the only sand available where I live seems to be silica sand, the only one I can find without any salt and with angular shapes.
I've tried to do samples of mortar with round edged sand (silica too I guess) seems to work all right.
I would like to know the differences between the river sand and the silica sand, the round edges and the sharp ones, what are the consequences on the mortar in terms of solidity, workability, etc...