The wrong choice of fireplace brick can have fatal consequences. Materials that aren't suitable for extreme heat will break down, potentially causing a gas fire in your home. Not all concrete bricks are created equal. While you can find some that work for fireplaces, basic concrete does not pass the heat test.
You know it gets hot inside a fireplace — up to 1,500 degrees Fahrenheit. To safely construct a working fireplace, you need a brick that can handle these super-hot temperatures without cracking or sustaining damage. If the bricks can't withstand heat, gas from the fire can seep into cracks in the material. If enough gas gets through the brick, it can make contact with the wood frame of your house and combust.
A typical concrete block will not cut it for fireplaces. These large concrete slabs are air-dried when finished, not fired in a kiln. This lack of heat processing makes the bricks ill-suited to withstand the high temperatures inside your fireplace, and they can break or crack over time. You can have traditional concrete blocks near the fireplace if you cover them with firebrick or another heat-resistant material. On the other hand, if you have a nonworking fireplace just for show, concrete blocks offer inexpensive coverage.
If you enjoy the look of concrete, look for heat-resistant concrete firebrick. Heat-resistant concrete contains naturally heat-resistant materials, such as calcium aluminate, which can stand up temperatures higher than 1,000 degrees Fahrenheit. It's generally a lighter-weight concrete. Avoid concrete that contains quartzite, which gets damaged in fires. If you're not sure whether a concrete brick is fire-friendly, don't use it.
Red clay bricks work just fine for a fireplace, as they're exposed to temperatures of up to 3,000 degrees Fahrenheit during construction. These are the typical red bricks used in construction. You can also use medium-duty firebricks, which are fire-resistant and capable of withstanding temperatures above 2,000 degrees Fahrenheit. Both will serve well in a home fireplace.