Pykrete is a composite material made of approximately 14 percent sawdust or some other form of wood pulp (such as paper) and 86 percent ice by weight. Its use was proposed during World War II by Geoffrey Pyke to the Royal Navy as a candidate material for making a huge, unsinkable aircraft carrier. Pykrete has some interesting properties, notably its relatively slow melting rate (due to low thermal conductivity), and its vastly improved strength and toughness over unmodified (crystalline) ice, actually closer to concrete. Pykrete is slightly more difficult to form than concrete, as it expands during the freezing process, but can be repaired and maintained from the sea's most abundant raw material: water.
The carrier concept was abandoned when the U-Boat threat receded and it was decided to concentrate on more conventional means of combating them.