Sandpaper requires specific grades of corundum; Ontario happens to have several notable large deposits of extremely fit-for-purpose corundum. The Canadian deposits were also a conveniently close source for what would become America's largest abrasives products producer, 3M, after its attempt to mine corundum in Minnesota failed (3M stands for Minnesota Mining and Manufacturing company).

I bet we could do fairly well. Hundreds, maybe even thousands of years. We've learned a lot about how to form exceptionally long lasting cement. We just choose not to do it that way, most of the time.

Wood is a composite material, strong under both compression and tension. It's the fibres in wood (lignans) which provide the latter. Stone (and unreinforced concrete) lack such fibre elements, and are strong only under compression.

With stone it's necessary to build compression structures such as arches and domes for heavy load-bearing, and taller structures must be significantly flared out at the base (or utilise buttresses, as with gothic cathedrals) for stability. Structures under tension can be far more lightweight and compact.

Steel-box construction and reinforced concrete both offer tension-based strength, but are also susceptible to metallic oxidation (rust), which limits the life of such structures. A nonmetallic fibre (natural or synthetic) might offer an alternative to this, and I've seen some work investigating this, though there may be other issues (e.g., depolymerisation of plastics over long periods of time).