Is there a term for words that are a combination of abstract and concrete nouns?
For example: Mental floss, Pipe dream, Relational glue
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See reifying at Merriam Webster. The process is called Reification. It is defined as
to consider or represent (something abstract) as a material or concrete thing : to give definite content and form to (a concept or idea)
Read this Wikipedia link. It is pretty elaborate. It explains different constructs.
You'll also find concretism to be interesting which seems to define what you're looking for, though all definitions I found are related to poetry.