[laputan_machine]

Sand is cheaper per kilo than used coffee grounds.

"Building-grade sand" is sharp sand. It is the cheapest sand you can buy. Roman's built things out of their concrete which was full of random bits of rubble, and some of these structures still stand today.

[hkt]

Correction: they used lime mortar. Modern concrete was created about a century and a half ago. The random bits were chunks of lime which increased the building's hydraulic qualities - lime self heals, concrete doesn't