This is my first post. I am building a house in a high, wooded region in Central Argentina. The climate is arid in winter and humid in summer. I have already built a stem wall with stone collected on site, laid in a lime/sand mortar with some volcanic dust added. No cement so far. Yet here in Argentina, even those that build in cob (or with unfired clay bricks and a cob mortar) tend always to lay a sand/cement course between the stem wall and the first laying of cob. They call it 'la capa'. Despite having read that impermeable cement membranes are generally not recommended in lime/earth construction, most here seem to do it anyway, as a kind of 'just in case' measure, not only to top off the stem wall but also in the floor, by always adding cement to the sub-floor mix instead of making up a pure cob or a pure lime-crete mix.
If you check out some of my other comments on Permies, you will see that I pretty much detest portland cement. Lime is a complement to adobe mixes no matter the type or recipe. Cement tends to trap moisture where you don't want it does not match the flexibility of the other building materials.