Actually the sand is a good thing in that it will mix with the clay and
help its breakup allowing additional organic matter to be more easily
introduced.. Also worked wonders for the draingage of my yard (we are on
the receiving end of a slight slope in our neighborhood). My neighbor has
standing water after a heavy rain-I have green grass..




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

--
Travis in Shoreline (just North of Seattle) Washington
USDA Zone 8b
Sunset Zone 5