"Travis" wrote :

I don't know about gypsum but I have heard that clay+sand=cement.

Nope. Sand (aggregate actually) + portland cement = concrete.
Sand+clay=clay with sand in it. The problem comes when people work the clay
while it's too wet. THAT leads to the concrete-like texture you describe.
To get any noticeable effect from the addition of sand, you've gotta add a
ton of it.

Jason