Hi Salmo,
Sorry to be contradict you, but many people get a bit confused about lime and what it is.
The natural base mineral for lime is calcite, CaCo3, found very pure in limestones and, less pure, in chalks. Both of these can contain a proportion of magnesium (magnesian limestones).
Limestone is not lime, but is used for making lime. It is sometimes used in agriculture because of its lower cost, but it's less effective and slower acting.
All lime is made by heating to produce quicklime (calcium oxide) which, as you say, is very reactive. It is sold both for agricultural and construction use. (As well as for lawn mowings XX ). Eye, skin and breathing protection needed during use. Because of health risk it can be obtained in granular form which contains less dust.
"Standard" lime is made by adding quicklime to water, which gives off a lot of heat during the reaction, and gives hydrated lime Ca(OH)2, which is the product most often used in building work.
If you buy builder's lime it will have no additives, excet, as I said, the lime/cement mixes sold for DIY, but which are labelled.
As far as I know, gypsum is never added, but plaster (made from heated gypsum) is added to lime for specialist restoration work, although this use is now discouraged. I don't think you'll find these in DIY outlets, though.
A couple of interesting links:
http://www.ima-eu.org/fileadmin/downloads/minerals/Lime-fact-sheet.pdfhttp://www.britishlime.org/lime_con01.phpOut of interest, gypsum (and plaster) isn't good on clay soils as it deflocculates clay, making it very sticky, whereas lime flocculates clay, making it easier to work.
Sorry if this was a bit long,
Tony