I want to create a necklace made from concrete, but use a brass crimp tube for the chain hole. Is there anything chemically wrong with doing that? Will either the concrete or the brass corrode over time by mixing them?
Different metals have different issues. For example, aluminum is highly reactive with the alkaline concrete (the reaction is used to create foamed concrete). Iron rusts and decomposes all the way through. Besides losing its integrity, the rust can crack the concrete.
Brass is much less affected by the concrete. The concrete can accelerate development of the patina and pitting on the surfaces in contact, which would not be visible. This won't affect the integrity of either the brass or concrete. And concrete covering one portion of the brass won't affect the exposed, visible brass.
Perhaps a more important issue is just that brass tarnishes, even when it's well protected by a finish. If the finish is at all permeable to air, the brass will tarnish underneath and the finish just makes it harder to polish the brass again. If you want the patina as part of the intended design, great. But if you want the brass to look polished, the concrete will make life difficult. Concrete is porous, and cleaning the brass later will stain the concrete. You could try to seal the concrete to facilitate cleaning the brass later.
If you go this route, don't embed the brass in the concrete when you mold the concrete; that would make it almost impossible to seal the concrete where they meet (and they expand and contract at different rates). Instead, mold or drill a hole for the brass. Seal the concrete, and you may want to seal the brass. Then put the brass in the concrete. I wouldn't try to crimp the brass after it's in the concrete, as it will create stress that could break the concrete (it has very little tensile strength). I would glue the brass in place if it is necessary to hold it there.