In general, I agree that if you have a concrete slab it can be a good place to store energy. You also mention building block by block, so I assume you are using concrete block, again a pretty good storage medium.
The only problems are whether these are well insulated and whether you can control the use of the heat. Because of this, there are advantages to having the storage separate where you can insulate better and control when the energy is used. And, virtually any mass would work. Concrete would certainly work, but why use some thing so expensive? Gravel, or even soil can be used and would be much cheaper.
Then, the same storage could be used for hot water and heating. I agree with the others that it makes sense to use the system for hot water as well. Unless the water is stored above ground it is unlikely it will be warm enough for bathing, let alone for dishwashing, etc. Besides, you will have lots of surplus energy in the summer.
I don't know if you plan on air conditioning, but if so you could use the same system for this by running it at night when you can radiate heat into the cool sky. Or, if this is not enough cooling in your location, it is possible to cool using solar heat. Again, the same mass could be used to store cooling.
For more details, see my blog at
www.energy-guru.blogspot.com