fill under concrete?

I have an old garage (28'x24') that had a concrete floor, which sank due to the fill that was used under that concrete floor, I think. I removed the old concrete and about 12" of loose dirt mixed with gravel. My questions are: What type of fill should I use. Should I rent a tamper and tamp that fill? There is a limestone quarry not far from here.
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First, if any of the existing fill look soft or otherwise unsuitable, that fill should also be removed. As a subgrade for a concrete garage floor, I'd go with crushed stone. Note that this is NOT the same thing as gravel. (Gravel is typically river stone with rounded edges and is not a great structural material.) The crushed stone should be pieces about 3/4" to 1-1/2" in size. Some compaction will be possible, but it should be fairly stable right out of the truck.
Joe F.
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