4.1: Reading/Media

The Roman aqueduct at Pont du Gard, France, built with ancient Roman concrete in the first century CE. The longevity of the Roman aqueduct is in part due to the self-healing property of the Roman concrete, a long-standing mystery resolved in the second/third decades of the 21st century. (Photo by Benh Lieu Song, Wikimedia.)

Self-Healing Concrete for Aqueduct in Imperial Rome, 2,000 BP

Excerpts from "Hot mixing: Mechanistic insights into the durability of ancient Roman concrete" by L. M. Seymour et al. Science Advances 9, eadd1602 (2023). CC BY

Introduction

Excerpt: For many centuries, and throughout the entire ancient Roman Empire, architectural elements, such as walls and foundations, and infrastructure systems, including aqueducts, roads, and bridges, were created from unreinforced concrete. This concrete was typically composed of volcanic tuff and other coarse aggregates (caementa), and bound by a mortar based on lime and pozzolanic materials such as volcanic ash (pulvis), the detailed formulations of which were tailored to their specific intended applications. Whereas aerial lime mortars relied on the uptake of CO₂ from the air to harden, hydraulic mortars combined lime and water with reactive silicates and aluminosilicates (pozzolanic materials) to form cementitious hydrates [e.g., calcium alumino-silicate hydrates (C-A-S-H)].

By developing these hydraulic mortars, the Romans were able to create a stronger, more durable material that allowed them to build larger, more complex-shaped architectural structures for purposes that were not previously possible (Fig. \(\PageIndex{1}\)), including constructions in the sea. The production process for Roman mortar began with the calcining of lime from a source such as limestone, marble, or travertine (all predominantly calcite, CaCO₃) to form quicklime [calcium oxide (CaO)]. This lime-based material, which can be hydrated using water (a process known as slaking) or added directly (a process known as hot mixing), was then mixed with volcanic ash, ceramic fragments (cocciopesto), or other pozzolana, sand, and water to form the hydraulic mortar.

Figure \(\PageIndex{1}\): "The Baths at Caracalla", painting by Lawrence Alma-Tadema, 1899. Bathhouses were among the urban infrastructure built with Roman concrete. (Public domain image.)

Studies focusing on the durability of Roman concrete constructed in marine environments, for example, evidenced the dissolution of lime and vitric tuff clasts at high pH, followed by the precipitation of C-A-S-H–containing reaction rims and, subsequently, the post-setting crystallization of Al-tobermorite and phillipsite in the matrix. In both Augustan and Imperial era architectural concretes, a similar C-A-S-H precipitation and subsequent crystallization and growth of platy strätlingite crystals in the perimeters of scoriae and the cementing matrix were observed. More recently, both Al-tobermorite and strätlingite crystals were found in the mortars of the Augustan period (ca. 30 BCE) tomb of Caecilia Metella. The prolonged reactivity of volcanic aggregates and their potential role in the long-term durability of these materials has thus been an ongoing focus of recent studies on Roman concretes.

In addition to the features described above, aggregate-scale relict lime clasts (Fig. \(\PageIndex{2}\)) , also referred to as remnant lime or lime lumps, are a ubiquitous and conspicuous feature of both architectural and maritime Roman concretes. The presence of these distinctive bright white features has been previously attributed to several scenarios including incomplete or over-burning during the calcining of lime, carbonation before concrete preparation, incomplete dissolution during setting, or insufficient mixing of the mortar.

In maritime structures, these lime clasts can be heterogeneous in composition; may contain calcite, vaterite, brucite, ettringite, hydrocalumite, C-A-S-H, tobermorite, and Al-tobermorite; and can be categorized into one of the following three groups: (i) geologic, (ii) partially dissolved, and (iii) fully dissolved (transformed). Geologic inclusions are calcite-bearing aggregates that did not fully calcine during the production of quicklime. The other two, partially and fully dissolved clasts, have provided insight into the chemical evolution of maritime concretes. For example, partially dissolved clasts show a C-A-S-H–containing reaction rim, gradating toward a calcite-rich core. Fully dissolved clasts, in contrast, exhibit C-A-S-H throughout and, in some cases, the formation of Al-tobermorite. In both partially and fully dissolved clasts, the clast exteriors contain hydrocalumite and ettringite, attributed to hydration with seawater. Although these clasts are well characterized in maritime Roman concretes, less is known regarding the microstructure and chemical composition of relict lime clasts in open-air Roman constructions (structures on land) and the role that they might play in processes associated with the durability of these structures.

To address these yet unresolved questions, we report on the chemical characterization of relict lime clasts found in 2000-year-old Roman concrete samples obtained from the archaeological site of Privernum, Italy. The investigated samples are compositionally consistent with other architectural mortars encountered throughout the Roman Empire, and were sourced from the masonry mortar of the city wall, an open-air structure. We characterized the composition of the lime clasts and their surrounding matrix using large-area scanning electron microscopy and energy dispersive x-ray spectroscopy (SEM-EDS), powder x-ray diffraction (XRD), and confocal Raman imaging. The results of these analyses provide compelling evidence for hot mixing of Roman mortar using quicklime instead of, or in addition to, slaked lime. From these findings, we propose that persistent, aggregate-scale, high surface area lime clasts that result from this process could serve as a source of reactive calcium for long-term pore and crack filling and therefore provide a chemically dominated intrinsic self-healing mechanism.

Figure \(\PageIndex{2}\): The test samples came from the archaeological site of Privernum, near Rome, Italy (A), and shown as a photogrammetry-based three-dimensional reconstruction (B). The architectural mortar samples were collected from the bordering concrete city wall (C). Large-area EDS mapping of a fracture surface (D) reveals the calcium-rich (red), aluminum-rich (blue), silicon-rich (green), and sulfur-rich (yellow) regions of the mortar. Further imaging of polished cross-sections (E) shows aggregate-scale relict lime clasts within the mortar (the large red features denoted by asterisks). The colored arrows in (E) denote the pore-bordering rings visible in the EDS data that are rich in calcium (red) or sulfur (yellow), which are enlarged at right to show additional detail. Photo credits (B and C): Roberto Scalesse and Gianfranco Quaranta, Associazione AREA3, Italy. (From Seymour et al. 2023).

Self-healing mechanism in Roman concrete

Excepts: Previous evidence suggests that in ancient Roman cementitious structures, the relict lime clasts can react over time with other mortar components, forming both amorphous (C-A-S-H) and crystalline (e.g., Al-tobermorite and strätlingite) phases. These maturation pathways, however, are not the only ones observed in Roman cementitious materials and, as shown here, can also follow different trajectories depending on the local environmental conditions. While the polished cross-sections of the ancient Roman concrete samples described in the present study do show clear inclusion of silicon and aluminum in the lime clast, suggesting that some conversion to C-A-S-H and/or its crystalline homologues occurred, calcium carbonate, predominantly as calcite, remains within their cores.

Whereas previous evidence supports the formation of Al-tobermorite minerals in lime clasts of maritime structures due to the heat of the pozzolanic reaction, the results presented here suggest that, by contrast, when quicklime is introduced via hot mixing in terrestrial structures, the reaction is limited to the outer rim of the lime clast, encapsulating calcium-rich core structures within mortar matrix. This particular configuration can be observed in the comparison of Roman and modern samples (Fig. \(\PageIndex{2}\)) , both showing the same type of lime clast inclusions, hydration-driven cracking, and distinctive reaction rims. The initially matrix-incorporated clasts would then undergo a slow transformation into calcium-rich, highly porous phases of various polymorphs of calcium carbonate.

Inspired by these discoveries, it is thus likely that the high abundance of aggregate-scale lime clasts in ancient Roman mortars could thus serve as a source of calcium for post-pozzolanic processes in a pore- and crack-filling “self-healing” mechanism that combats the progressive degradation of these cementitious materials. Over time, as cracks and pores form, the intrusion of water causes the dissolution of calcium-based phases in the relict lime clasts carrying them into the pore network. As the calcium-rich fluids leach into the cracks or the connected pore network, many pathways exist for potential post-pozzolanic reactions. For example, excess pozzolanic material, such as volcanic ash that did not react during the initial setting and curing, or aggregates of volcanic origin, can now dissolve and react with Ca-rich fluids originating from the lime clasts to form C-A-S-H phases, thus reinforcing the interfacial zones between volcanic aggregates/ash and the binding matrix. This strengthening is associated with the consolidation of the interfacial zones and increased mechanical performance of the C-A-S-H compared to its precursors. Another possible pathway is the recrystallization of CaCO₃ phases within the pore/crack space. This pathway, in which secondary calcium carbonate is precipitated through a mechanism similar to the one occurring in the formation of calthemites, relies on the wetting and drying cycles experienced through normal weathering conditions. These processes have been observed previously in both modern and ancient carbonate-based mortars. In contrast to these previous studies, however, we suggest here that the hot mixing–transformed lime clasts act as a calcium source for these processes and, furthermore, the precipitation of calcium carbonate as a crack-filling mechanism is already a known pathway for autogenous healing in other ancient lime-containing mortars. In the present study, we demonstrate the calcium enrichment of matrix phases adjacent to the lime clasts, supporting the hypothesis that the lime clasts are a source of calcium for leaching and recrystallization within the pore space of the mortars (Fig. \(\PageIndex{2}\)) . Microcrack filling by calcite has been recently observed in ancient Roman mortars from the tomb of Caecilia Metella, and the self-healing tests carried out on our modern samples described in the present study further support this hypothesis.

Key Geologic Concepts:

Carbonate; calcite; mineral growth in water; volcanic ash.

Supplemental Materials:

Article: "Phillipsite and Al-tobermorite mineral cements produced through low-temperature water-rock reactions in Roman marine concrete" by Marie D. Jackson, et al. American Mineralogist 102, 1435 (2017). CC BY

Podcast: "The significance of the ancient Roman concrete for a decarbonizing world", University of Utah.

Article: "Scientists may have found magic ingredient behind ancient Rome’s self-healing concrete", Science.

Article: "Reinventing Concrete, the Ancient Roman Way", New York Times, October 19, 2024.