent of the CO₂ emissions worldwide.¹³ Outside of the energy production and transportation industries, the cement industry is the largest producer of CO₂.¹⁴

In addition to CO₂ emissions, concrete is used to create hard surfaces that contribute to surface runoff, heavy air pollution  and respiratory symptoms.¹⁹

Greening Concrete
There are several ways in which the concrete industry is starting to lessen the environmental impacts of its operations. For example, researchers at Kansas State University are experimenting with concrete that uses less calcium carbonate cement. They replaced 20% of the cement in concrete mix with silicate material called high lignin residue, which is a waste from the process used to make cellulosic ethanol .²⁰

More significant is the increase in concrete recycling. Instead of sending concrete debris directly to landfills, concrete is now often collected from demolition sites and put into a crushing machine and used as gravel in newer construction projects.²¹ The LEED ® Green Building Rating System recognizes recycled concrete in its point system,²² and more and more government agencies now approve the use of recycled concrete.²³ Think about how amazing this is: concrete recycling makes the waste product of one industry into a substitute for virgin raw material of another industry, thereby reducing the environmental impact of both.

Greening the concrete production process will be even more important in the future given the increasing demand for concrete as worldwide development booms and civil infrastructure in the U.S. demands replacement. This is exemplified by the findings of one study that of the 597,340 bridges in the US, 73,784, or about 12.4 percent, are structurally deficient. The demand for concrete is expected to grow to approximately 18 billion tons (16 billion tonnes) a year by 2050.²⁵ In order to meet this demand we simply cannot keep extracting additional raw materials and filling our landfills up with concrete. Recycling concrete will be essential to continuing to hold our lives together with concrete.

To get information about where you can recycle your concrete: Construction Waste