Built to Last

SLOW Magazine - - Contents - Text: Gary Muir Images © istockphoto.com

Ro­man con­crete is a mar­vel of the an­cient world. Un­for­tu­nately, the recipe for it was lost in his­tory dur­ing the in­va­sion of Rome. The Pan­theon is still the world’s largest un­re­in­forced con­crete dome in the world – some 2,000 years af­ter it was built. A re­cent dis­cov­ery has helped sci­en­tists un­der­stand more about the meth­ods and in­gre­di­ents be­hind this al­most in­de­struc­tible ma­te­rial.

The Ro­man au­thor Pliny the El­der (23 AD – 79 AD) wrote in his Nat­u­ralis His­to­ria that con­crete struc­tures in har­bours, which were ex­posed to the con­stant bar­rage of salt­wa­ter, be­come “a sin­gle stone mass, im­preg­nable to the waves and ev­ery day stronger”. Pliny’s ob­ser­va­tion stands true to this day: While most mod­ern marine con­crete struc­tures crum­ble within a few decades, cen­turiesold Ro­man piers and break­wa­ters re­main stand­ing, and are, in fact, stronger today than when they were first built cen­turies ago.

Ro­mans used their highly ro­bust con­crete in sev­eral ar­chi­tec­tural builds, in­clud­ing the Pan­theon and Tra­jan’s Mar­ket in Rome. It was used to cre­ate mas­sive marine struc­tures which pro­tected har­bours from the open sea, also act­ing as an­chor­ages for ships and ware­houses.

Marie Jack­son, a ge­ol­o­gist at the Uni­ver­sity of Utah in the United States, has spent years study­ing the min­er­als and

mi­croscale struc­tures of Ro­man con­crete as she would a sam­ple of vol­canic rock. Jack­son first took an in­ter­est in Ro­man con­crete dur­ing a sab­bat­i­cal year in the Ital­ian city. She stud­ied tuffs (nat­u­rally ce­mented vol­canic ash de­posits) and was fas­ci­nated by the role such ma­te­ri­als played in pro­duc­ing the re­mark­able and durable Ro­man con­crete.

To­gether with her col­leagues, Jack­son be­gan study­ing what it was that made ar­chi­tec­tural con­crete in Rome so re­silient. Jack­son and her team soon dis­cov­ered that sea­wa­ter fil­ter­ing through the con­crete re­sulted in the growth of in­ter­lock­ing min­er­als, and it was de­cided that th­ese were re­spon­si­ble for the in­creased co­he­sion of the ma­te­rial. Once they’d fig­ured this out, it re­mained to be un­cov­ered ex­actly what the Ro­mans put into their ce­ment in the first place.

Ac­cord­ing to his­to­ri­ans, the Ro­mans made con­crete by mix­ing vol­canic ash with lime and sea­wa­ter to make a mor­tar, be­fore adding chunks of vol­canic rock – the “aggregate” in the con­crete. The com­bi­na­tion of ash, wa­ter, and quick­lime pro­duces some­thing called a poz­zolanic re­ac­tion – named af­ter the city of Poz­zuoli in the Bay of Naples. It is thought that the Ro­mans may have ob­tained the idea for this mix­ture from tuff, which was quite com­monly found in the area. To put the Ro­mans’ recipe into per­spec­tive, mod­ern Port­land ce­ment con­crete also uses rock aggregate but with one im­por­tant dif­fer­ence: The sand and gravel par­ti­cles are in­tended to be in­ert, as any re­ac­tion with the ce­ment paste could form gels that ex­pand and crack the con­crete.

A project con­ducted by Jack­son be­tween 2002 and 2009 called the ROMACONS project, un­cov­ered a re­mark­ably rare min­eral, alu­mi­nous to­ber­morite (Al-to­ber­morite), in the marine mor­tar. Jack­son was sur­prised by the find see­ing as this min­eral was par­tic­u­larly dif­fi­cult to make. She noted that syn­the­sis­ing it in a lab­o­ra­tory re­quires high tem­per­a­tures and yields only small quan­ti­ties. Jack­son’s study of the drill cores be­gan to get quite tech­ni­cal, the ex­am­i­na­tions re­veal­ing that Al-to­ber­morite and a re­lated ze­o­lite min­eral, phillip­site, formed in pumice par­ti­cles and pores in the ce­ment­ing ma­trix. How­ever, it was de­cided that some­thing else had caused the min­er­als to con­tinue grow­ing, and for so long af­ter the con­crete had hard­ened.

Af­ter fur­ther ex­ten­sive study, Jack­son and her team con­cluded that when sea­wa­ter per­me­ated the con­crete, it dis­solved com­po­nents of the vol­canic ash, al­low­ing new min­er­als – par­tic­u­larly Al-to­ber­morite and phillip­site – to grow from the highly al­ka­line leached flu­ids, and in so do­ing, in­creas­ing the con­crete’s re­sis­tance and strength. Jack­son said of this find­ing: “We’re look­ing at a sys­tem that’s con­trary to ev­ery­thing one would not want in ce­ment-based con­crete. We’re look­ing at a sys­tem that thrives in open chem­i­cal ex­change with sea­wa­ter.”

With the se­cret to this ma­te­rial’s su­per­strength now re­vealed, the ques­tion arose as to why this con­crete is not used more of­ten in mod­ern times. The an­swer is sim­ple: be­cause the recipe has been com­pletely lost.

De­spite Jack­son’s ex­ten­sive scru­tiny of an­cient Ro­man texts, the pre­cise meth­ods for mix­ing the marine mor­tar were nowhere to be found. How­ever, even if the recipe was lo­cated, the type of rock the Ro­mans worked with is un­com­mon in most of the world, and substitutions would thus have to be made. Jack­son is de­ter­mined to find a work­able so­lu­tion, how­ever, and is cur­rently col­lab­o­rat­ing with ge­o­log­i­cal en­gi­neer Tom Adams to de­velop a re­place­ment recipe, al­beit one that uses ma­te­ri­als from the United States and sea­wa­ter from the Berke­ley Ma­rina in Cal­i­for­nia.

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