Abstract: A method for forming a composite structure, using a mandrel that is later removed from the composite structure, involves production of a mandrel by depositing a particulate mixture, including an aggregate and a binder, into a mold and removing the mandrel from the mold. The mandrel may be treated while still in the mold by heating, curing with an agent, microwave energy, or by some combination thereof. Once finished, the mandrel can be used in manufacturing polymer and/or composite components. The mandrel can also include materials that can be easily removed from the finished composite structure by water, shakeout, chemically dissolving, or by some combination thereof. The mandrel can be a self-expanding mandrel, and can be used in a process or system for the manufacture of composite structures.