This locution is popularly interpreted as "a good and a bad", and there were even debates to agree if in the mortar mixture the lime was the good and the sand, the bad. Beyond the fact that the proportions change according to the final use that is going to be given, the truth is that the phrase is colloquial and does not have a relationship of proportion or opposition; the "good or bad" was added by the use since they are two different materials and can also be understood as "of one thing and the other".
