Bet most of you never really thought about this.  But since Portland cement (i.e., concrete) is responsible for about 8% of our CO₂ emissions- and often needs repair, it’s been on my horizon, even if it’s not one of my primary interests.  Especially since we produce about 12 billion tons of concrete (which yields 12 metric tons of carbon dioxide) a year.  And, while we consider concrete to be a solid mass, it’s really pretty porous (consider a bunch of small balls packed together, with voids between the balls, akin to a dry sponge.  And, when rain or snow slips among the voids0 and then freezes- kerplow, the concrete splits apart.

About a dozen years ago, I reported to y’all about self-repairing concrete. Developed at the University of Newcastle, their concept was introduced at an MIT competition.   They called their concoction “BacillaFilla:, bacterial spores that germinate – some produce calcium carbonate crystals, others polysaccharides (bacterial glue), and others produce filaments.  That basically lets the concoction fill and repair the concrete.  Except, it doesn’t seem that this proceeded to commercial availability.

But…

Why is it that these Roman aqueducts, coliseum, and arenas are still around.  What did these folks know that we don’t.  (A lot, obviously).

Six professors had the same question.  Drs. LM Seymour, J. Maragh, and A Masic (MIT), Dr. P. Sabatini (DMAT, Italy), L Di Tommaso (Institute of Mechanics and Materials, Switzerland), and J. C. Weaver (Harvard) published their analysis in Science Advances.           The article, Hot mixing: Mechanistic insights into the durability of ancient Roman concrete, in which they found that hot mixing and switching quick lime (calcium oxide)for slaked lime, the Romans were able to produce a concrete for the millennia.  (Remember- these Roman structures have proven to be durable in seismic zones, a variety of climates, and with direct contact with seawater.)

It turns out the Romans substituted hydraulic mortars (instead of aerial lime mortars), they produced stronger materials, that also afforded greater versatility with architectural structures.  The Romans produced quicklime via hot mixing (instead of slaking, which means it would have been added to water)- and then it was mixed with volcanic ash or ceramic (known as pozzolana).

We know that humidity is a prime factor in the setting of concrete.  When the humidity is below 20%, the calcium oxide covers to amorphous calcium carbonate.  As it rises to 40%, conversion to aragonite, vaterite, and calcite occurs.  But once the humidity exceeds 60%, only calcite is observed.

Nevertheless, the big issue is the Roman production methods are still ambiguous.  But, it is clear that the hydraulic mortar rendered the produced concrete capable of self repair.

I am guessing its time to revert to this ancient process to keep our concrete in usable conditions.

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