Multi-scale imaging, strength and permeability measurements: Understanding the durability of Roman marine concrete

Abstract

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 low permeability of concrete samples was linked to mortar-dominated microstructures showing no debonding with the aggregates. 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 polymerized alkali-alumino-silicate. 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.