Compared to modern concrete which can waste away in decades, Roman concrete can survive for millennia
Mystery of long-lasting Roman concrete and why it matters
Throughout its long history, the Pantheon has endured modifications and changes, largely emerging intact as one of the best-preserved monuments of the ancient world. In comparison, structures built of modern concrete start to waste away in mere decades, with very few such structures surviving for over a century. For decades, the mystery of Roman structures and the concrete used to build them has fascinated and confused scholars. Up until recently, it was assumed the magical ingredient behind the endurance of Roman concrete was pozzolana, a mix of volcanic ash, largely procured from the Italian city of Pozzuoli and the caustic substance known as lime.
Later, research done on ancient Roman piers noted the presence of minerals such as aluminous tobermorite and phillipsite in the concrete, adding to the theory that Roman concrete grows stronger over the centuries due to various chemical reactions within the construction material itself. But, as the aforementioned recent study has noted, this may not be the complete picture. The team at MIT that carried out the joint study examined pieces of 2,000-year-old concrete taken from the archaeological site of Privernum and found that beyond the materials used, the techniques employed by the Romans may also have made a huge difference in the strength and endurance of their structures. During their examination, the team found small chunks of lime in concrete that are referred to as lime clasts. These lime clasts were previously overlooked as simply being a product of poor mixing of mortar or evidence of sub-par quality construction material. But the team at MIT had a different take. Admir Masic, one of the authors behind the study was quoted as saying that the whole idea of lime clasts being a product of low-quality control bothered him. After all, the Romans were known to be extremely precise when it comes to the recipes used to make construction materials and it would be inconceivable that they would not be bothered by such lapses in quality. As it turned out, there was more to this ‘imperfection’ than meets the eye. Concrete, generally speaking, is made by mixing cement (a binding substance made of limestone, water and finely crushed rock or sand) with water and coarsely crushed stone. For a long time, it was assumed that Romans made use of the less-reactive slaked lime or calcium hydroxide in making the binding agent that would then be mixed to form concrete. But, after carrying out spectroscopic analysis of the lime clast within the Roman concrete, researchers at MIT have suggested that the Romans instead used calcium oxide or quicklime for the purpose instead, a far more reactive and dangerous substance that they say was not first mixed with water to form slaked lime but instead directly added to the volcanic ash and other materials for the concrete before water was added. The process, referred to as ‘hot mixing’ created extreme heat and may have actually been the key behind the durability of Roman concrete. As explained by Admir Masic in a news release, hot mixing had two distinct benefits. “First, when the overall concrete is heated to high temperatures, it allows chemistries that are not possible if you only used slaked lime, producing high-temperature-associated compounds that would not otherwise form. Second, this increased temperature significantly reduces curing and setting times since all the reactions are accelerated, allowing for much faster construction.” Beyond this, the team also suspected that the lime clasts thus formed during the process had some part to play in healing cracks within the concrete that are formed due to weathering. To test out their theory, the team carried out an experiment using two slabs of concrete that were deliberately cracked. One of these slabs was made using modern methods, the other using the supposed Roman method. Then, for two weeks, the team ran water through the cracks in the two slabs of concrete. After two weeks, it was found that the slab made using the Roman method had completely healed itself and was no longer allowing water to flow through. The study suggested that the water flowing through the cracks would dissolve the lime clast and cause it to seep and re-crystalise within the cracks, sealing them up and, in effect, ‘healing’ them. Why this is a big deal Beyond the excitement of cracking open a mystery of the ancient world, the study is significant for two major reasons. First, the current method of using reinforced concrete for construction creates structures that require regular maintenance, repair and even rebuilding. The wide-ranging use of concrete means that these maintenance costs can be astronomical. In his 2011 book, Concrete Planet, author Robert Courland estimated that the cost of rebuilding and repairing concrete structures for the US alone would run into trillions of dollars, a cost that likely continues growing. Having more robust, self-sealing concrete can therefore serve to cut down on these costs incurred during the lifespan of a concrete structure. Beyond the economic benefit, more robust concrete is also good for the planet. By volume, concrete production is the third-largest source of carbon dioxide emissions in the world. Concrete is also hard and relatively expensive to recycle, meaning that concrete often constitutes the largest proportion of construction waste produced by the world at large. More robust concrete would require the production of less concrete in general, reducing the climate impact of construction activities.