Instant stone (just add water!)

post by jasoncrawford · 2019-11-13T22:33:39.903Z · score: 75 (30 votes) · LW · GW · 9 comments

This is a link post for

Originally posted on The Roots of Progress, January 6, 2018

From the time that humans began to leave their nomadic ways and live in settled societies about ten thousand years ago, we have needed to build structures: to shelter ourselves, to store our goods, to honor the gods.

The easiest way to build is with dirt. Mud, clay, any kind of earth. Pile it up and you have walls. A few walls and a thatched roof, and you have a hut.

Earthen hut with thatched roof in Sudan - Petr Adam Dohnálek / Wikimedia

But earthen construction has many shortcomings. Dirt isn’t very strong, so you can’t build very high or add multiple stories. It tends to wash away in the rain, so it really only works in hot, dry climates. And it can be burrowed through by intruders—animal or human.

We need something tougher. A material that is hard and strong enough to weather any storm, to build high walls and ceilings, to protect us from the elements and from attackers.

Stone would be ideal. It is tough enough for the job, and rocks are plentiful in nature. But like everything else in nature, we find them in an inconvenient form. Rocks don’t come in the shape of houses, let alone temples. We could maybe pile or stack them up, if only we had something to hold them together.

If only we could—bear with me now as I indulge in the wildest fantasy—pour liquid stone into molds, to create rocks in any shape we want! Or—as long as I’m dreaming—what if we had a glue that was as strong as stone, to stick smaller rocks together into walls, floors and ceilings?

This miracle, of course, exists. Indeed, it may be the oldest craft known to mankind. You already know it—and you probably think of it as one of the dullest, most boring substances imaginable.

I am here to convince you that it is pure magic and that we should look on it with awe.

It’s called cement.

Let’s begin at the beginning. Limestone is a soft, light-colored rock with a grainy texture, which fizzes in the presence of acid. Chalk is a form of limestone. What distinguishes limestone and makes it useful is a high calcium content (“calcium” and “chalk” are cognates). Specifically, it is calcium carbonate (CaCO3), the same substance that makes up seashells. In fact, limestone, a sedimentary rock, is often formed from crushed seashells, compressed over eons.

Limestone from a quarry in southern Germany - Hannes Grobe / Wikimedia

Limestone can be used for many purposes, including fertilizer and whitewash, but its most important industrial use is in making cement. When it is heated to about 1,000 °C (e.g., in a kiln), it produces a powder called quicklime. Chemically, what’s going on is that burning calcium carbonate removes carbon dioxide and leaves calcium oxide (CaCO3 + heat → CaO + CO2).

Quicklime is a caustic substance: touching it will burn your skin (hence “quick”, meaning active, “alive”). But perhaps its strangest property is that when mixed with water, it reacts, giving off heat—enough to boil the water! The result, called “slaked” or “hydrated” lime, is calcium hydroxide (CaO + H2O → Ca(OH)2 + heat).

Further, if you pour a lime-water slurry into a mold, not too thick, and expose it to the air, a still more amazing thing happens: in a matter of hours, the mixture “sets” and becomes once again as hard as stone. The calcium hydroxide has absorbed CO2 from the air to return to calcium carbonate (Ca(OH)2 + CO2 → CaCO3 + H2O), completing what is known as the “lime cycle”.

In other words, by mixing with water and air, this powder—a basic cement—has turned back into rock! If this technology hadn’t already existed since before recorded history, it would seem futuristic.

The product of a pure lime cement is too brittle and weak to be very useful (except maybe as a grout). But we can make it stronger by mixing in sand, gravel or pebbles, called “aggregate”. Cement, water and sand produce mortar, a glue that can hold together bricks or stones in a masonry wall. Adding gravel or pebbles as well will make concrete, which can be poured into molds to set in place. (The terms “cement” and “concrete” are often conflated, but technically, cement is the powder from which mortar and concrete are made; concrete is the substance made by adding aggregate and is what constitutes sidewalks, buildings, etc.)

Brick wall with cement mortar
Concrete wall with aggregate visible

This basic technology has been known since prehistoric times: the kilning of limestone is older than pottery, much older than metalworking, and possibly older than agriculture. But over the millenia, better formulas for cement have been created, with superior mixtures of ingredients and improved processes.

Pure lime cement needs air to set, so it can’t set if poured too thick, or underwater (for instance, on a riverbed to form the base of a column for a bridge). The Romans, who were great users of cement, discovered that adding volcanic ash, called pozzalana, to lime would produce a cement that sets even underwater; this is called a “hydraulic cement”. They used this “Roman cement” to build everything from aqueducts to the Colosseum. Another common hydraulic cement, called “natural cement”, is formed from a mixture of limestone and clay, which sometimes occur together in natural deposits.

Since the mid-1800s, the most widely used cement is a type called Portland cement. Without going into too much detail, this is made through an unintuitive process that involves heating a lime-clay slurry to the point where it fuses together into a hard substance called “clinker”. Clinker was originally considered waste material, a ruined product—until it was discovered that grinding it into powder produced a cement that is stronger than Roman or natural cement. (!) Today a wide variety of cements are available on the market, optimized for different conditions.

No matter the formula, however, all cements have one shortcoming: they are very strong under compression, which is the kind of strength needed in a column or wall, but weak under tension, which comes into play, for instance, when a beam buckles under load. The Romans dealt with this problem using arches, which direct forces into compression along the arch. Medieval builders created the pointed Gothic arch, which could stretch even higher than the round Roman ones, and the flying buttress, which added support to the walls of their tall cathedrals.

Pont du Gard, a Roman aqueduct bridge near Nîmes, France
Gothic window, Church of St. Helen, Lincolnshire, England - Spencer Means / Flickr


But in the twentieth century, a new way of building took over: reinforcing the concrete with steel. Steel, unlike concrete, has high tensile strength, so this “reinforced concrete” is strong under both compression and tension. The reinforcement bars created for this purpose are called “rebar.” Reinforcement allows concrete to be used not only for foundations, walls and columns, but for cantilevered structures such as the decks of Fallingwater.

Fallingwater, by Frank Lloyd Wright - Mathieu Thouvenin / Flickr

This is cement. We start with rock, crush and burn it to extract its essence in powdered form, and then reconstitute it at a place and time and in a shape of our choosing. Like coffee or pancake mix, it is “instant stone—just add water!” And with it, we make skyscrapers that reach hundreds of stories high, tunnels that go under the English channel and the Swiss Alps, and bridges that stretch a hundred miles.

If that isn’t magic, I don’t know what is.

Sources and further reading: Concrete Planet: The Strange and Fascinating Story of the World’s Most Common Man-Made Material,, Minerals Education Coalition, Portland Cement Association, and many pages on Wikipedia. Thanks also to Doug Peltz of Mystery Science for helpful conversations.


Comments sorted by top scores.

comment by quanticle · 2019-11-14T02:21:43.815Z · score: 25 (10 votes) · LW(p) · GW(p)

Rebar has one fatal flaw: rust. As water permeates into the concrete, the rebar inside rusts. The iron expands as it rusts, which tends to burst the concrete open from the inside, a phenomenon known as concrete cancer. However, recently researchers have built a pair of bridges using glass and carbon-fiber rebar, in an attempt to make a building material that has the strength of reinforced concrete with the durability of plain concrete.

comment by elityre · 2019-11-15T03:55:12.711Z · score: 9 (5 votes) · LW(p) · GW(p)

This was Awesome. Thanks for sharing.

comment by shanecan · 2019-11-15T23:00:30.645Z · score: 3 (2 votes) · LW(p) · GW(p)

Concrete has the "advantage" of being un-recyclable. If you build your temple from stone blocks then it can be dismantled and re-used to build homes or heathen temples. Trying to disassemble a concrete structure will only yield either a crumbling mess, or a fireworks show in the case of implosion.

Ghost towns built from wood disappear after a couple generations. Youtube has some wonderful videos of people exploring abandoned soviet era concrete structures that I can stronlgy recommend.

I have considered advocating for the placement of concrete tubes as an alternative for tent cities, but concluded that it would backfire poltically.

comment by jasoncrawford · 2019-11-15T17:06:15.274Z · score: 3 (2 votes) · LW(p) · GW(p)

Not totally unrecyclable. You can crush concrete and re-use it as aggregate for other concrete, I think.

Not sure if you can re-kiln it to extract fresh lime, but that seems possible in principle. Might just not be worth it right now, given the availability of limestone deposits.

Recycling is not always better than alternatives, it's just one option among many. If the economics don't make sense then there's no reason to do it.

comment by Douglas_Knight · 2019-11-15T20:58:06.418Z · score: 3 (2 votes) · LW(p) · GW(p)
This basic technology has been known since prehistoric times: the kilning of limestone is older than pottery, much older than metalworking, and possibly older than agriculture.

This seems to refer to lime floors in the mideast "Pre-Pottery Neolithic" (Göbekli Tepe, Jericho) but that doesn't mean that it predated pottery. Some ceramics are very old, much older than agriculture. All around the world pottery is seen before agriculture, but it seems to appear and disappear.

comment by jasoncrawford · 2019-11-15T21:40:21.303Z · score: 1 (1 votes) · LW(p) · GW(p)

According to Concrete Planet, by Robert Courland, the archaeological site at Göbleki Tepe, c. 9600 BC, shows evidence of lime products (plaster, mortar, and/or concrete). Fired-clay figures (not even pottery) don't show up until Nevali Çori, c. 8600 BC. At least, according to the table on p. 48. On that same page he says that “fired ceramics make an appearance soon after the invention of the limekiln.”

comment by jasoncrawford · 2019-11-15T21:45:57.281Z · score: 1 (1 votes) · LW(p) · GW(p)

The Venus figurine you linked to is interesting. I knew there were carved figurines that old but not fired ceramic. Maybe Courland is wrong, or maybe he's just talking about kilning (presumably this figurine, dating from over 27 kya, would have been fired on a campfire, not in kiln).

In any case, I wouldn't call the figurine pottery, so maybe what I wrote is still technically correct?

comment by Douglas_Knight · 2019-11-16T03:14:12.099Z · score: 2 (1 votes) · LW(p) · GW(p)

Courland (and the many others who say the same thing) probably means that there weren't ceramics in the mideast before lime. But there were ceramics elsewhere.

What is your technical criterion? That it isn't a pot? Maybe that is what people mean by "Pottery Neolithic," but this seems to me a stupid criterion. Anyhow, there were ceramic pots in many places in East Asia before this. The general world-wide trend is that ceramic pots predate local agriculture, with the odd exception of the mideast. Here is pot from China, dated there 20-10kya, together with a fragment said to have a more precise dating of 20kya. Here is a Japanese pot. Here is a Siberian potsherd. I think South American pottery was pots, but I'm not sure. It's pretty recent, but I think it predated Andean agriculture, although not Mexican. African bowls predate agriculture and seem contemporaneous with Çatalhöyük, perhaps Göbekli Tepe. [This is list is simply the second paragraph of wikipedia on pottery, which was the basis for my previous claim about "all around the world," but now I've tracked down the individual examples.]

The Venus figurine might be uniquely old, but Croatia had a bunch of paleolithic ceramic figurines.

comment by Chris Hibbert (chris-hibbert-1) · 2019-11-16T18:53:10.140Z · score: 2 (2 votes) · LW(p) · GW(p)

The next stage in the evolution of building with concrete is also a wonderful innovation. "pre-stressed" concrete is another solution to the problem that concrete is stronger in compression than in tension. To make pre-stressed concrete, you start by laying out the rebar in sections where you will pour the concrete with the ends of the rebar sticking out. Then, just before pouring the concrete, you put the re-bar under tension, pulling it from the ends. After the concrete sets hard, you release the tension. the re-bar pulls the concrete, putting the entire slab under compression. Then when you use it to bridge over a gap, any resulting tension is partly mitigated by the pre-existing squeeze.