From Halopedia, the Halo wiki
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 explosives, which can easily destroy buildings composed of polycrete, making it useless in building military structures. Polycrete is also unable to stop a Gauss round from the M99 Stanchion Gauss Rifle.
List of appearances