Wikipedia quotes ASTM C71 on the definition of refractory: “non-metallic materials having those chemical and physical properties that make them applicable for structures, or as components of systems, that are exposed to environments above 1,000 °F.” They are commonly used, for instance, to line kilns and other high-temperature furnaces.
Castable refractories can be mixed with water, formed, and solidified like cement, but the process of drying the solid before full-temperature use is critical. When working with castable refractory, be mindful of the hazard of heating trapped water too rapidly and causing a steam explosion. If using a commercial mixture, follow the manufacturer’s directions closely. If using a DIY formula, use a gradual “bake out” process in which the cast material is brought up to final operating temperature very gradually, in a series of slow, prolonged stages, with full cooling phases in between.
There are several recipes for “homemade” refractories floating around the web, but Lionel Oliver’s tutorial over on BackyardMetalcasting.com is one of my favorite resources on the subject. Lionel has been experimenting with and writing about home foundry work online for more than a decade, now, and his recipe uses no hard-to-find materials. By volume, it is 3:3:4 Portland cement:perlite:silica sand, mixed together thoroughly, then combined with 4 parts fireclay. The dry ingredients are then moistened to the texture of “stiff cookie dough,” packed into a form, and allowed to dry thoroughly before bake-out.
If you can get your hands on ready-made furnace cement, preferably of the “black” 3000 °F variety used to make repairs on wood-burning stoves, a simpler recipe is provided by John A. Wasser:
You will need about 1 part (by volume) of Furnace Cement for each 4 parts (by volume) of Perlite so for a two gallon bag of Perlite you will need a half gallon of Furnace Cement. If you use much less than four volumes of Perlite for each volume of Furnace Cement all of the passages between Perlite beads will be sealed and it will take a long time for the cement to set (it needs contact with air). If you use much more than five volumes of Perlite for each volume of Furnace Cement the resulting material will be quite weak. You will also want to have some Furnace Cement to use as a sealing coat on your lining. The Furnace Cement has about the consistency of roofing tar and is very sticky. It is MUCH easier to work with if you add about 2 cups of water per gallon of cement. This makes it more like a thin plaster.
John’s site doesn’t have any pictures, but here’s a page from Dan’s Workshop showing John’s refractory mixture in use in the construction of a small electric furnace.