The material they used (water mixed with wood, frozen) is called pykrete. > Blocks of ice containing as little as four percent wood pulp were weight for weight

The material they used (water mixed with wood, frozen) is called pykrete. > Blocks of ice containing as little as four percent wood pulp were weight for weight as strong as concrete; in honor of the originator of the project, we called this reinforced ice "pykrete". When we fired a rifle bullet into an upright block of pure ice two feet square and one foot thick, the block shattered; in pykrete the bullet made a little crater and was embedded without doing any damage. My stock rose, but no one would tell me what pykrete was needed for, except that it was for Project Habakkuk. (from I Wish I'd Made You Angry Earlier by Max Perutz, via Wikipedia: https://en.wikipedia.org/wiki/Pykrete https://en.wikipedia.org/wiki/Pykrete)