The concrete may provide evidence beyond material evidence (e.g. DNA, etc.)
It probably was Sakrete or a similar product. Although that isn't hard to come by, as has been mentioned, the sources are still constrained to building supply type businesses. Forensics will be able to pin down manufacturer, brand, lot, and so forth.
Most companies that use these products don't generally keep much on hand unless they plan to use it quickly, because it really doesn't keep well unless it is carefully stored. I won't go into details. You'll have to trust me. So it may not be too likely that it was acquired from a construction firm or apt, complex. What someone did have would probably be noticed missing.
If it can be shown that one of these people purchased any it will go a long way toward nailing them. They would have to account for its use, and that would be hard to fake.