From KGB Wiki
Caulk is one of the fundamental components of Booth. It is supposed to be used for filling in cracks, but it has been misapplied as both fake-mortar and a structural component. Caulk is the butt of many jokes.
Despite being similar in shape and function to mortar, you should not confuse it with any other senses of the word "mortar", irregardless of any suggestive actions the antecedent of such a sense might be capable of.