Can Flex Paste be used to reattach stones that have fallen off outside concrete walls of home?

Hello there and thank you for reaching out to us! Flex Paste will do a fantastic job in adhering to many different surfaces including stone and concrete so it should work well for this application. Please make sure all surfaces are clean of dirt and debris, use a joint knife to apply the product easily and evenly, and apply the stone. It's that easy and after 24 hours it will cure to a hard rubber-like material that will last for years.