49 J. Válek et al. (eds.), Historic Mortars: Characterisation, Assessment and Repair, RILEM Bookseries 7, DOI 10.1007/978-94-007-4635-0_5, © RILEM 2012 Abstract Roman hydraulic maritime concretes of the central Italian coast have pumiceous volcanic ash, or pulvis Puteolanus, from the Bay of Naples as mortar pozzolan. Petrographic and mineralogical analyses of cement microstructures in relict lime, tuff, and pumice clasts suggest that pozzolanic reaction at high pH produced gel-like calcium-aluminum-silica-hydrate cements. Orthorhombic 11 Å-tobermorite, with unit cell dimensions a = 5.591(1)Å, b = 3.695(1)Å, c = 22.86(1)Å, developed in the residual cores of portlandite clasts and in certain pumiceous clasts, as well. M.D. Jackson (*) Department of Civil and Environmental Engineering, University of California at Berkeley, Berkeley, CA, USA e-mail: mdjackson@berkeley.edu G. Vola CTG Italcementi, Bergamo, Italy e-mail: gabriele.vola@gmail.com D. Všianský Institute of Geological Sciences, Masaryk University, Brno, Czech Republic e-mail: daliborv@centrum.cz J.P. Oleson Department of Classics, University of Victoria, Victoria, Canada e-mail: jpoleson@uvic.ca B.E. Scheetz Larson Transportation Institute, Pennsylvania State University, University Park, PA, USA e-mail: bscheetz@engr.psu.edu C. Brandon Pringle Brandon Architects, London, UK e-mail: Chris-Brandon@pringle-brandon.co.uk R.L. Hohlfelder Department of History, University of Colorado, Bouder, CO, USA e-mail: Robert.Hohlfelder@Colorado.edu Cement Microstructures and Durability in Ancient Roman Seawater Concretes Marie D. Jackson, Gabriele Vola, Dalibor Všianský, John P. Oleson, Barry E. Scheetz, Christopher Brandon, and Robert L. Hohlfelder