Landscape Architect & Specifier News

AUG 2017

LASN is a photographically oriented, professional journal featuring topics of concern and state-of-the-art projects designed or influenced by registered Landscape Architects.

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The ruins in Caesarea, Israel, built by the Romans 2,000 years ago, include the oldest theater in Israel, an amphitheater, hippodrome and the concrete bath at Caesarea's Promontory Palace (pictured). Some of the marine concrete still remains standing. PHOTO: CREATIVE COMMONS ATTRIBUTION-SHARE ALIKE The Secret of the Durability of Ancient Roman Marine Concrete? Seems to be Volcanic Materials Scientists* are studying why the ancient Roman concrete used for marine structures actually became stronger over time and still remains standing after 2,000 years. By contrast, modern concrete marine structures begin to deteriorate after only several decades. The ancient Romans made concrete by mixing volcanic ash with lime and seawater, then adding chunks of volcanic rock as aggregate. This combination produced a "pozzolanic" reaction, i.e., the silica rich ash, which has no cementing properties, is converted to a calcium silicate, which has good cementitious properties. The longevity of the Roman concrete, say the scientists, "seems to result from the long-term durability of poorly crystalline calcium-aluminum-silicate-hydrate (CASH) binder in the mortar. The mortar of modern concretes partially replaces Portland cement with natural pozzolan to reduce CO2 emissions and produce a resilient CASH binder. The CASH binder in the ancient Roman concrete was produced through reaction of seawater, calcium oxide (CaO) from limestone and zeolitized volcanic ash, mainly from the Campi Flegrei volcano. (Note: Zeolites are crystals created when volcanic rocks and ash react with alkaline groundwater or seawater.) Modern Portland cement concrete also produces a pozzolanic reaction, but of course uses sand instead of volcanic ash, and gravel instead of volcanic rock. The alkali-silica (pozzolanic) reaction in concrete, unfortunately, over time causes cracks to form. In the Roman marine structures, seawater seeping through the concrete dissolved the volcanic crystals and glass and creating aluminous tobermorite and phillipsite crystals in their place. It's these minerals, say the authors, that helped reinforce the concrete and prevent cracks from growing. Although aluminous tobermorite is a rare calcium-silicate hydrate mineral, it is routinely found in the Roman marine concrete, but does not occur in conventional concretes. *American Mineralogist, July 2017. August 2017 69 I n f o r m a t i o n R e q u e s t # 5 0 1

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