Phillipsite and Al-tobermorite mineral cements produced through low-temperature water-rock reactions in Roman marine concrete
Marie D. Jackson1,*, Sean R. Mulcahy2, Heng Chen3, Yao Li4, Qinfei Li5, Piergiulio Cappelletti6, and Hans-Rudolf Wenk7
1Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84112, U.S.A. 2Geology Department, Western Washington University, Bellingham, Washington 98225, U.S.A. 3School of Materials Science and Engineering, Southeast University, Nanjing 211189, People’s Republic of China 4Center for Advancing Materials Performance from the Nanoscale (CAMP-Nano), Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China 5School of Transportation Science and Engineering, Harbin Institute of Technology, Harbin 150090, People’s Republic of China 6Dipartimento di Scienze della Terra, dell’Ambiente e delle Risorse (DiSTAR), Università degli Studi di Napoli Federico II, Naples I-80134, Italy 7Department of Earth and Planetary Sciences, University of California, Berkeley, California 94720, U.S.A.
Volume 102, pages 1435–1450, 2017
Pozzolanic reaction of volcanic ash with hydrated lime is thought to dominate the cementing fabric and durability of 2000-year-old Roman harbor concrete. Pliny the Elder, however, in first century CE emphasized rock-like cementitious processes involving volcanic ash (pulvis) “that as soon as it comes into contact with the waves of the sea and is submerged becomes a single stone mass (fierem unum lapidem), impregnable to the waves and every day stronger” (Naturalis Historia 35.166). Pozzolanic crystallization of Al-tobermorite, a rare, hydrothermal, calcium-silicate-hydrate mineral with cation exchange capabilities, has been previously recognized in relict lime clasts of the concrete. Synchrotron-based X‑ray microdiffraction maps of cementitious microstructures in Baianus Sinus and Portus Neronis submarine breakwaters and a Portus Cosanus subaerial pier now reveal that Al-tobermorite also occurs in the leached perimeters of feldspar fragments, zeolitized pumice vesicles, and in situ phillipsite fabrics in relict pores. Production of alkaline pore fluids through dissolution-precipitation, cation-exchange and/or carbonation reactions with Campi Flegrei ash components, similar to processes in altered trachytic and basaltic tuffs, created multiple pathways to post-pozzolanic phillipsite and Al-tobermorite crystallization at ambient seawater and surface temperatures. Long-term chemical resilience of the concrete evidently relied on water-rock interactions, as Pliny the Elder inferred. Raman spectroscopic analyses of Baianus Sinus Al-tobermorite in diverse microstructural environments indicate a cross-linked structure with Al3+ substitution for Si4+ in Q3 tetrahedral sites, and suggest coupled [Al3++Na+] substitution and potential for cation exchange. The mineral fabrics provide a geoarchaeological prototype for developing cementitious processes through low-temperature rock-fluid interactions, subsequent to an initial phase of reaction with lime that defines the activity of natural pozzolans. These processes have relevance to carbonation reactions in storage reservoirs for CO2 in pyroclastic rocks, production of alkali-activated mineral cements in maritime concretes, and regenerative cementitious resilience in waste encapsulations using natural volcanic pozzolans.