It has been an enduring mystery that in contrast to the brief lifespan of today’s OPC – Ordinary Portland Cement – the most common of construction materials, Roman mortars and concrete have endured earthquakes, weather and seawater for millenia. Scientists have long known from chemical analysis and from Roman writers such as Vitruvius, Pliny and Cato, that the ingredients of Republican and Imperial period concrete were lime from limestone or marble, pozzolanic (volcanic) materials such as ash, sand and tuff (a rock- like substance made of volcanic ash, tiny pieces of rock and pumice). But the exact mixing process remained unknown. The first step in the making of concrete is heating limestone in a kiln (calcinating) which forms quicklime. Quicklime must be hydrated with water. One way is to first add water to the quicklime before mixing everything together. This is called slaked lime. The other method is to add quicklime to all the other ingredients, including water, all at once. This is called hot mixing. Because slaked lime makes a more homogenous mix with smaller particles of lime, people have tended to accept this as the method employed by the Romans. But, in fact, large clasts (chunks) of lime are found in Roman concrete. This has been put down to incomplete or overburning in the kiln, carbonation before the preparation, incomplete dissolution during the setting, or plain improper mixing. In 2021 - 2022 , a group of scientists and researchers from MIT, DMAT Italy, IMM Switzerland, Wyss Institute and Harvard University, created a study to find out the exact ingredients and mixing method of Roman concrete. 1 They excavated mortar samples from Privernum, a town near Rome which was occupied from the second century BCE. These were analysed using SEM-EDS, a mapping technique which uses an electron microscope and X-ray Spectroscopy to produce detailed colour images which map the elements of the mortar sample. I found these images very beautiful - red (calcium from the limestone), blue (aluminum), green (silicon) and yellow (sulfur).
https://www.science.org/doi/10.1126/sciadv.add1602
Diagrams from Science Advances , vol 9, no.1 above: Optical micrographs showing the conspicuous bright white color of the lime clasts, which can easily be identified from large-area elemental mapping via SEM-EDS below: Lime clasts are morphologically distinct from other calcareous aggregate material (denoted by a yellow asterisk) and exhibit a distinctive particulate microstructure
https://www.science.org/doi/10.1126/sciadv.add1602
1 ‘Hot mixing: Mechanistic insights into the durability of ancient Roman Concrete’. Seymour, Linda M, and Janille Maragh, Paolo Sabatini, Michel di Tommaso, James C Weaver and Admir Masic. Science Advances , vol 9, no.1, Jan 2023.
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on site review 48 :: building materials
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