New findings by a team of scientists led by MIT and Harvard shed more light on exactly how a lost technology may be finding its way back into the modern world. And the secret turns out to be quicklime and hot mixing.
Discovery of the hot lime
One of the elements of Roman concrete that until recently was thought to be a secondary effect of hand mixing or an error in execution are the so-called limestone clasts. Further study of extant Roman buildings, as well as a re-reading of notes by Vitruvius and Pliny about the requirement that the concrete be pure white, has scientists looking more deeply into these formations. They sampled the concrete from the archeological site in Priverno, Italy, and analyzed it by elemental mapping, which showed the presence of complex chemical compounds with a predominant content of calcium, silicon and aluminum, as well as evidence of crack-filling by secondary mineral phases, t .is on the ability of Roman concrete to recover.
Historically, Roman concrete was always thought to have been made by combining lime with water in a process known as slaking, but scientists say this process should not have led to the formation of the limestone clasts observed. After spectroscopic analysis, they also noticed traces of formations that are only possible at high temperatures. Thus, one of the leaders of the research – Admir Masić, came to the idea that it is possible that the slag was used in its more reactive form of quicklime, and that the process was the so-called hot mixing.
Although not ancient, current roads and paths could benefit from old technology Photographer: Anton Chepilski
During the hot mixing, the scientists explain in their report, the limestone clasts form their characteristic friable nanostructure, creating an easily disintegrating and reactive source of calcium, which also gives the mixture its crucial “self-healing” ability. Due to the placement of the clasts throughout the mixture, any cracks that potentially formed would pass through them as well. Thus, once water from external sources such as rain or moisture is added, the material can react with it and create a calcium-rich solution that will then crystallize as calcium carbonate and fill the cracks or react with the pozzolana to further harden the composite material. This reaction naturally and independently stops the cracks before they can widen and spread.
