Old mortar& plaster upcycling; what minerals in them are good for the soil?

I'd have a look at the Wikipedia entries for 'plaster' and 'portland cement'. They are good on the chemical composition/reaction, plus, importantly, the safety angle.

I don't think there is a problem using gypsum-based plaster on the land, provided it really is solely composed of gypsum (i.e. Plaster of Paris that has been reacted with water.) If, on the other hand, it is dry Plaster of Paris, then you have to be very careful how you handle it, as when it reacts with water it produces a lot of heat, so can cause burns.

Quote from Wikipedia:- 'The chemical reaction that occurs when plaster is mixed with water is exothermic in nature and, in large volumes, can burn the skin. In January 2007, a student sustained third-degree burns after encasing her hands in a bucket of plaster as part of a school art project. The burns were so severe she required amputation of both her thumbs and six of her fingers.'

Gypsum, the product of this reaction, is calcium sulphate and is only slightly soluble in water and pretty safe to handle. Both calcium and sulphur (from the sulphate ion ) are important plant nutrients, so a sparing application of gypsum, rather than plaster of Paris, mixed into the soil shouldn't do any harm, but will be unnecessary if your soil is already chalky (calcareous) with a pH >7.

Personally, I wouldn't use a bagged lime-based plaster on the land, because of the dust hazard and its highly alkaline properties. You are talking about forming Calcium Hydroxide here. Whilst this compound is also only sparingly soluble in water, a solution will still have a pH of >10. Not only can you burn your hands, but it is likely to make the soil solution far too alkaline, at least locally, and damaging to plant roots. Raising the pH of the soil beyond 8.5 is never a good idea.

Likewise, I definitely wouldn't use bagged Portland cement on the land. Again, you are looking at both a dust hazard and a potentially highly alkaline situation.
Quote from Wikipedia:- 'when traditional Portland cement is mixed with water the dissolution of calcium, sodium and potassium hydroxides produces a highly alkaline solution (pH ~13):'
Plus you have the possibility of a significant addition of Aluminium salts to the soil. (see wikipedia chemical composition). Aluminium is not something you want to encourage plants to absorb, as in high concentrations it is toxic to both plants and humans!!!

I'd still be cautious about about using too much of the 'cured' products. In the case of lime plaster and cement, you are talking about reactions with carbon dioxide to produce carbonates (e.g. pure calcium carbonate (chalk) in the case of lime plaster). But with cement, I'd still not be happy adding anything with added Aluminium in it to the soil.

That's great! Thought I'd never get rid of the bricks but smashed up a load of them and used in containers for the med hebs. I binned the mortar chippings and the well-out-of-date portland cement bags.
Probably better binning the plaster it to prevent bioaccumulation of Al (and othe unknown constituents) in my veg. The manufacturer never got back to me with an MSDS and the product is possibly superceded.

Oh well, was trying to save money and my energy. It's such a trek to get to the garden centres, let alone shlep 25kg bags of horticultural grit, gravel and soil amenedments home.

Thank you very much.
Sorry for the late response, I had something on all weekend and an exam this week