Solid building material (8)

I believe the answer is:

concrete

I believe this clue is a double definition.

'solid' is the first definition.
(similar in meaning)

'building material' is the second definition.
(concrete is a kind of building material)

(Other definitions for concrete that I've seen before include "factual type of evidence?" , "Substantial, real" , "Definite; building material" , "Unsightly building material" , "Real, not abstract" .)

I've seen this clue in the Irish Times (Simplex) and The Sun.

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