doen't concrete count as a moveable solid because it doens't splits etc, then you only need a code that makes it not affected by velocity or air in general then you have moveable solid.
A pixel falls, if it hits something, it stops and go sideways if there's nothing, easy.
A movable solid, it falls, it hits something on one or more pixel, the weight of the pixels makes the solid spins one way or the other depending when it hit, with which velocity, and where it hit. If the solid hit on a flat surface, it won't spin because the gravity of each pixel makes it stay. That's without reactions, without pressure and no newtonian gravity.
@hollywoodninja The problem is with Powder Toy that it's pixel based. (It checks for nearby pixels each and every frame) and all powder movement is based on this. Liquids aren't a problem as well, cause pixels move separate. Moving solids is a problem cause that means moving multiple particles as one, doing checks for moving nearby particles every single frame. That will give a whole new meaning to the word "lag". [ANNOUNCING THAT MY LAPTOP IS FIXED!]