Ice is strange stuff: brittle when struck suddenly, yet malleable when pressured over a period of time. With low but steady pressure, this plastic deformation can continue indefinitely. Above all, ice is unpredictable. Molded into a beam, it will fracture at loads anywhere from 5 kg/sq cm to 35 kg/sq cm. Because it fails at unpredictable loads, it is not ideal as a building material...
Pykrete is a super-ice, strengthened tremendously by mixing in wood pulp as it freezes. By freezing a slurry of 14 percent wood pulp, the mechanical strength of ice rockets up to a fairly consistent 70 kg/sq cm. A 7.69 mm rifle bullet, when fired into pure ice, will penetrate to a depth of about 36 cm. Fired into pykrete, it will penetrate less than half as far—about the same distance as a bullet fired into brickwork. Yet you can mold pykrete into blocks from the simplest materials and then plane it, just like wood. And it has tremendous crush resistance: a one-inch column of the stuff will support an automobile.
"The Floating Island", Cabinet Magazine
More on the story of a failed, secret WWII project to build ships out of ice.
