What type of concrete slab do martial artist use to break in demonstrations?

There's concrete and then there's cinderblock.

Never use concrete: there are so many variations, and some are made for high tensile work and are embedded with tiny steel fibers. You could use an 8lb iron mallet and still have a hard time breaking it.

You want to be using cinderblock. That is what most people in martial arts use when they break "concrete".

well have to ask have ever broke a concrete block before and to what I know when you train in breaking any thing its from the practice... also depends on the strike .. be it a punch, palm or etc.. it seems to me your trying to do some thing you have never done before.. so its best you don't try with out the teaching of the art and knowing how to strike the right way... but to say most in training start small and work to bigger block of concrete.. same thing with wooden boards..