They are very hard; very strong; very stable; very sharp ~ you must wear heavy gloves to handle them.
I think the only mortar for which the recipe was lost during the Early Middle Ages was that for hydraulic or marine cement. It sets up under water.
Every now and then you'll see this on DISCOVER where they're looking over an old fortress or castle in Europe and they mention that the mortar seams are almost invisible.
Do you mean “cinder blocks”, as we use in constuction, or something else?
Modern cement cures better under water than in air - because it cures slower.
Pre-cast and pre-stressed bridge sections, eg, are cured in a water bath.