I hope all of you had a Merry Christmas and a Happy New Year, and 2023 is shaping up well for you. They say the end of the year is a time to look back, so why not look really back? Quite some time ago, I visited Rome, and I have always been fascinated by the Roman civilization, so why not start this year by looking that far back?
Perhaps one of the more rather remarkable buildings is the Pantheon, which has the world’s largest unreinforced concrete dome. That was built under the direction of Marcus Vipsanius Agrippa, the “get-things-done” man for Augustus. No reinforcement, and it lasted that long. Take a look at modern concrete and as often as not you will find it cracks and breaks up. Concrete is a mix of aggregate (stones and sand) that provides the bulk, and a cement that binds the aggregate together. We use Portland cement, which is made by heating limestone and clay (usually with some other material but the other material is not important) in a kiln up to about 1450 degrees Centigrade. The product actually depends to some extent on what the clay is, but the main products are bellite (Ca2SiO4) and alite (Ca3SiO5). If the clays contain aluminium, which most clays do, various calcium aluminosilicates are formed. Most standard cement is mainly calcium silicate to which a little gypsum is added at the end, which makes the end surface smoother.
Exactly what happens during setting is unknown. The first thing to note is that stone does not have a smooth surface at close to the molecular level, and further, stones are silicates, in which the polymer structure is perforce terminated at the surface. That would mean there are incomplete bonds. An element like carbon would fix this problem by forming double bonds but silicon cannot do that so these “awkward” surface molecules react with water to form hydroxides. What I think happens is the water in the mix hydrolyses the calcium silicate and forms silica with surface hydroxyls, and these eliminate with hydroxyls on the stone, with the calcium hydroxide also taking part, in effect forming microscopic junctions between it and stone. All of this is slow, particularly when polymeric solids cannot move easily. So to make a good concrete, besides getting the correct mix you have to let it cure for quite some time before it is at its best.
So what did the Romans do? They could not make cement by heating clay and lime up to that temperature easily, but there were sources where it was done for them: the silicate around volcanoes like Vesuvius. The Roman architect and engineer Vitruvius used a hot mix of quicklime (calcium oxide) that was hydrated and mixed with volcanic tephra. Interestingly, this will also introduce some magnesiosilicates, which are themselves cements, but magnesium may fit better than calcium onto basaltic material. For aggregate Vitruvius used fist-sized pieces of rock, including “squared red stone or brick or lava laid down in courses”. In short, Vitruvius was selecting aggregate that was much better than ordinary stone in the sense of having surface hydroxyl groups to react. That Roman concrete lasted so long may in part be due to a better choice of aggregate.
A second point was the use of hot mixing. One possibility is they used a mix of freshly slaked lime and quicklime and by freshly slaking the mix became very hot. This speeds up chemical reactions, and also allows compound formation that is not possible at low temperatures. By reacting so hot it reduced setting times. But even more interestingly, it appears to allow self-healing. If cracks begin to form, they are more likely to form around lime clasts, which can then react with water to make a calcium-rich solution, which can react with pozzolanic components to strengthen the composite material. To support this, Admir Masic, who had been studying Roman cement, made concretes using the Roman recipe and a modern method. He then deliberately cracked the samples and ran water through them. The Roman cement self-healed completely within two weeks, while the cracks in the modern cement never healed.