MANILA, Philippines?The ongoing debate on whether concrete is green or not continues to hound the building industry. After all, concrete is the most widely manufactured material in the world with over 10 billion tons produced annually. It is valued for its durability, compressive strength, versatility and almost limitless design possibilities. At the same time it accounts for about 5 percent of total carbon dioxide emissions that contribute to global warming.

In the United States, the cement industry has adopted strategies to reduce this environmental impact concrete. They have set goals to increase energy efficiency 20 percent by the year 2020 and reduce CO2 emissions by 10 percent per ton of product, based on a 1990 baseline.

How concrete is made

The first step in the production of cement, the binder in concrete, is the collection of limestone and other raw materials. These are analyzed, blended and ground. The next step is to heat these ingredients at temperatures up to 3,400 degrees in a kiln where they react and partly fuse.

This phase is the most energy intensive part of the process and adds the most to greenhouse gas emissions. This process is known as calcination. It produces a substance called clinker which is marble-sized. These are cooled and ground with gypsum and limestone to produce cement. Concrete is produced when cement is mixed with sand, aggregates and water.

Greenhouse gas emissions

Ninety-five percent of greenhouse gas emissions in cement manufacturing come from calcination and the burning of the fossil fuel needed to keep the kiln temperatures.

The emissions associated with calcination are difficult to reduce unless a substitute is found for limestone. Studies are now being conducted by several nongovernment organizations to create stronger, more durable, environment-friendly material.

Cutting down on cement use

One way to cut down on the use of cement is by using supplementary materials such as slag cement, fly ash and silica fume?all products of other industrial systems. Slag is derived from the steel-making process. Fly ash is derived when coal is burned at power plants and similar facilities. Silica fume is a by-product of silicon and ferrosilicon metal production. By using these materials, less virgin materials are used and fewer materials are sent to the dumpsites.

Benefits

By using supplementary materials to replace cement, the physical property of concrete is enhanced. Take slag for example; by its addition, a denser and more durable concrete is developed as it becomes permeable to water.

One possible disadvantage with using a large percentage of slag is that it takes longer for the concrete to gain strength. However, it can result in higher compressive strength than conventional concrete.

Today more and more concrete structures contain at least small quantities of supplementary materials, such as slag and fly ash. Some researchers are now hoping that these supplements may completely replace cement in the future.

New generation of concrete

The new breed of concrete known as ultra-high-performance concrete was developed several years ago. It incorporates metallic or organic fibers and does not need reinforcement. It is also 6 to 8 times stronger than conventional concrete. Since it is possible to use smaller structural members, it is then associated with fewer gas emissions.

Another new breed of cement that has environmental benefits was introduced in Europe recently. It contains titanium dioxide. Originally it was used to maintain the appearance of a building?s exterior. Scientists later found that the product facilitated a chemical reaction using light to improve air quality by breaking down pollutant particles.

In many developed countries, residential communities alongside busy roadways are shielded from noise pollution by sound barriers. Some of these barriers are made of very expensive Plexiglas so as not to cut visual access.

In the United States a special concrete panel that has high-absorbing quality has been introduced as a barrier to cut down light, sound and pollution in heavily traveled highways. They are also studying how it can help reduce pollution within urban areas.

Whereas steel may be recycled in its original or near-original state, concrete can be reused mostly in a downgraded form such as filling material.

(E-mail the author at amadodejesus@gmail.com.)