If you're looking to build a pizza oven, you may be wondering if you can use regular concrete bricks. The answer is no - if the bricks are made of clay and baked in a kiln (refractory bricks or red clay bricks), they can be used for a pizza oven, but if they are concrete bricks, you should stay away. Clay bricks are the best option for a pizza oven, as they will do the job and if you're only using it a few times a year, it makes sense to go this route. You can even use reclaimed bricks or bricks taken from a structure you're demolishing. Concrete is often used in the construction of wood-fired brick-type pizza ovens.
If the hearth, dome or other high-impact “hot face” areas are constructed of concrete, it's necessary to use a mixture composed of refractory products such as crushed refractory bricks called “grog” or refractory cement. This is mixed with water and creates a high-quality cementing material for the construction of the pizza oven. Construction mortar or refractory mortar together with Portland cement-based concrete or refractory concrete is widely used to build high-quality pizza ovens. A serious domestic fire can generate enough heat to damage and weaken concrete and steel reinforcing bars in footings, slabs, and stem walls of footings. To ensure your outdoor pizza oven is strong and level, the supports should be made of concrete, breeze blocks, house bricks or even old railway sleepers.
Place a piece of plastic sheet on a smooth surface or mix the refractory cement on a smooth concrete surface. Before firing up the furnace for the first time, it's important that the concrete and mortar are completely cured and dry. As with setting concrete, setting mortar in the furnace must be slowed down so that it doesn't happen too quickly. When concrete is above 77° F, the hydration process accelerates and generates internal heat. Use some scrap wood to level the concrete and make sure it's flat so that your pizza oven rests on a level base.
Commercially, fireproof concrete is manufactured by mixing fly ash - a by-product of Portland cement production. Cement and concrete are less likely to be used in the construction of a cob kiln, since the main building material is usually cob (mud and straw). Taking the right measures before buying or installing a fire pit is your best defense against concrete damage. Placing the fire pit on concrete is safe, but cracks can occur due to high temperatures. The concrete members will expand and contract when exposed to cold and hot ambient temperatures. Dry cement contains calcium oxide, which isn't normally dangerous but when mixed with water turns into calcium hydroxide, which is dangerous.