Is a concrete or abstract noun?

"Concrete" can be both a concrete noun and an adjective, depending on how it's used.

* Concrete noun: When "concrete" refers to the material itself, it's a concrete noun. For example: "The building was made of concrete."

* Adjective: When "concrete" describes something as being specific and definite, it's an adjective. For example: "We need concrete evidence to support our claims."

So, it's important to look at the context to determine which part of speech it is.