Caementiciae Structurae: Ancient Roman Concrete Structures Fabricated with Reactive Volcanic Rock

Roman architectural and marine concrete structures, caementiciae structurae , have remained intact and functional for two millennia. A mortar fabricated with scoriaceous or pumiceous volcanic tephra aggregate ( harenae fossiciae , pulvis ) binds cobble-sized volcanic or carbonate rock and/or ceramic coarse aggregate ( caementa ). These clasts form a conglomeratic framework that reinforces the concrete. An early pozzolanic cementitious system, in which aggregates react with lime ( calyx ) hydrated with freshwater or seawater, binds the concrete. The components of this system then react with pore fluids to produce post-pozzolanic hydrated silicate phases, mainly strätlingite, Al-tobermorite, and phillipsite crystals, which remodel and toughen the concrete. A heterogeneous permeability structure facilitates these beneficial fluid-concrete interactions. The Ses Llumetes shipwreck provides a window into transport of pulvis pumiceous tephra as aggregate in marine structures. Roman natural scientists recorded accurate empirical observations and hypotheses for these dynamic cementitious processes, which have great relevance to self-sustaining marine concrete infrastructure and alternatives to cement-based concrete. ▪ Advanced analytical methods validate Roman hypotheses for mechanisms of self-sustaining resilience in ancient architectural and marine concrete. ▪ An early pozzolanic system consolidates and strengthens the concrete, mainly through production of C-A-S-H (calcium-aluminum-silicate-hydrate) binding phase in a cementing matrix. ▪ Post-pozzolanic reactions with pore fluids produce silicate mineral cements and remodel the cementing matrix, toughening the concrete. ▪ A heterogeneous permeability structure derived from reactive aggregates, mainly volcanic tephra and cobble-sized caementa , aids these processes. ▪ Roman designs in modern marine concrete could produce beneficial interactions with saltwater, improving structural functionality and resilience.