Concrete and cement

Cement and concrete. Hydraulic and non-hydraulic binders and the artificial stone made with them — lime, natural cements, Portland cement, clinker, and the concrete, mortar, grout, render, and precast units formed from them. The commodity here is the binder and the bound mass, not the stone it imitates.

It is the only commodity on this list that is shipped as a powder and consumed as a rock. Cement leaves the works as a manufactured good with a shelf life, sold by the ton and degraded by damp; it arrives on a site, is mixed with water and aggregate, and becomes an artificial stone that cannot be shipped at all. That transformation is the category. It is also why neither of the adjacent tags can absorb it: Industrial chemicals describes the powder and not the mass, Rocks, sand, and gravel describes the mass and not the powder, and what is actually being traded is the capacity to turn one into the other at a distance from where the powder was made.

The practice is Neolithic, which surprises people who assume concrete is Roman. Lime-concrete floors were laid at Yiftahel in the southern Galilee around 7000 BCE, and at Lepenski Vir in Serbia by about 5600 BCE. The date is a judgment rather than a discovery, because authors draw the line between lime plaster, lime mortar, and lime concrete in different places, and moving that line moves the anchor by millennia. What is not in doubt is that burning limestone to make a binder, and binding stone with it to make a floor, is one of the older manufacturing processes in the record — older than copper, older than salt as a trade, and contemporary with the first dairying and the first fermented drink.

What Rome added was hydraulicity, which is what the word cement technically denotes: a binder that sets under water. Roman builders achieved it by mixing lime with volcanic ash, and the results are visible in the harbor works around Baiae before the close of the second century BCE, with general use from about 150 BCE and some scholars placing the development a century earlier. Vitruvius, writing around 25 BCE, specifies one part lime to three of pozzolana for building and one to two for work under water. The detail that matters for a list of commodities is what happened next: when Herod built the harbor at Caesarea between 22 and 15 BCE, the pozzolana was shipped to Judaea from Puteoli, on the Bay of Naples. They moved volcanic ash across the Mediterranean because only that ash would set in seawater — which is the moment the binder stops being a local practice and becomes a traded good with a named source and a freight cost.

Portland cement was patented in 1824, and the industry that followed made concrete the most consumed material on earth after water. The consequence now attached to that scale is chemical rather than industrial. Making cement means calcining limestone, and calcination drives carbon dioxide out of the rock itself, independently of whatever fuel heats the kiln. Cement accounts for something on the order of eight percent of global carbon dioxide emissions, and a substantial part of that comes from the reaction rather than from the furnace — which means it cannot be abated by changing fuels or electrifying the plant, as most heavy industry can. That is why cement is generally described as among the hardest emissions in the economy to eliminate, and it is the point at which this tag meets Emissions allowances: a commodity whose emissions are a property of its chemistry is the limiting case for any instrument that prices them.

One further thing distinguishes this tag, and it is the reason it was added to the list at all. It is the only commodity here that displaced a large part of another commodity on the same list. In taxonomic terms, the aggregate half of Rocks, sand, and gravel consumed the dimension-stone half of the same tag — crushed stone displacing cut stone, with the binder as the agent. Until this category existed, that agent had no name in the Atlas, and the displacement could be described from the losing side only. The displacement itself was narrower than the story usually told about it, and better evidenced for being narrow. Concrete took paving outright: granite blocks ran at over forty million a year in the early 1920s and were down to 208,000 by 1954, though the proximate cause was rubber tires replacing the steel-shod horse-drawn truck, which changed what a street surface had to survive. Its larger effect was not substitution at all — large buildings moved from solid stone masonry to thin veneer hung on steel and concrete frames, which cut the volume of stone per building without replacing stone with anything. American dimension stone peaked in quantity in 1925 and stood at little more than half that by 1955, while by 1956 crushed and broken stone outweighed it a hundred and twenty times over and outsold it five to one. The two quarry districts usually named as concrete’s victims do not survive examination as such: Catskill bluestone’s trade migrated some forty miles west to Delaware County, where it still operates, and the standard account of Connecticut brownstone’s end gives changing architectural fashion and the stone’s own failure in service rather than cement. The clearest clean substitution concrete achieved in the period was against roofing slate, whose production fell from $5,186,167 in 1908 to $2,588,971 by 1956 after asphalt-base roofing surfaced with slate granules arrived around 1910. Concrete did not so much beat cut stone as make most of it unnecessary.

Boundaries. The matrix governs: a cement-matrix composite is tagged here however it is reinforced, so reinforced concrete and glass-fiber reinforced concrete both belong here, while the reinforcement itself is tagged separately — bar as Iron and steel, alkali-resistant glass fiber as Glass. A polymer-matrix composite is not tagged here even where it imitates cast stone; those are Plastics and polymers, on the mirror of the same rule. The aggregate bound into concrete is Rocks, sand, and gravel, and so are the limestone and clay burned to make clinker. Fired clay articles, architectural terra cotta among them, are Ceramics, whether glazed or not.