Last week I was in the crawlspace underneath the bedroom addition and noticed daylight shining in. After picking out a piece of the loose mortar, I saw that there exists about a 5/8" gab between the original (1930) concrete block basement foundation and the add-on (1980?) concrete block crawlspace foundation. Someone had attempted to put mortar in the gap, however due to settling or concrete shrinkage this was now loose, leaving gaps or outright falling out.

What is the best approach to use to fill the gap? It's letting in a fair amount of cold, and is a possible path for rodents as well. I've seen some talk of polyurethane or semi-rigid epoxy sealers, but am not sure of the best approach.