Thanks everyone for entertaining my posts about welding, generators and offgrid electric power, etc., etc.
I have come in and looked at replies.
Silence has been I have been lime-mortar'ing, and using lime in another niche application I came across - "mud" mortar - subsoil-mortar - used in local natural stone construction here in Cornwall (and around Britain historically, it seems).
Lime-mortar you can buy as a premix in sacks. Apparently the lime component is already slaked through to CaOH - which is dried so is a powder - but which otherwise holds all the right characteristics. Giving you an easy-to-use much less hazardous material to work with. Took me ages coming from nothing to having some idea what to use. Then at a basic level the meaning of the terminologies.
The "subsoil-mortar" had me doing "hot-mix lime". Lime component goes in as the quicklime, CaO, as it comes out of a lime-kiln. Its heat of hydration is notorious - leading to name "hot-mix". Seeing is understanding...
I at least somewhat see now how in Roman "hydraulic mortars" - mortars which have a self-setting reaction "within themselves" after hydration, not solely relying on drying and/or reaction with carbon-dioxide in the atmosphere (as lime tries to revert to limestone) - are "clever". Roman mortar on quays / harbours in seawater have lasted 2 millennia and remain sound. It was thought unreacted quicklime was "careless" hot-mixing. I can now at least kind-of see what has recently been realised - that feature is deliberate. It gives a self-healing mechanism, by having a reservoir of quicklime to chemically react and self-heal if there is structural settlement etc.
So have been preoccupied. While being grateful for help & guidance which came-in on topics I have asked about.