Polycrete

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

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