Sailingdog hit many of the functional issues with concrete ballast. I personally helped a fellow who owned a Buccaneer with concrete ballast that had gotten wet and frozen many times over the years. The concrete had been reduced to loose rocks that you could easily remove with your fingers surrounded by water saturated dust.
Of course the technique for constructing concrete ballast varies widely from simply dumping cement into the keel encapsulation envelope as was done on the Buccaneer, to casting a mix of boiler punchings and concrete in a separate mold and setting it like you would a cast lead keel as Bristold did on the Bristol 24 (AKA Corsair)
The other issue with this technique is never knowing what density the keel will end up being. The production manager at Bristol mentioned Corsairs where they accidentally left out the boiler punchings, and my family looked at a Cheoy Lee Frisco Flyer that turned out to have the a similar problem.
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Curmudgeon at Large- and rhinestone in the rough, sailing my Farr 11.6 on the Chesapeake Bay