Over the years I have run into so many non-metallic boxes secured in block or brick wall with some type of grout added and the box pushed into it to secure the box. I wonder what amount of that substance, contaminates the romex sheath. I did it myself for a long time. Now I use spray can expanding foam to hold the box in the wall. It has worked out excellent, none have worked loose. My feeling is that it is alot better on the cable than a caustic cement. Even better is to run pvc down the void of the block if it is possible, but a lot of remodel does not allow for that.
