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 strät-lingite 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 clastsupported conglomeratic fabric of the concrete presents further resistance to fracture propagation at the structural scale.Roman concrete | volcanic ash mortar | fracture toughness | interfacial zone | strätlingite

Time resolved alkali silicate decondensation by sodium hydroxide solution

“…The chemistry of disolved silica has been widely investigated for decades because of its relevance to various fields such as chemistry, geology, biology, and manifolds fields of application including construction, gas separation, ceramics and hydrogels [1][2][3][4][5]. For instance, to produce accessible and rock-like materials, mankind started making calcium silicate cement from limestone and pozzolan, and extracting aluminosilicate clays.…”

Recent studies of cements and concretes by synchrotron radiation crystallographic and cognate methods

“…For instance, this technique has been very recently applied to study pyroclastic aggregate concrete of Trajan's Markets (1900 years old) as well as their reproductions (experimental archaeology): hydrated lime-volcanic ash mortar that binds decimeter-sized tuff and brick aggregates. [150] The mortar reproduction gains fracture toughness over 180 d through progressive coalescence of C-A-S-H gel binder and crystallization of strätlingite and katoite at ≥ 90 days, after pozzolanic consumption of hydrated lime was complete. Key experimental details were, beam energy: 10 keV, beam size: 8 × 2 µm, and sample thickness: 300 µm.…”

Continuous Synthesis of Nanominerals in Supercritical Water

“…Also, tobermorite is understood to be ak ey ingredient in the longevity of ancient undersea Roman concrete. [61,62] Nowadays, tobermorite has been proposed to be used as seedinga ddition for cement composites and as as tabilizing agent for the trap of impurities. [63][64][65][66][67] Three structuralv arieties are distinguished, depending on the amount of water in the interlayers pace:1 4 tobermorite, 11 tobermorite (the most common structure under ambient conditions) or 9 tobermorite.…”