Instacrete
From Halopedia, the Halo wiki
Summary
The instacrete bunker was mentioned in Halo: First Strike, during the SPARTAN-IIs ground op on Reach. It seems to be a structure made of a form of fast-drying concrete, which can be easily shaped and set up. This is the only time in the books or the games that it is mentioned. Not much more is known about it.
