Polycrete

Polycrete was a very common material used by Human civilians and military personnel to build structures of many kinds.

It is used for building numerous things, particularly in the Outer Colonies of the UNSC. Many of the buidlings of Harvest were comprised of polycrete, and many structures on Tribute were as well. However, polycrete is used for durability, not protection. It is vulnerable to explosive ordinance, which can easily destroy buidings composed of polycrete.