Instacrete

Instacrete is a material invented by UNSC scientists for use in combat zones. Instacrete is easy to shape and build with, incredibly strong, and dries quickly, allowing strong, permanent buildings to be hastily erected when they are needed. It is primarily used for creating bunkers and defensive walls. It can be inferred that the instacrete will take quite a lot of punishment even while in a semi-solid state. It is believed that instacrete can withstand quite a bit of small arm's plasma fire before being completely burned away. Its estimated dry time to full hardness and strength is several hours depending on temperature and humidity.

History
During the Battle of Reach, Charlie Company was forced into creating a hastily made instacrete bunker which was described as having not fully hardened by the end of the First wave of Covenant attack.