Concrete cavity - fill it up?

Often times when you install plumbing valves or electrical boxes on an exterior concrete block wall you have to chip out concrete. Since those are block wall once you chip it you have a six inch deep hole.
So after you installed everything, what do you do with the cavity behind it? Do you fill it somehow?
Someone told me I should spray some foam filler in it...I am not so sure, I have seen some of those when expanded basically "glue" onto any surfaces, I think if I have an overspray it may expand to seal off the shutoffs for a valve and if it's an electrical box it may be flammable?
Any advise? leave it alone or fill it?
Thanks in advance,
MC
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On Apr 7, 2:06�pm, snipped-for-privacy@hotmail.com wrote:

use expanding foam to fill voids, after cementing around at least one side of the entrance.
you dont want air leaks, expanding foam helps keep cement from falling in cavaties
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On Mon, 7 Apr 2008 11:06:58 -0700 (PDT), snipped-for-privacy@hotmail.com wrote:

If the device will need service in 10 years then you stuff it with insulation. If not, then the foam will be fine.
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