Permacrete, also known as duracrete, was a very strong and heat resistant material. It was used mostly in the construction of landing pads and pits for spaceports. It was usually not used in spacecraft because of its weight; when it was, it was usually hydrofoamed to reduce its density. Instead, the slightly weaker but lighter durasteel was the wiser choice. Untreated duracrete came in liquid or sludge form. A cheaper, weaker, but quicker-forming alternative to permacrete was stresscrete.