Wednesday, July 29, 2009

Green Concrete? (Part 1)

I must admit, when I think of concrete, Green is not what comes to mind.

In fact cement production is extremely energy intensive; some estimates calculate it contributes as much as 7% of global carbon dioxide emissions. Demand for cement worldwide continues to grow, with more than 3 billion tons produced each year.

Several cement manufacturers are advertising "green" cement formulas as an alternative to the standard quarried cement mixes of the past. But questions still remain as how "green" they really are.

Basic cement is composed of six components: Cement, coarse aggregate (rock), fine aggregate (sand), water, fly ash, and chemical admixtures (for different specialty applications).

Green concrete formulas support their environmental claims in two principal ways:

1. Using recycled aggregates
Green concretes often contain a percentage of coarse and fine aggregates rather than digging up virgin materials. This is slightly greener due to the reuse of the otherwise quarried materials, yet the processes involved in preparing these recyclables often use a comparable amount of energy (and fossil fuels) to produce.

2. Fly ash
Here is the sticky wicket. Fly ash is a waste byproduct from the chimneys of coal fired power plants and is composed primarily of silicon dioxide and calcium oxide, but may also contain a range of other toxic compounds. Before governmental mandates regarding air quality, this fly ash was released into the atmosphere. Now, much of it is captured and placed in landfills. This creates new environmental challenges, as evidenced by the Knoxville, TN coal spill that occurred earlier this year.

The concrete industry uses a higher percentage of fly ash in Green cement mixes than in standard concrete. Trade organizations and certification programs in green building are touting this as a great benefit, since using fly ash in concrete diverts it from landfills.

However, we still don't understand the possible health risks associated with using higher quantities of fly ash. To date there have been no environmental standards adopted for the use of concrete mixes with a higher percentage of fly ash.

Buyer beware when choosing Green concrete! In my next post, I will discuss the pros and cons of this concrete and safer options for the use of these products.

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