Green concrete has two meanings. The term most often refers to concrete that has not gone through the curing process by which it becomes a hardened, strong building material used throughout the world. More recently, the term green concrete has become a reference to new types of concrete and concrete products that are designed to be environmentally friendly.
Sand, rock, Portland cement and water are the usual ingredients of standard concrete. The strength of concrete is determined by the ratios, or the mix design, of these components and can vary greatly. Concrete hardens as the result of a chemical reaction between the water and cement in the mix. What many people think of as drying is actually called curing.
The curing process produces heat, which can cause the concrete to release its moisture too quickly and result in an inferior product. Freshly poured concrete must be protected to keep it from drying too quickly. Often the sides and bottom of the concrete are protected by forms and by the surface on which it was poured, but the top must be covered by a sheet of plastic, a coat of spray-on plastic or the traditional burlap, which is kept wet until curing is complete. Typically, concrete curing takes seven days.
Concrete can set, or become hard to the touch, within a short period. Foot traffic is usually safe within three to four hours. Concrete is typically considered green for a day or so. During this green period, joints can be cut and forms can be easily removed. After a week, light auto traffic may be allowed, and after a month, the concrete should have reached the point where heavy traffic can be permitted.
The most widely used fabricated building material in the world is concrete. It is used for roads, bridges, buildings, swimming pools and parking lots. Unfortunately, it has one serious drawback. Manufacturing the Portland cement that is a major component of concrete requires heating limestone and other elements to high temperatures. This process releases significant amounts of CO2 into the atmosphere.
Some architects, builders and engineers lean toward environmentally responsible green concrete construction with new concrete formulations that help conserve non-renewable resources and reduce the problem of CO2 emissions. Another benefit of green concrete products is that they are made from recycled industrial products that would otherwise have gone to landfills. Stanford professor Brent Constantz says he has invented a green cement that will eliminate the problem altogether. Constantz claims his new process does not generate any CO2 and reduces plant emissions by trapping CO2 in the cement.