The Mystery Finally Solved: Why Has Roman Concrete Been So Durable?

Image by Ben­jaminec, via Wiki­me­dia Com­mons

Rome may not have been built in a day, but it was built to last — or at least its con­crete was, giv­en that the pieces of the Roman Empire that have stood to our time, in one form or anoth­er, tend to have been built with it. That mate­r­i­al has proven not just durable but endur­ing­ly fas­ci­nat­ing, hold­ing a great deal of not just his­tor­i­cal inter­est but tech­ni­cal inter­est as well. For ancient Roman con­crete appears to out­last its much more tech­ni­cal­ly advanced mod­ern descen­dants, and the com­plex ques­tion of why is one we’ve fea­tured more than once here on Open Cul­ture. Just this year, researchers at MIT, Har­vard, and lab­o­ra­to­ries in Italy and Switzer­land have found what seems to be the final piece of the puz­zle.

“For many years, researchers have assumed that the key to the ancient concrete’s dura­bil­i­ty was based on one ingre­di­ent: poz­zolan­ic mate­r­i­al such as vol­canic ash from the area of Poz­zuoli, on the Bay of Naples,” writes MIT News’ David L. Chan­dler. “Under clos­er exam­i­na­tion, these ancient sam­ples also con­tain small, dis­tinc­tive, mil­lime­ter-scale bright white min­er­al fea­tures.”

Pre­vi­ous­ly assumed to be noth­ing but imper­fec­tions in the process or the mate­ri­als, these “lime clasts,” in light of this most recent research, con­sti­tute evi­dence of “hot mix­ing,” which involves heat­ing to a high tem­per­a­ture ingre­di­ents includ­ing quick­lime (or cal­ci­um oxide), a pur­er and more reac­tive form of lime.

Under­go­ing hot mix­ing, “the lime clasts devel­op a char­ac­ter­is­ti­cal­ly brit­tle nanopar­tic­u­late archi­tec­ture, cre­at­ing an eas­i­ly frac­tured and reac­tive cal­ci­um source” that “could pro­vide a crit­i­cal self-heal­ing func­tion­al­i­ty.” In prac­tice, this means that “as soon as tiny cracks start to form with­in the con­crete, they can pref­er­en­tial­ly trav­el through the high-sur­face-area lime clasts. This mate­r­i­al can then react with water, cre­at­ing a cal­ci­um-sat­u­rat­ed solu­tion, which can recrys­tal­lize as cal­ci­um car­bon­ate and quick­ly fill the crack, or react with poz­zolan­ic mate­ri­als to fur­ther strength­en the com­pos­ite mate­r­i­al.” Here we have a con­vinc­ing expla­na­tion of the reac­tions that, in ancient Roman con­crete, “auto­mat­i­cal­ly heal the cracks before they spread.”

No such self-heal­ing hap­pens in mod­ern con­crete, the pro­duc­tion of which has not involved quick­lime for a very long time indeed — but per­haps it could once more. Dur­ing their research process, writes Dezeen’s Rima Sabi­na Aouf, the team “pro­duced sam­ples of hot-mixed con­crete using both ancient and mod­ern for­mu­la­tions, cracked them, and ran water through the cracks. With­in two weeks, the cracks had healed and water could no longer flow through, while iden­ti­cal con­crete blocks made with­out quick­lime nev­er healed.” Such find­ings “could help increase the lifes­pan of mod­ern con­crete and there­fore mit­i­gate the noto­ri­ous envi­ron­men­tal impact of the mate­r­i­al,” and the researchers “are now work­ing to com­mer­cial­ize their more durable con­crete for­mu­la.” Even in the twen­ty-first cen­tu­ry, the build­ing indus­try could well ben­e­fit by doing as the Romans did.

via MIT News

Relat­ed Con­tent:

How Did the Romans Make Con­crete That Lasts Longer Than Mod­ern Con­crete? The Mys­tery Final­ly Solved

How to Make Roman Con­crete, One of Human Civilization’s Longest-Last­ing Build­ing Mate­ri­als

How Did Roman Aque­ducts Work?: The Most Impres­sive Achieve­ment of Ancient Rome’s Infra­struc­ture, Explained

How the Ancient Romans Built Their Roads, the Life­lines of Their Vast Empire

The Beau­ty & Inge­nu­ity of the Pan­theon, Ancient Rome’s Best-Pre­served Mon­u­ment: An Intro­duc­tion

Roman Archi­tec­ture: A Free Course from Yale

Based in Seoul, Col­in Marshall writes and broad­casts on cities, lan­guage, and cul­ture. His projects include the Sub­stack newslet­ter Books on Cities, the book The State­less City: a Walk through 21st-Cen­tu­ry Los Ange­les and the video series The City in Cin­e­ma. Fol­low him on Twit­ter at @colinmarshall or on Face­book.

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