From Halopedia, the Halo wiki
Instacrete is a construction material used by the UNSC in combat zones. Instacrete is easy to shape and build with, incredibly strong, and dries quickly, allowing strong, permanent buildings to be quickly 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. Its estimated dry time to full hardness and strength is several hours depending on temperature and humidity.
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.
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