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i'm no expert but i would say the cement wall is good in the mass catagory but you need some space for the low freqs to dissolve. build another wall in front of it with a minimum 1" gap between the walls. do not let them touch.
put a 1/4" strip of rubber under that 2nd wall between the 2x4 and floor. sheet rock BOTH sides of that wall and insulate it with regular insulation -the pink stuff works well (minimum r-13)
that should do ya
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