Concrete components Photo courtesy of Portland Cement Association

Cements are the glue that make concrete strong, durable, safe, long-lasting, versatile, and economical in all forms of construction—from homes to infrastructure. From mining to end use there are several attributes that make cement sustainable, and many more choices that the industry is investigating and implementing to make the process more sustainable.

The raw materials for cement manufacture are widely available, which reduces the need for long-distance transport. The quarries that supply the necessary raw materials should have a mitigation/restoration plan so that they can limit the impact on land use. Restoration can include creating parks and small lakes, or redevelopment for new communities.

The manufacture of cement is an energy-intensive process. The raw ingredients are mined or recovered from other industries, then ground and blended. This raw feed is then heated to approximately 2600°F (1450°C), where the chemical reactions to form hydraulic cement take place. In addition to CO₂ from fuel used to heat raw ingredients of cement, the cement manufacturing process releases CO₂ from limestone (a primary raw material) as an integral part of the subsequent chemical reactions to take place in the kiln, a process known as calcination.

Energy consumption data (1988 to 2007) for the cement industry in the U.S. (Portland Cement Association 2008)

Modern cement plants generate a relatively small amount of by-products themselves, and often use by-products from other industries as a portion of their energy supply as well as raw materials, thereby reducing the use of virgin materials and processing fuels, in addition to reducing the amount of material sent to landfills. Waste oils, medical waste, and scrap tires are commonly used as fuels in cement manufacturing, helping remove these products from the waste stream.  Performance-based specifications are allowing the use of increased amounts of uncalcined limestone and other process additions (often flyash or slag), which reduces the emissions associated with cement production.  Other materials used in cement manufacturing include copper slag, foundry sand, mill scale, sandblasting grit, and synthetic gypsum. Blended cements, as well as Portland cement with supplementary cementitious materials are proving to be effective in construction performance, reduction of calcination-source CO₂ and use of industrial by-products. Process improvements have led to a 13% drop since 1988 in energy usage to produce 1 ton of clinker.

Concrete is an extremely durable material, and the longevity of structures is an important sustainability attribute which affects all three pillars of sustainability: it reduces the need for additional materials, energy demand, and waste due to replacement; it offers better economic value through a lower annual cost of ownership; and less frequent replacement of buildings and pavements reduces the downtime and inconvenience associated with repairs and replacement.

CO₂ - Contribution of materials in concrete to the total CO₂ and embodied energy (Portland Cement Association 2006).

Durability is a function of mixture proportioning, mixing, placement, and curing. Deficiencies in these areas increase concrete permeability and create a corresponding decrease in concrete durability. The cementitious material content and water content of a concrete mixture are particularly critical because they directly influence permeability. The use of appropriate cementitious materials can improve the durability of concrete by providing—among other attributes—sulfate resistance, freezing-and-thawing durability, deicer scaling resistance, and enhanced corrosion resistance.

White cement clinker is manufactured with select raw materials so that it contains less than 1% ferric oxide (Fe₂O₃). It can be an integral part of innovative sustainability strategies in two ways. First, white cement concrete can deliver architectural benefits that replace less durable cladding or coating materials. White cement is essentially the same as gray cement with the exception of its color, so concrete made with white cement retains all of the durability and benefits of standard gray concrete. Because it facilitates a broad pallet of architectural colors and finishes difficult to achieve with ordinary gray cements, it is an aesthetic choice for architects and designers.

The second innovative, sustainable application for white cement is its bright color and reflectivity. These characteristics can be used to reduce lighting needs and unwanted heat gain. Concretes that use white hydraulic cements have significantly higher reflectance than gray cement concrete. This reflectance can reduce lighting needs by more than 30%, thereby saving the electrical power for the light itself and reducing a building’s cooling load. In exterior applications, white cement concrete’s higher reflectivity reduces solar heat gain and thermal heat islands. It should be noted that, although white cements are significantly lighter colored than gray cements, all concrete does a very good job of reflecting solar energy. Marceau and VanGeem (2007) found that a wide range of concrete mixtures were eligible for LEED sustainable sites credit.

Citation

Marceau, M. L., and VanGeem, M. G., 2007, “Solar Reflectance of Concretes for LEED, Sustainable Sites Credit: Heat Island Effect,” PCA R&D Serial No. 2982, Portland Cement Association, Skokie, IL, 94 pp.

For More Information

Portland Cement Association – www.cement.org
American Coal Ash Association - www.acaa-usa.org
Slag Cement Association - www.slagcement.org