So I was looking at paleoarts' new clubs and I wondered, especially for warclubs, how well would concrete do instead of stone? Concrete, while comparatively new, has been around for at least 2 thousand years. Instead of pecking or grinding a stone into shape you simply cast it into shape. So if you had both options, would there be an advantage to using stone over the concrete?

I've never really had any problems breaking up concrete with a hammer, I usually have to work pretty hard to break chunks of rock down.  The rocks are used for knapping and they are pretty tough.  I think a concrete warclub head will last a long time if all you hit is flesh and bone but if your target is wearing metal armor, I don't think it will last long, nor will it stand up to blocking and parrying other weapons and shields.

Just like anything else, there can be a whole spectrum from garden variety to high tech and exotic.
I used to be dubious about the strength of concrete, and then I had to do a penetration through a Spancrete floor ... I needed some serious equipment and really had to WANT to get through that stuff.

This idea hasn't stopped nagging in the back of my head, so I got a book about concrete. The author makes the argument that a form of concrete can be argued to predate permanent villages and agriculture. He also argues that the quality varies a lot, the concrete that the pantheon was made of is superb. It will set underwater (called hydraulic) and has enormous strength. Archeologist Auguste Perret, said "go to Rome and try to break the old Roman concrete with an axe; you will only dent the steel".
 
I worked precast and pre-stressed concrete one summer. We made pieces for bridges and stadiums. The mixes (type amount of sand, type and amount of aggregate, various catalysts)varied from job to job as does the amount and types of reinforcement.  And each piece has stronger and weaker places. I really don't know. Overall the book I'm reading argues that most of the concrete we know is inferior to Roman concrete and will need to be replaced. We avoided rediscovering better concrete because we added reinforcement which the Romans did not and it's only comparatively recently that we have rediscovered the stronger type of concrete. If you used reinforcement, a good aggregate and hydraulic concrete I really would be interested in seeing how well a club made up of it would compare to stone.  
 
Depending on what my conclusions are after finishing the book I might have to procure and compare the two.

I'd have to agree that modern concrete doesn't match roman concrete. AFAIK, we've had the technology to recreate their formula for a long time but it's just not cost effective to go brought all that trouble because we don't use our concrete in the same way the Romans used their.