Dry clay soil
Not sure what good plaster boards and sand would do 
Are you thinking of the gypsum it contains to improve the soil? Plaster board nowadays also contains loads of other stuff and I believe it is controlled environmental waste - I wouldn't use it on my garden.
You just need organic matter and lots of it. Clay soil can be the most fertile in the world if it is improved ... every year add layers of farmyard manure, home made compost, whatever organic matter you can get your hands on.
When Pa bought a run down farm on the clay land of High Suffolk in the 50s it was considered to be poor land and worth little - virtually impossible to far because the soil had so much clay that machinery became stuck in it after just a little rain ... 30 years later it fetched the highest price for agricultural land in Suffolk that year, because it had been improved with year after year of good straw-based manure from the pig and cattle yards.
Lay the manure on top of the soil in the autumn and let the worms drag it down over the winter, then fork it over in the spring when the weather improves and the land's not wet - that way you don't break up the soil structure.
Last edited: 01 August 2016 07:22:06
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Organic material is the only thing that will do it. It holds water like a bath sponge. Add as much compost, finely shredded bark or well rotted farm yard manure as you can get.If you can get used mushroom compost from a mushroom farm, that is good, but it contains lime so don't use it on acid lovers such as azaleaa or rhododendrons. Adding sand or grit is useless, it just makes concrete.