Mechanical resilience and cementitious processes in Imperial Roman architectural mortar

Abstract

The pyroclastic aggregate concrete of Trajan's Markets (110 CE), now Museo Fori Imperiali in Rome, has absorbed energy from seismic ground shaking and long-term foundation settlement for nearly two millenia while remaining largely intact at the structural scale. The scientific basis of this exceptional service record is explored through computed tomography of fracture surfaces and synchroton X-ray microdiffraction analyses of a reproduction of the standardized hydrated lime-volcanic ash mortar that binds decimeter-sized tuff and brick aggregate in the conglomeratic concrete. The mortar reproduction gains fracture toughness over 180 d through progressive coalescence of calcium-aluminum-silicate-hydrate (C-A-S-H) cementing binder with Ca/(Si+Al) ≈ 0.8-0.9 and crystallization of strätlingite and siliceous hydrogarnet (katoite) at ≥ 90 d, after pozzolanic consumption of hydrated lime was complete. Platey strätlingite crystals toughen interfacial zones along scoria perimeters and impede macroscale propagation of crack segments. In the 1,900-y-old mortar, C-A-S-H has low Ca/(Si+Al) ≈ 0.45-0.75. Dense clusters of 2- to 30-µm strätlingite plates further reinforce interfacial zones, the weakest link of modern cement-based concrete, and the cementitious matrix. These crystals formed during long-term autogeneous reaction of dissolved calcite from lime and the alkali-rich scoriae groundmass, clay mineral (halloysite), and zeolite (phillipsite and chabazite) surface textures from the Pozzolane Rosse pyroclastic flow, erupted from the nearby Alban Hills volcano. The clast-supported conglomeratic fabric of the concrete presents further resistance to fracture propagation at the structural scale.

Keywords: Roman concrete; fracture toughness; interfacial zone; strätlingite; volcanic ash mortar.

Fig. 1.

Markets of Trajan concretes. (A) Great Hall, vaulted ceiling and brick-faced concrete walls; reprinted with permission from Archives, Museo Fori Imperiali. (B) Drill core with Pozzolane Rosse volcanic ash (harena fossicia) mortar and conglomeratic aggregate (caementa). (C) Fractures in vaulted ceiling, Grande Emiciclo: 1, crack follows caementa perimeter; 2, crack traverses caementa. Wall concrete contains ∼88 vol % pyroclastic rock: 45–55% tuff (and brick) as caementa, ∼38% volcanic ash pozzolan, and ∼12% lime paste, with 3:1 ash:lime volumetric ratio (de Architectura 2.5.1) in the mortar (18).

Fig. 2.

Cementitious components, SEM-BSE images, and compositional analyses. (A) Cementitious matrix at 28 d. (B) Cementitious matrix at 180 d. (C) Great Hall mortar, interfacial zone along scoria perimeter. (D) SEM-EDS analyses of A and B, Ca/(Si+Al) vs. Al/(Si+Al) as atomic percent ratios of total Ca+Si+Al+Na⁺K+Mg+Fe+Ti (Table S4). (E) bulk composition of cementitious matrix (<74-µm powder) as weight percent oxides (Table S5).

Fig. 3.

X-ray microdiffraction analysis (Debye diffraction rings, Table S3) and SEM-secondary electron images. (A and B) cementitious matrix and scoria perimeter at 180 d. (C and D) Trajan’s Market foundation, relict scoria. (E and F) Trajan’s Market foundation, cementitious matrix. Debye ring traces of higher-intensity d-spacings of coarse-grained crystals (C and E). Short dashes, katoite; dots and dashes, strätlingite; long dashes, åkermanite.

Fig. 4.

Fracture testing of the wall mortar reproduction. (A and B) Tomographic studies of 28- and 180-d specimens. (C and D) Axial zone tomographic slices at 28 and 180 d. a, b, and c are crack segments. (E) experimental (solid) and computational (dashed) results of load-CMOD curves (18): region (1) before the curve reaches its peak (2) signals growth of microcracks, a smooth postpeak descent (3) indicates stable growth of a critical macrocrack and weakening of the specimen (4), similar to fracture of quasi-brittle materials (30) in present-day concrete (31, 32). Reprinted from ref. . (F) Fracture energy (G_f) measured from mapped fractures compared with G_F previously determined through FEA (18) (Table S2).