Construction Physics

There's a new paper out exploring some of the chemical mechanisms at work in Roman concrete. As per usual, it’s triggered a round of enthusiastic discussion of Roman concrete, and how its ability to last for millennia puts modern concrete (which often fails after a few decades) to shame.

These sorts of comparisons generally miss the important differences between modern and Roman concrete that are key for understanding why their lifespans are so different.

The paper investigated chemical structures that are frequently found within Roman concrete called lime clasts - essentially, small lumps of lime and other chemicals. Lime clasts have been studied previously in Roman maritime structures such as harbors, but this is (apparently) one of the first studies that looked at lime clasts in Roman concrete structures on land.

The chemical analysis of the clasts is beyond my ability to evaluate, but the researchers determined that the lime clasts might be the result of lime added directly to the concrete mix as quicklime (CaO), rather than lime that had been mixed with water (slaked lime, Ca(OH)2). ( The MIT article suggests that the idea of Romans using quicklime rather than slaked lime in their concrete was novel, but the paper itself references several papers suggesting this mechanism, and I was able to find others as well.)

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