I did martial arts for years and often helped out with the classes as an adult. We used to buy the cheapest concrete brick/paver we could get for breaking. Something similar to this:

https://www.homedepot.com/p/16-in-x-8-in-x-1-75-in-Pewter-Co...

There is a lot of variance in bricks. And honestly a lot of the cheap ones would break if you dropped them, or loading and loading them in the van...

Not to mention people would accuse us of baking them, or freezing them, leaving them outside for months or years, all kinds of stuff. But honestly we would buy a bunch of them from the home improvement store on the way to the demo at the mall or a school. You'd be surprised at how easily you can break one with the simple tap of a claw hammer.

It's just physics. The longer the brick, the easier it will break. Some demo rocks are much longer. The trick is to hit it fast, use the smallest impact point you can, and follow through. If you don't follow through you won't even be able to break a pine board. Theres some kind of psychological thing prevent most people from punching through a target, whether it's a brick, punching bag, or a board.

Drywall, however, is an exception to this rule.