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Why is 2000 year old concrete from Ancient Rome harder than modern concrete?

Credits: J. P. Oleson

One of the fascinating mysteries of Ancient Rome is the impressive longevity of some of its concrete structures. Beaten by waves for 2,000 years, these structures still remain standing, as resistant as ever, and much more so than Portland cement. So what was their secret? 

While our modern marine concrete structures erode in just a few decades, 2000 year old pillars and breakwaters still stay standing, and seem to be more resistant than ever. So what magic chemistry is hiding behind this phenomenon? Could there be a sort of building related old wives recipe that has been lost for at least two millennia? Not only is Roman concrete more durable than anything we can make nowadays, but it also gets stronger over time. For Marie Jackson, a geologist in the University of Utah, the miracle ingredient may be none other than sea water, which, by seeping through the concrete, leads to the deposit of minerals that help hold the Roman concrete together.

Modern concrete is generally made from Portland cement, a mix of silicone sand, limestone, clay, chalk and other ingredients. At the end of production, this substance should be inert, as any undesirable chemical reaction could cause cracks in the concrete, leading to erosion and structural breakdown. This is why modern concrete does not have the same longevity as natural rocks. However, this is not how Roman concrete worked.

The Romans in fact made their concrete by mixing volcanic ash with lime and sea water to form a mortar. They then incorporated pieces of volcanic rock to form the aggregate in the concrete. This was a recipe inspired by volcanic ash which was naturally deposited on the outskirts of Naples. The substance formed was used in numerous architectural structures, including the Pantheon steps and Trajan’s Column in Rome. Huge maritime structures also protected the ports and served as anchor points for ships and warehouses.

To compare the two types of concrete, the researchers collected Roman maritime concrete samples from along the Italian coast. They then charted the samples with the help of an electronic microsope, and carried out microdiffraction and microfluorescence analyses in the Lawrence Berkely National Laboratory in California. Using advanced techniques, they were able to identify all of the mineral products used in ancient concrete over the past few centuries.

By charting the minerals present, the succession of various crystals produced and their crystallographic properties, the researchers observed that Al-tobermorite and another material called phillipsite combined with zeolite, and fused together in the cement matrix. Difficult to synthesise in the laboratory, these minerals develop in the concrete due to the sea water that surrounds them, which slowly dissolves the volcanic ash on the inside and creates space for a reinforced interlayer of crystals to form.

So based on the principle that Roman concrete is more resistant and more durable than modern concrete (which also produces significant carbon dioxide emissions), why don’t we simply take up the old method again? “Because the recipe for Roman concrete is lost forever”, explains the geologist. The Romans in fact had the advantage of using a certain type of volcanic rock that is very rare nowadays. “We therefore need to find alternatives with other rocks,” notes Mary Jackson. “But you will never get the same results.”

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