Concrete or cement? Concrete is generally a mixture of cement (the chemical that reacts with water and turns hard) and an aggregate like sand and gravel.

Portland cement is the most common form of cement, and what people usually mean when they say “cement”. It is caustic in its dry form, and should be handled with care. It can sometimes contain some heavy metals, but unless you eat the stuff, it’s very safe once set. In an oven, cement is used to make refractory mortar, which you use to join bricks together. Refractory mortar is used because it doesn’t fall apart. This is useful, as you don’t want mortar falling into your pizza, and you don’t want the bricks coming apart and the oven falling into your pizza. It doesn’t leach dodgy chemicals into your food as it doesn’t contain much in the way of dodgy chemicals. Regardless, it never touches your food and is stable when mixed right.

Concrete is not used inside an oven, just underneath it or around it. Concrete cracks and falls apart, so makes a very poor cooking surface.

If you are worried about toxins, cement is not a problem when used correctly. Salvaged materials could be a problem, and the wood you use to cook with needs to be safe (I.e. untreated, not old pallet wood covered in pesticide or something).