Why modern mortar crumbles, but Roman concrete lasts millennia

Authored by sciencemag.org and submitted by Project_Progress
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Why modern mortar crumbles, but Roman concrete lasts millennia

Modern concrete—used in everything from roads to buildings to bridges—can break down in as few as 50 years. But more than a thousand years after the western Roman Empire crumbled to dust, its concrete structures are still standing. Now, scientists have finally figured out why: a special ingredient that makes the cement grow stronger—not weaker—over time. Scientists began their search with an ancient recipe for mortar, laid down by Roman engineer Marcus Vitruvius in 30 B.C.E. It called for a concoction of volcanic ash, lime, and seawater, mixed together with volcanic rocks and spread into wooden molds that were then immersed in more sea water. History contains many references to the durability of Roman concrete, including this cryptic note written in 79 B.C.E., describing concrete exposed to seawater as: “a single stone mass, impregnable to the waves and everyday stronger.” What did it mean? To find out, the researchers studied drilled cores of a Roman harbor from Pozzuoli Bay near Naples, Italy. When they analyzed it, they found that the seawater had dissolved components of the volcanic ash, allowing new binding minerals to grow. Within a decade, a very rare hydrothermal mineral called aluminum tobermorite (Al-tobermorite) had formed in the concrete. Al-tobermorite, long known to give Roman concrete its strength, can be made in the lab, but it’s very difficult to incorporate it in concrete. But the researchers found that when seawater percolates through a cement matrix, it reacts with volcanic ash and crystals to form Al-tobermorite and a porous mineral called phillipsite, they write today in American Mineralogist . So will you be seeing stronger piers and breakwaters anytime soon? Because both minerals take centuries to strengthen concrete, modern scientists are still working on recreating a modern version of Roman cement.

*Update, 5 July, 12:30 p.m.: A previous version of this story said the Roman Empire fell "thousands of years ago." It has been updated to say “the western Roman Empire" fell "more than a thousand years ago.”

Veritas3333 on December 3rd, 2020 at 19:37 UTC »

They also massively overbuilt things with concrete. They didn't have rebar, so everything was just solid concrete with no internal reinforcement. Since there wasn't internal reinforcement, they had to make things much thicker.

MrMathemagician on December 3rd, 2020 at 19:17 UTC »

Do we have any indication if they knew this was better or if they just were like, “We need ash for our cement; hey, the volcano makes a ton.”

slugbabyrat on December 3rd, 2020 at 19:07 UTC »

This reads as if scientists have been trying to transform into aluminium tobermorite for decades, and I love that.