In our world of plastic consumer products of every variety, indoor air quality (IAQ) has become an increasingly important consideration when designing living spaces. Many materials which have been used for many decades have only recently been discovered to leach potentially harmful substances into our homes and businesses. And in the world of fireplaces, where objects and surfaces are heated dramatically, IAQ impact can be of even greater concern – for what better way to encourage material off-gassing than to heat it up with a fireplace! Thankfully, for fireplace design, there are many good options.
Cast concrete, assuming it contains no acrylic or latex additives, is a very stable, long-lasting and, when it comes to IAQ, inert material for fireplace applications. Modern concrete is, by design, made to have low reactivity. This is because in most applications – roads, bridges, buildings – it is subjected to all kinds of environmental abuse and attack from chemicals. The raw materials in concrete (sand, stone, cement, and organic pigments) must be chosen carefully to ensure that the concrete will stand the test of time which, in some applications, can be centuries. To achieve this level of durability, the constituents of concrete must be completely stable, particularly when mixed together. This is good news for interior uses such as fireplace mantels, where the amount of wear and tear is minimal.
Concrete is created through a hydration reaction. This means that the active ingredient, cement, reacts with water. The result is the formation of a hard, crystalline substance that binds all the aggregate together, and the generation of heat. This reaction is what is referred to as ‘curing’ and happens rapidly at first, continuing for months at a progressively diminishing pace. The only byproduct that is emitted in a gaseous form is the excess water that is not used in the chemical reaction. Accordingly, most of this water escapes during the early stages of the concrete reaction. By the time a concrete fireplace mantel or tile is installed in a home, the amount of water vapour that is being emitted is virtually undetectable.
Its chemical stability and the fact that the only substance it off-gases is water, make cast concrete ideal as an interior finish.