  Roman-era concrete is the iconic embodiment of long-term physicochemical
resilience. We investigated the basis of this behavior across scales of
observations by coupling time-lapse (4-D) tomographic imaging of macroscopic
mechanical stressing with structural microscopy and chemical spectroscopy on
Roman marine concrete (RMC) from ancient harbors in Italy and Israel.
Stress-strain measurements revealed that RMC creeps and exhibits a ductile
deformation mode. The permeability of specimens from Italy were found to be low
due to increased matrix-aggregate bonding. Structural and chemical imaging
shows the presence of well-developed sulfur-rich, fibrous minerals that are
intertwined and embedded in a crossbred matrix having the chemical traits of
both a calcium-aluminum-silicate-hydrate and a geopolymer. This latter likely
reflects the ultra-alkaline volcanic nature of the primary source materials. We
hypothesize that the fine interweave of sulfur-rich fibers within this
crossbred matrix enhances aggregate bonding, which altogether contributes to
the durability of RMC.
