... entirely out of salt.
The salt is not held immobile in any way. Yamamoto makes his creations completely out of loose granules he pours out through a squeeze bottle. Here's a video of him in action, filling a whole room up with his designs inch by painstaking inch. If he sneezed, he'd instantly erase a month of work.
Stefan Worring/Motoi Yamamoto
That's why, after several years, he reluctantly abandoned pepper sculpting.
Yamamoto also defies physics to create 3D sculptures out of the same fine, powdery dust that we can't even manage to wipe completely off the kitchen counter, much less carefully stack into a quarter-scale model of that stairway from The Lord of the Rings.