In case you're interested

There's a terrific little entry on strange matter in the Wikipedia, pointed out to me by a friend who shares my opinion that the evidence that the universe will end in heat death is a major bummer.

Strange matter is composed of strange quarks bound to each other directly, in a similar manner to how neutronium is composed of neutrons; a strange star is essentially a single gigantic nucleon .... Some theories suggest that strange matter, unlike neutronium, may be stable outside of the intense pressure that produced it; if this is so, then small substellar pieces of strange stars (sometimes called "strangelets") may exist in space in a wide range of sizes ....

Well, heat death may be a bummer but strange matter is cool. Obviously, this is the solution to the problem of what the Ringworld's floor material is made of.

The material the Ringworld is made of, scrith, is fictional. The stresses such an artifact would undergo are so strong that the tensile strength of scrith approaches that of the strong nuclear force, which holds atoms together.