Micro CoGen.

Yes.

You can buy High Strength concrete mix at most building supply stores and at most True Value Hardware stores (Yes, I do work for one.) There are also additives that can take the place of some of the water in the mix.  Be careful of how much water goes into the powdered mix; watery mix makes weaker concrete.  The additives can make the concrete less brittle, and thus less prone to cracking. Concrete continues to harden over a long period.  The more time you can give it before stressing the stronger it will be. Cover the top of the block with polyethylene for a few days to slow water loss and promote stronger concrete. If it is warm and/or dry you may have to mist the block with water for a few days to allow a longer and stronger cure.

Central Massachusetts