Slurry

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A suspension of a solid in a liquid. 

If an underground tank is to be permanently taken out of service, but allowed by the local jurisdiction to remain in the ground, it must be filled with an inert material. Sand may be used for this purpose. If sand is placed in the tank, however, a fairly large hole must be cut in the top to permit the introduction of the sand. Moreover, as the tank deteriorates the sand may migrate into the surrounding soil.

But if a liquid-like material, that will later set up into a solid, can be pumped into the tank, the job becomes somewhat easier, as well as far more permanent. For this purpose, a slurry is sometimes used. In its liquid state, the slurry can be pumped in through a small opening in the top of the tank. Later, when it sets up, the slurry becomes a solid. 

In addition to water, material used to create a slurry may consist of cement, polyurethane foam, or other chemicals.