Hi John, thanks for asking questions about rocket mass heaters. What you are proposing is possible but may not function as well as hoped. Some more information may be helpful as well--what is the size of the shop what are the low temps going to be like when you are trying to heat the shop? Is the shop insulated?
So I first would recommend insulating under the entire slab as this will hugely improve the rmh ability to heat the space. Even if you insulate under and to the sides of the pipe, the heat will distribute through the concrete and be sapped by the available mass beneath it.
Second, if you do insulate to the sides and under the pipe (flue) then the ability for the stove to shed its heat to the mass will be greatly reduced and defeat the point of the stove. With the insulation keeping heat from permeating on 3/4 of the sides, and then the cement floor heating up above it, it won't build up and store your heat for later release--instead it will just continue on out of the building out of your chimney.
The better option if space is the issue, is to build an rmh with a belled bench which tend to take up less space then the traditional piped mass, then have it exit the building. Belled benches also tend to have more radiant immediately released heat than a piped mass which will hold heat longer but take longer to release that heat into the space.
As for the cement--temps above 212 will start to break down cement. It can be used in parts of the mass, but shouldn't be used in the stove itself where combustion is much too high.
Here is a link to a thread discussing heating floors with the RMH-- https://permies.com/t/5831/underfloor-heating-system-rocket-mass