Concrete is a mix of water, cement and usually something like sand. The cement is what raises the eyebrows of environmentally conscientious people. Making and distributing cement is very energy intensive: Global cement production accounts for anywhere from 2.5 to 8 percent of man-made carbon dioxide released in the air [source: U.S. Building Council's Green Home Guide].
The good news for concrete is that cement is only a small percent, around 12 percent, of concrete countertops, and fly ash can actually be used in its place. In the concrete-forming process, fly ash becomes an inert substance, so it won't emit VOCs or other toxins into the air. Another positive for concrete is that using a sealant to protect the surface will not create any offgas effects; it just increases your counter's durability.
Speaking of durability, when the concrete countertop's life span is over, it is very easy to use it again. The material can be crushed, cut up into pieces or reused as whole slabs. This is an excellent benefit because it eliminates the need to produce new concrete.