Jumping on @jamesqf's answer, this is physically difficult (but not impossible) and what most suggestions have said so far is somewhat incorrect realistically. This is because of Newton's Laws (Specifically the Third here)
- For every action, there is an equal and opposite re-action
This means in our example two things:
1) When a mass moves linearly, as in "rocket-firing" it, it also creates an equal force in the opposite direction. So look at the ground here:
That air and fuel is moving with a lot of force! Neglecting the exhaust problem of a steam rocket, a human weight (~62 kilo says google) hammer hitting forwards at fast velocities would hit a human forwards with an imparted force, but the imparted force is equally applied to the wielder backwards, including joints and bones, and in the opposite direction. So hitting forwards with some force would push you backwards with the same force. Breaking concrete? Not useful for a human wielder bluntly, unless you had some sort of intricate counter weight.
2) When a mass moves from a "tether" like a handle, it exerts an opposite rotation force. For example, Mario has some interesting ways of killing megafauna, swinging them by the tail!
He's got some strong legs! Suppose my hammer "jerks" forward from my arm by rotation. Something has to counter-balance that, and it's not going to be my wrist if the force could break bones. Maybe my muscles, but probably not from an arm all the way down...
My conclusion is that a blunt "warhammer" would not work for this with a human wielder holding the force. And about that concrete: it's a fun fact that a human femur, the largest bone in the human body, is about as strong as concrete. Not stronger: you won't do it bluntly.
Needs to be effective in hurting and killing humans, mega-fauna, and break through concrete and steel.
The more gears, the better.
But of course, I neglected two thirds of the important laws.
- Force is equal to the change in momentum per change in time: Force is equal to mass times acceleration
Are you familiar with flywheels? You could use a flywheel churned by a steam engine and TONS of gears to do a torque differential - then impart its momentum by having it strike something or "gear stop" suddenly. Say the flywheel is ~150-200lbs, the bullet-shaped hammer head is ~30lbs, that's a lot for the rest of the contraption - but it would be more like a siege weapon.
Something that would break concrete could be sharp, but not steel. The steel would bend under mass but jackhammers are no good, even if it was ultra-hard like Tungsten. It would need to be blunt because steel doesn't chip away to my knowledge, and steam isn't hot enough to do something heat wise to steel (It's at least 212 F, but steel melts > 1000 F, that steam would cut out of pipes and slice people but even water has a hard time cutting steel under pressure - unless they found and rigged up a water-jet).
Another idea is two hammers on a pivot that rev up and hit, maybe on some sort of tank. But that would be a lot bigger, I should think - maybe something you could put on trained mega-fauna or a tank of some sort.
Anyways, this is my mostly realistic answer. But this isn't the Physics SE.