Sorry, late to the party! Following @1st ade invitation, I tend only to get involved when restoration and the local authority heritage conservation officer collide. As a general rule we don't know the composition of the mortar on particular structures without a sample analysis. Two observations, the original would be very local in origin, and subsequent repairs may not have been faithful to the original. Loathe as I am to criticise the local authority officer, we do get some bizarre ideas. One asked for mortar to match the original on a boundary wall in Frome (Dissenters Cemetery) - analysis showed it contained ash from Frome Power Station. The officer struggled with why we couldn't get this any more...
Ystalyfera aqueduct on the Swansea Canal was thought to be one of the earliest canal structures to use hydraulic lime - link below, however I think earlier ones may have now been noted. Even when the claim was made it was acknowledged the Romans had used it, but we then forgot the technology
Afon Twrch Aqueduct Ystalyfera - Welsh monument database