Pykrete, also known as picolite, is a composite material made of approximately 14% sawdust (papershreds or wood pulp) and 86% water by weight then frozen. It is invented by Max Perutz during World War II.
The material was proposed to United Kingdom’s Royal Navy, by Geoffrey Pyke, as a candidate material for making a huge, unsinkable aircraft carrier, Project Habakkuk. Steel and aluminium were in short supply and required for other purposes. Pyke realized that the answer was ice, which also could be manufactured for only 1% of the energy needed to make an equivalent mass of steel.
Pykrete has some interesting properties, notably its relatively slow melting rate (due to low thermal conductivity), and its vastly improved strength and toughness over pure ice, actually closer to concrete, while still being able to float on water. Pykrete is slightly harder to form than concrete, as it expands while freezing, but can be repaired and maintained from the sea’s most abundant raw material. It has a crush resistance of greater than 21 megapascals (3,000 psi) so a very short 25 by 25 mm (1 inch by 1 inch) column could support the weight of a typical car. The wood pulp also makes the pykrete stable at higher temperatures. Pykrete can be easily formed using water and any porous and fibrous material, such as shredded paper or sawdust. Anything that can be molded with this wet pulp can be frozen into a strong and non-brittle solid.
Max Perutz (1914-2002)
Sawdust or shredded paper and ice.