Is there a name for sand-clay mixture?
The clay-sand mixture used in casting is known as green sand, foundry sand, or molding sand.


  Green sand is an aggregate of sand, bentonite clay, pulverized coal and water. Its principal use is in making molds for metal casting.
  via Wikipedia


Different proportions of sand, clay, and other compounds result in slightly different properties, but green sand can be packed tightly to hold its shape and therefore not move when molten metal is poured into a mold.

A walkthrough of the sand casting process can be found at Foundry 101: Step-by-Step Sand Casting Instructions.