Thanks. Yeah, I've poured a few slabs and built a few walls, but construction engineering is waaay out of my normal competencies, so I appreciate the vocabulary corrections.

You say "aggregate shape doesn't count for bugger all". So why does river / crushed sand work, and desert sand doesn't? It sounds more like there are different aggregate mixes / types of aggregate material that work for particular applications (hadn't thought that through, but it makes perfect sense), and that "rough" sand is better for the most-common concrete use-cases. Do you know of specific types of concrete where "smooth" sand would be preferred?