Tree patch

I have an old Dogwood tree that has a bit of cancer where a limb had been pruned off sometime back and now has developed into a "hollow" entering the main trunk of tree. What would be acceptable to use to fill this area before it becomes deadly to the tree? My granddad use to use cement ( as I remember) to repair such defects, but I always thought that probably not the best thing to do........ I think a lot of his trees died!! :) Suggestions? Thanks

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