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Roman concrete has kept harbours, domes and aqueducts standing for 2,000 years, and small white lumps of lime inside it are one reason. When water seeps into a crack, calcium from these lumps can form new crystals that seal it.

Science Advances, 2023

EngineeringArchitectureHistoryScience

The story behind the card

For a long time these white lumps, called lime clasts, were taken as a sign of careless mixing. A 2023 study in Science Advances by a team at MIT led by Admir Masic proposed another explanation. The Romans, it argued, added quicklime, a highly reactive form of lime, directly to the mix, where it heats up as it reacts; this method, called hot mixing, left the lumps behind. To test the idea, the team made concrete the same way, cracked it and ran water through the cracks. Within about two weeks the cracks had sealed, while in a sample made without quicklime they did not. Volcanic ash from the Bay of Naples also added strength. A 2017 study of Roman harbour walls, led by the geologist Marie Jackson, found a different process: seawater seeping through them reacts with the volcanic ash and grows rare minerals that reinforce the concrete.

Türkçe oku
Roma betonuyla yapılmış limanlar, kubbeler ve su kemerleri iki bin yıldır ayakta. Betonun içindeki küçük beyaz kireç topakları bunun nedenlerinden biri: Bir çatlağa su sızdığında bu topaklardaki kalsiyum yeni kristaller oluşturup çatlağı kapatabiliyor.
Science Advances, 2023

Bu beyaz topaklar uzun süre özensiz karıştırmanın izi sayıldı. 2023'te Science Advances dergisinde yayımlanan ve MIT'den Admir Masic'in ekibinin yaptığı bir çalışma başka bir açıklama getirdi. Çalışmaya göre Romalılar, tepkimeye çok yatkın bir kireç türü olan sönmemiş kireci doğrudan karışıma katıyordu; kireç tepkimeye girerken karışımı ısıtıyordu. “Sıcak karıştırma” denen bu yöntem topakları geride bırakıyordu. Ekip fikri sınamak için aynı yöntemle beton yaptı, betonu çatlattı ve çatlaklardan su geçirdi. Yaklaşık iki hafta içinde çatlaklar kapandı; sönmemiş kireç katılmadan yapılan örnekte ise kapanmadı. Karışıma katılan Napoli Körfezi'nin volkanik külü de betonu güçlendiriyordu. Jeolog Marie Jackson'ın yönettiği bir ekip ise 2017'de Roma liman duvarlarında başka bir süreç buldu: Duvarlara sızan deniz suyu volkanik külle tepkimeye giriyor ve betonu güçlendiren nadir mineraller oluşturuyor.

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