Post #1840390

River sand is the right amount of jaggedness to where it can pour and settle into the right density in cement to have the right strength in the finished concrete. Ocean/beach sand works, too, but needs to be rinsed with fresh water, and is usually pretty valuable where it is (for beach resorts and what not). They're testing for how to use different types of sand (desert sand, manufactured sand, recycled sand) and testing the pouring characteristics and resulting concrete strength, so that they can make reasonable decisions on when it's worth using substitutes.