When building a chimney, concrete blocks are probably the least favorable type of masonry to use. Because they are made from poured concrete instead of natural stone or a kiln-fired material, concrete blocks have almost no tolerance to extreme heat. As a result, a concrete block chimney is subject to cracking after only a few uses. This problem can be reduced somewhat by filling the concrete blocks with a heat-resistant concrete, but the risk cannot be completely eliminated. However, if your chimney gets little use, this can still be a feasible material, and it can be more customizable than other building materials for something more decorative and unique.
Rather than using unfired concrete to build a chimney, most experts recommend using natural stone, or a fired stone, such as clay bricks. Both natural stone and kiln-fired bricks have already been exposed to high heat and pressure, and will not be damaged by the heat generated by a chimney. Stone construction also has the benefit of allowing the chimney to be circular rather than square or rectangular, which is an appealing alternative to the common chimney designs.
Concrete Block Chimneys
If concrete blocks are the material you want to use regardless of the cracking hazard, lay out a chimney plan that is 32 inches by 32 inches. This allows for the blocks to form an internal square of eight inches by eight inches, which can accommodate standard chimney flue. Other sizes are available, but this is the most common found in most areas.
Filling Concrete Blocks
When you build a chimney out of concrete, you will need to fill the blocks after they have been set. For best results, use delivered concrete instead of mixing it yourself from bags, and hire a concrete pump. A concrete pump uses a powerful pump and long flexible hoses to move concrete from ground level to anywhere it is needed. As the cells of the blocks are filled, a concrete vibrator should be used to eliminate pockets of air trapped within the cells and ensure that the blocks are filled solidly.
After the blocks have set and fully cured and the chimney has been constructed, you have some options for customization of color with paint. Be sure whatever you use has heat-resistance qualities since the concrete will heat all the way through when in use, even to the outside.
Local Building Codes
Before you start building a chimney, check the local building codes for your area. Many locales require permits before a chimney can be built, and demand meticulous inspections during the construction to make sure that the chimney conforms to area building codes. If this law is in effect in your area, you may be prohibited from building your own fireplace unless you have a contractor's license. Failure to get the required permits could result in fines and other legal action by the local zoning board.
The Best Solution
A chimney is not generally a project for an amateur builder. In addition to the complexities inherent in a concrete block chimney, you also need to have the experience to drop a hole down through the roof and the ceiling, which means extensive roofing and interior repairs as well as a high probability that wiring or plumbing will also have to be moved to allow chimney access. In short, building a chimney is a highly complicated procedure, and should probably be done by a certified contractor, whether they are required by law in your area or not.