A brief history of the world's second most consumed product
Last week Veja magazine released another one of those articles that piques the curiosity of any construction enthusiast: "why is Roman concrete more resistant than modern concrete?" The article is based on an article published by the American Mineralogist, which has a great focus on the reaction of the chemical components of concrete, and how the Romans already at that time had managed to create a durable and resistant material. But to understand this story we need to travel back in time, but not without first understanding the composition of our beloved concrete.
Concrete is an artificial stone, basically composed of 4 components: Binder (cement or lime ), Coarse Aggregate ( gravel/stone), Coarse Aggregate ( sand ) and water. The product of the mixture of binder, coarse aggregate and water is called mortar. The modern conventional structures, when using concrete, are made of: Reinforced Concrete, Prestressed Concrete or Precast concrete. Concrete is produced by the chemical and mechanical reaction of its components, which produce a porous mass of high resistance. The pores are produced by the evaporation of water in a reaction that releases heat (exothermic), and its strength increases as the concrete ages, the calculation strength is usually based on 28 days.
Reinforced concrete is nothing more than a structural material where the solidarization between concrete and steel occurs, given by the addition of simple concrete with a passive reinforcement. Prestressed concrete works in an analogous way, but with an active reinforcement, which is given by the prestressing or "pre-tension" normally caused by cables or wire ropes subjected to a tensile force. This reinforcement acts to reduce or even eliminate the stresses that may arise when the concrete begins to suffer the effects of the load to which it is subjected. Finally, we have precast concrete, which is usually prestressed concrete made in industrialized plants specialized in this technique, which provide standardized parts for construction.
Notice therefore that there is a difference between modern and Roman structures, since modern structures make use of steel solidarization, and the Roman ones used only plain concrete. And yet we are not talking about the same type of concrete.
Remember we mentioned that the binder can be cement or lime? Nowadays we use Portland Cement, the most common being CP II E 32. Our Roman ancestors used a concrete with Pozzolana instead of lime. Pozzolana is nothing more than a volcanic ash, from Bahia de Napoli in Italy, in the Campo Flegrei region, more specifically in the city of Pozzuoli, next to Bacoli. This ash improved the resistance of lime and sand mortars, even when subjected to the tides and the action of time, and was considered a precious possession of the Roman Empire. But this was not an exclusivity, since it is known that the Greeks of Santorini also used volcanic ash for their constructions... Since Napoli was founded by Greeks, it is not difficult to suspect that the Roman technique is perhaps nothing more than an inheritance from its Greek founders...
It turns out that the region is limited, and it was not always possible to extract such ashes. As a solution the Romans sometimes used products from baked clay, such as bricks, vases, etc. With the Roman decline also occurred the decline in the use of Pozzolana, and consequently in the quality of mortars, a fact that only improved from the XVII century on. And it was in 1726 that our first Civil Engineer, John Smeaton, realized that the limestone with clay addition presented superiority to the pure limestone for the production of binders.
However, it was only in 1824 that Portland Cement appeared, created by Joseph Aspdin, a brickmaker from York County, who gave it its name thanks to the appearance that the material produced had with the stones of the Portland region. Joseph Aspdin's "Cement" was actually an artificial Hydrated Lime, but the process he used gave rise to cement as we know it today, and it is because of this that cement still gets the name Portland.
Perhaps the great evolution of cement was not in the aspect of strength, but in the production and application of the material. Smeaton and Aspdin were looking for practical solutions that would create a resistant material, economical to manufacture and that could meet the demands imposed on them. It is worth remembering that we are talking about a time of industrial revolution and that pozzolanic ash is a finite and expensive commodity, on which industry cannot be dependent.
Yet it is still used in cement today. Remember I said that the most used cement is CP II E 32? CP stands for Portland Cement, the Roman numerals represent the type of cement, the letter represents which type to add, and the number represents the concrete's compressive strength at 28 days.
Out of curiosity, the Brazilian Standard - NBR 11578, says that the composition of pozzolanic cement should be
Clinker: 76 to 94%
Pozzolana: 6 to 14%.
Limestone: 0 to 10%.
For the curious, we have taken from the Brazilian Association of Portland Cement a table with the different types of cement available in the Brazilian market:
So take it easy, the Roman product was not such a showstopper. The article unveils one of the mechanisms by which Roman pozzolanic concrete "self-refining". To put it very simply: while the reaction of portland cement concrete under conditions analogous to Roman produces expandable gels that degrade the structure, Roman pozzolanic concretes produce mineral cements that further refine the pore space, and improve the bond of the concrete.
Remember that concrete's strength increases with age? Yes you read that right, the older the older the stronger the concrete....This is true as long as favorable conditions exist for the internal reactions of concrete to occur naturally. External interferences from aggressive environments, such as the marine environment, can lead to undesirable reactions, causing unwanted expansions, loss of strength, and other harmful pathologies. This article is part of a series of studies that have been developed worldwide, in obtaining regenerative concretes, concretes that are able to respond positively to adverse situations. In this context, we have just discovered the operation of a product that we have been using for a long time, but which we have never completely mastered.
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