| Knowledge of "Roman cement" roughly speaking was never really lost. How do you think the quay walls in medieval Dutch and Flemish harbours were built? The mortar was made with Trass [0], a natural puzzolanic from the Eifel, transported along the Rhine. "Roman cement", also known as hydraulic lime [1] mortar, was in widespread active mainstream use in many places in Europe until the 1920's, and in some the 1930's. In Latvia for example, large scale Roman cement production only came to an end with the second world war flooding specific quarries. The main problem with these hydraulic cements is they cure/set much more slowly. That's very much guaranteed to limit their use in speed focused modern construction. There's still a market for hydraulic lime mortars. There's even a euro norm for it, EN-459. These products have excellent breathability and moisture resistance properties. The entire field is seeing a revival in monument restauration and ecological construction. Most straw bale construction in Europe for example is plastered using hydraulic lime plasters. Some interesting products: - natural hydraulic limes (NHL), like Saint-Astier from France - formulated/artifical hydraulic limes (HL), often using blast furnace fly ash - natural puzzolanic additives, like trass from the volcanic Eifel, has been used continuously since the Roman era! [0] https://en.wikipedia.org/wiki/Trass [1] https://en.wikipedia.org/wiki/Hydraulic_lime |