Sand, cement and concrete: geology of the building blocks of construction

Sand is everywhere. Deserts and beaches are full of it. But next to water, sand and gravel are amongst of the most exploited natural resources. Along with cement, they are indispensable for making concrete, a process that across all stages of production has a major impact on the environment and climate. Since the Romans built the Pantheon, the global concrete mill has been turning at an ever-accelerating pace. Can it be slowed down?

Author: Kathelijne Bonne.

I cannot imagine materials less inspiring than concrete and cement. In the past it may have been used to create magnificent structures like the Pantheon in Rome, but today it forms the skeletons of soulless speculative construction, unnecessary paving and other absurd forms of brutalism. The Las Marías II building project (Torrelodones, Madrid) is a sobering example of the concrete paradox in which, in times of drought, erosion and floods, the most pressing needs of the Anthropocene Era are ignored. So-called luxury villas – cheaply slapped together – pop up like mushrooms while the surrounding area is being paved with concrete only to serve as a base for artificial lawns.

"Nature comes to live with you," the slogans promise.

Orwell's concrete dystopia

Las Marías is now a dystopia of concrete sprawl. I recently ventured in it, after months of avoiding it. As I walk among tower cranes, concrete mixers and construction debris, I feel transported into George Orwell's 1984. What was here before, nature, rocks, bushes, moss and walking paths, has simply been 'dissolved'. I can't remember the cork oaks and pine trees that stood here last year. They have been erased, evaporated from my mind.

I walk around a massive new roundabout in the middle of nowhere. Less than a year ago there was a lovely gravel path leading into nature. Now it looks like a motorway. I wonder why the new concrete footpath is about thirty feet wide. No one will want to walk in this sea of cement.

As the November sun burns my freckled forehead and the white cement blinds my eyes, unfit for so much light and whiteness, I get overwhelmed by paradoxical thoughts about concrete and cement.

They create an idea of cleanliness while nature gets mutilated.

They seem the epitome of civilization but shake it to its core.

And the same sand mined to produce concrete is a source of silicon in computer chips, which now do thinking work for us so that we no longer ask ourselves questions.

Like my fellow geologist/writer Roseanne Chambers, I am deeply concerned about concrete, which she has already written about in her article Concrete Concerns (see biography).

Eternal sand

Ultimately, all of concrete's 'building blocks' also come from nature, as it is made from sand and limestone, but not without first undergoing industrial transformations and creating emissions. Paradoxically, the granite landscape of the Guadarrama Mountains near Madrid is itself a natural source of sand. This type of rock, granite, is rich in quartz, one of the most durable minerals and key component in sand. Rock is ground to loose sediment through physical weathering: Water in rock crevasses freezes and expands, as such fracturing the rock, which crumbles and disintegrates into ever smaller pieces and eventually into gravel and sand. Wind and rivers will transport and redistribute these sediments, and finally it accumulates in sand deposits from which the sand can be mined. As such, the sand and the concrete made from it, often returns for building purposes to where it might once have been produced naturally.

Concrete consists of three ingredients: aggregate (mixture of sand and gravel), water and cement. Sand gives filling to concrete, its body and strength. There are many types of sand. Desert sands differ from river, beach or marine sands. Sands can be fine- or coarse-grained, each individual grain has some degree of roundness – from perfectly rounded to angular – , sand can be poorly or well-sorted, with individual grains possessing respectively varying or the same sizes. Single grains can be spherical, elongated or oblate. These are all characteristics, outcomes of nature – as are the sandstone landscapes of Narnia – that give materials their structural properties and strength. And the minerals that make up sand also vary according to the type of rock that produced the sands. Pure quartz sand is hard and white, and because quartz hardly weathers, quartz-rich sands defy eternity.

Cement origin

All those grains of sand have to stick together to obtain hard concrete. This requires cement, a substance that hardens irreversibly on contact with water. Cement binds things. Cement too is made from rocks like limestone, which in turn consists of shells and skeletons of marine organisms and coral reefs. These particles of the sea, beautifully portrayed by biologist Helena Cruz de Carvalho, end up in human structures. I can't help but feel some pain that these delicate little creatures end up as paving for driveways, parking lots, etc.

The limestone is crushed, ground, sieved, mixed with minerals such as silicon and other oxides, heated in blast furnaces at extremely high temperatures, until finally cement mixtures are obtained, which are stored in silos. The whole of this process accounts for 10% of global emissions. CO2 is released partly for the fuel to heat the kilns and drive the mechanical grinding processes, but also as a by-product of the process itself.

Pantheon and pozzolan

Besides shells, volcanoes can contribute to the production of concrete. The Romans perfected the cement recipes of the Greeks. They made their structures using mortar that contained pozzolan, a material that acquires cement-like properties in the presence of water and calcium hydroxide. The Romans mined it from volcanic tuff and ash, quarried near Pozzuoli, the heart of the Campi Flegrei volcano. Roman monuments like the Pantheon, which, like sand, weather the ages, owe their great strength both to the ingenuity of their master builders and the special properties of pozzolan. The dome of the Pantheon is still the largest dome made of unreinforced concrete. No earthquake, no barbarian invasion, no mass tourism, nor the slow ageing by the passage of time, affect the Pantheon. It stands there, glorious, unbidding. According to recent studies, the durability of Roman cement may be a result of its capability of repairing itself.

The Romans did not yet have to worry about climate or overconsumption (*), although philosopher-emperor Marcus Aurelius was already expressing his appreciation for nature: he saw beauty in the veins of a leaf. He would have been an incredible activist.

But now we do live in times when even the infinite is finite. Sand, for instance, is finite. There is a well-defined number of grains of sand on our planet. And there are fewer of them than stars in the universe, said space guru Carl Sagan. He meant to bring attention to how immeasurably many stars and worlds there are, but the statement has a different meaning in this context. There is indeed still a lot of sand, but it is being mined faster (50 billion tonnes annually) than it is being replenished by nature (15 bln tonnes) to keep the global concrete mill running. Deserts are indeed still full, but because of the weight of sand (and the cost of transportation), it is mined where the concrete is in demand. And locally, shortages can arise.

(*) Local/regional climate fluctuations may have been a factor in the disintegration of the Roman Empire. 

Wall around the earth

Every year, a wall 27 meters wide and high around the earth is produced from concrete. This requires 4.1 billion tonnes of cement and 50 billion tonnes of sand. Not only the production but also the mining has an impact. Opencast mining projects, like asbestos mines, leave big scars in the landscape. And when the mining is done along coasts, the beaches, mangroves, marine meadows, coral reefs, tidal flats and river beds are severely damaged. Natural coasts lose their protective buffering effect against tsunamis, storm surges and rising sea levels. Along rivers, soil erosion increases the effect of floods, more sediment is washed to downstream areas, and aquatic habitats area destroyed. There are sand shortages in India, and sand mafias have emerged in the form of illegal sand mining companies leading to conflict and even murder.

Sustainable concrete?

There are ways of existing with less concrete: Used concrete (such as concrete taken away in activities of depaving), construction waste and old bricks can be reused. Materials such as fly ash (by-product of coal-fired power plants) and slag (metal production) can serve as substitutes for cement and aggregates. Stricter regulations can help make sand mining more efficient and sustainable. Carbon dioxide emitted from concrete plants can be captured so it does not enter the atmosphere. And just paving less, and only including concrete in the building and not in gardens, parking lots and enclosures is even better. Pervious concrete, which the builders of Las Marías clearly don't know, is one solution to counter the impact of excessive paving. Researchers are still working out how the Romans made their concrete, as it is much more durable than modern concrete.

Creating awareness is perhaps the most important step. Ensuring that AI - which runs on silicon from grains of sand - does not take the place of our brains, although we can ask it for solutions. Ultimately, it is we humans, with our bodies and thinking abilities that have taken four billion years to evolve and thrive on this planet, who will have to face all that concrete and bear the consequences of its impact.

------

Concrete is one of the big emitters alongside meat consumption and transport. Read more about sustainable meat, or freshen up on planetary boundaries. Read more about soil salinisation, desertification, or the Tethys Sea, which also left thick layers of chalk that in our human age contributed to both Michelangelo's David and concrete.

Kathelijne: I am intrigued by how earth, life, air, ocean and societies interact on geological and human timescales.

Why I started GondwanaTalks.

Recent posts:

Do you like this article? Subscribe to my short newsletter (every couple of weeks, no large files or annoying gifs), to let you know I published something new.