Phillipsite and Al-tobermorite mineral cements produced through low-temperature water-rock reactions in Roman...

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Phillipsite and Al-tobermorite mineral cements produced through low-temperature water-rock reactions in Roman...

Messaggio da andrea oppicelli » dom 25 feb, 2018 16:50

Phillipsite and Al-tobermorite mineral cements produced through low-temperature water-rock reactions in Roman marine concrete
Marie D. Jackson Sean R. Mulcahy Heng Chen Yao LiQinfei Li Piergiulio Cappelletti Hans-Rudolf Wenk
American Mineralogist (2017) 102 (7): 1435-1450.
DOI: https://doi.org/10.2138/am-2017-5993CCBY
Published: July 01, 2017 Article history

https://gsw.silverchair-cdn.com/gsw/Con ... A4LVPAVW3Q

Abstract
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 rst 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 ( erem 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. Synchro- tron-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 uids 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. Ra- man 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 geo- archaeological prototype for developing cementitious processes through low-temperature rock- uid interactions, subsequent to an initial phase of reaction with lime that de nes 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 regenera- tive cementitious resilience in waste encapsulations using natural volcanic pozzolans.

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