Instant stone (just add water!)

by jasoncrawford4 min read13th Nov 201928 comments


Progress Studies
This post has been nominated for the 2019 Review
Write a Review

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.


27 comments, sorted by Highlighting new comments since Today at 12:45 AM
New Comment

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.

I notice I am confused.

I feel as though these type of posts add relatively little value to LessWrong, however, this post has quite a few upvotes. I don’t think novelty is a prerequisite for a high-quality post, but I feel as though this post was both not novel and not relevant, which worries me. I think that most of the information presented in this article is a. Not actionable b. Not related to LessWrong, and c. Easily replaceable with a Wikipedia or similar search. This would be my totally spot balled test for a topical post: at least one of these 3 must be satisfied. I have a few hypotheses for there’s an apparent demand discrepancy:

H1: The majority of users upvoting this content are upvoting the (high-quality and engaging) writing style and presentation of information.

H2: I’m (relatively) unique in my feeling that a gears-level understanding of the process of construction material is not particularly relevant to LessWrong

H3: The majority of users upvoting this content derived some broader understanding from the specific content of the post.

H4: the majority of users upvoting the content did so because they felt the post allowed them to make a tangentially related intellectual leap, realization, or connection that would have also been triggered with a 1-paragraph summary

H5: there is a small contingent of LessWrongers who REALLY enjoy progress studies, and routinely upvote all related posts in an effort to shift community norms or subconsciously.

If H1 is true, then I strongly support downvoting this post, regardless of its individual merit: these kind of posts, while potentially engaging on their own, should not be promoted to prevent proliferation. Featuring one in a review suggests, at the minimum, that this community specifically values these contributions. It also doesn’t improve the strength of the printed work: extraneous information is not particularly compelling, even if it is well-presented. I think this is the most likely hypothesis by elimination. I don’t know if this belief is due to the typical mind fallacy.

If H2 is true, then I’m fairly sure I’m not qualified to judge collation on the merits: P(I have an accurate picture of the merits of the post|I have an inaccurate picture of the relevancy of the post) seems very small. I’d suggest upvoting if you are someone who agrees with H2 by personal experience.

If H3 is true, than I’d still oppose curation, UNLESS you derived an insight that truthifies H3 for you AND you feel it is obvious/clear from the content of the post. If either of these conditions is false, I’m pretty sure that this post shouldn’t be curated. In the case that the post contains valuable insights that aren’t expressed clearly, which seems likely to irritate readers that don’t understand them, and isn’t exactly the type of content we’d like to promote. I assign a low but non-zero chance to this: my post comprehension is generally pretty good, but it would be incredibly stupid for me to believe that it has 0 probability. The fact that no commenters have expressed this type of realization decreases the chance of this.

If H4 is true, I’d oppose curation, for the same reason discussed in the previous discussion

If H5 is true, I don’t quite know what to say, and I’d probably oppose curation: I’d like to think curation is the product of broad community approval, rather than small subgroups, as I think the terminal impact of that is quite bad. I assign an extremely low probability to this being the primary reason, and a much higher probability to this being a secondary reason.

I’d reccomend based voting on what hypothesis you think is true; I think my preference is well-expressed above.

I strongly believe all of the following:

  • Progress Studies should be a core topic on LessWrong, and is directly relevant to LessWrong's central mission
  • On most of LessWrong's core topics, discussion is usually too abstract, and would benefit from more concreteness/object-level discussion
  • Expanding the set of topics regularly discussed on LessWrong to include more object-level science/history/economics would dramatically improve the quality of discussion on topics which are already common

I'll walk through each of those one-by-one.

First, progress studies. The current stated mission of LW's dev team is roughly "to accelerate the pace of intellectual progress". Even aside from that being the stated mission of the team, it seems like an obviously central goal of the rationalist community in central, as well as one of the main lenses through which to study rationality-in-groups. That makes progress studies about as core a topic as, say, social psychology or decision theory.

Second, concreteness. Concrete examples are the main evidence on which our intuition builds its models. In this case, we're thinking about questions like "what kind of technology changes induce lots of progress?", "what might be difficult about finding such technologies?", "to what extent do key technologies gradually develop over time?", etc. Concrete provides a nice concrete example relevant to all of these.

Now, I'm sure someone will object that we ought to be looking at data to answer these sorts of questions rather than individual examples. Counterargument: looking at data usually requires deciding beforehand what questions to ask or hypotheses to test. For questions like these, it seems likely that we're not even asking the right questions yet. Asking the right questions requires building more intuition, and in particular using real, concrete examples in order to build that intuition (we want our intuition-map to reflect the territory, so we have to look at the territory in order to make that map). In general, that's the sort of power which concrete examples/use-cases provide: they inform our intuition about what questions to ask. For pre-paradigmatic studies (including progress studies and alignment), that's much more of a bottleneck than hypothesis testing.

Third, expanding topics. Pulling from my own posts here, consider these two posts on biology. Biological evolution has been widely used in LW's discussions around alignment, usually with the message that we should not expect evolved systems to be legible. And yet, if we actually go study some biology, it turns out that biological systems are remarkably legible and modular. I can also say from experience that studying biological systems in greater depth provides lots of use-cases for testing out embedded agency models, which often reveal things which would be easy to miss when only thinking about e.g. ML systems. Organisms are embedded agents, it shouldn't be a surprise that studying organisms is useful for embedded agency. The relevance of progress studies is analogous: if the rationalist community wants to make tangible progress by thought or experiment, then it shouldn't be a surprise if studying the history of progress by thought or experiment (even unintentional experiment) is useful.

Concrete examples

I lol'd

(Well, I didn't lol, but I smiled in amusement)

((Okay I didn't even smile I just literally thought the words 'concrete example, lol'))

I think my comment in response to Raemon is applicable here as well. I found your argument as to why progress studies writ large is important persuasive. However, I do not feel as though this post is the correct way to go about that. Updating towards believing that progress studies are important has actually increased my conviction that this post should not be collated: important areas of study deserve good models, and given the diversity of posts in progress studies, the exact direction is still very nebulous and susceptible to influences like collation.

The most significant objection I have to the structure of this post is that I feel like it’s a primer/Wikipedia page, not a post explaining a specific connection. Both of the examples you provide explain the relevance of the natural system at hand to a core LessWrong discussion topic. The failure mode I’m worried about with this type of post is that there are a lot of things that have contributed to human progress, meaning that this type of historical brief could easily proliferate to an excessive extent. Like I mentioned to Raemon, I’d feel a lot better about this post if it discussed how this gear meshed with broader historical/progress trends, because then it would be a more useful tool for developing intuition. Using the spitballed three-prong test, the first post is definitely not replaceable with Wikipedia, and arguably relevant to LessWrong (though I think that condition is underspecified), and the section post is similarly acceptable, in my eyes. I’d support collation of more progress studies posts, just not this one.

Ok, I think our crux here is about how much posts should explicitly point out how their material connects to everything else.

Personally, I think there's a lot of value in posts which explicitly do not explain how they connect, because explaining connections usually means pulling in a particular framing and suggesting particular questions/frameworks. In a pre-paradigmatic field, we don't know what the right questions or frames are, so there's a lot of value in just presenting the information without much framing. It's the "go out and look at the world" part of rationality.

Now, a downside of this sort of post is that many people will come along who don't have any idea how the material relates to anything. There's no hand-holding in the interpretation/connections, so readers have to handle that part on their own, and not everyone is going to have enough prior scaffolding to see why the material matters at all. (I've definitely seen this on many of my own posts, when I present a result without explaining how it fits in with everything else.)

I think the best way to handle this sort of trade-off is to have some posts which present information (especially concrete examples) without much framing, and then separately have posts which try to frame that information and explain how things fit together (which usually also means positing hypotheses/theories). It's very similar to the separation of empirical vs theoretical work we see in a lot of the sciences. We already have a lot of the latter sort of post, but could use a lot more of the former. So e.g. "this type of historical brief could easily proliferate to an excessive extent" is something I'd consider a very positive outcome.

I think you’re mostly right. To be clear, I think that there’s a lot of value in unfiltered information, but I mostly worry about other topics being drowned out by unfiltered information on a forum like this. My personal preference is to link out or do independent research to acquire unfiltered information in a community with specific views/frames of reference, because I think it’s always going to be skewed by that communities thought, and I don’t find research onerous.

I’d support either the creation of a separate [Briefs] tag that can be filtered like other tags, and in that case, I’d support this kind of post, but at the moment, I don’t know what the value add is for this to be on LessWrong, and I see several potential costs.

Good suggestion, and I expect some mechanism along these lines will show up if and when it becomes significant.

(I disagree with this comment, but upvoted it because I think it does a good job exploring the question "how do we evaluate blogposts of varying types?" which I still feel pretty overall confused about)

There's maybe two separate question of "does this deserve a bunch of upvotes?" and "does this deserve to be in the 2019 Review Book(s)?"

I didn't upvote this post, but I might have, for a couple reasons. One major one is novelty. Right now there aren't that many LessWrong posts that explore object level worldmodeling. Rather than ask "is this a fit for LessWrong's main topics?" I think it's actually often useful to ask "does this expand on LessWrong's main topics in a way that is potentially fruitful?". I think intellectual progress depends in part on people curiously exploring and writing up things that they are interested in, even if we don't have a clear picture of how they fall fit together.

Separately, I do think Progress Studies are (probably) particularly important to what I think of as one of LessWrong's central goals: using applied rationality to put a dent in universe. I'm not sure this particular piece was crucial (I haven't re-read it recently). But, I think understanding how human progress works, in the general sense, is disproportionately likely to yield insight into how to cause more progress to happen in important domains.

I think that's valuable enough to consider upvoting, and valuable enough to consider it for a retrospective best-of Review. I think for in both cases it depends more on the specifics of the post, and whether it, in fact, led to some kind of later insight. (Part of the point of a retrospective review is you don't have to guess whether something would provide useful insight – you know whether it actually helped you in the past 1.5 years).

(strong-upvoted, I think this discussion is productive and fruitful)

I think this is an interesting distinction. I think I’m probably interpreting the goals of a review as more of a “Let’s create a body of gold standard work,” whereas it seems as though you’re interpreting it more through a lens of “Let’s showcase interesting work.” I think the central question where these two differ is exemplified by this post: what happens when we get a post that is nice to have in small quantities. In the review-as-goal world, that’s not a super helpful post to curate. In the review-as-interest world, that’s absolutely a useful facet to curate. I also think that while H5 might not be true in this case, we’d have opposite recommendations of it was true, but I could be wrong about that.

Separately, I’m not sure that even given that we want to be endorsing gears-level pieces on progress studies, this is the specific work we want to curate: I’d like to see more on the specific implications and consequences of concrete and how it “meshes” with other gears (i.e. for an unrelated field, agriculture, this probably would involve at least tangential discussion of the change in societal slack brought on by agriculture). I suspect this would go a long way towards making this piece feel relevant to me.

OP here. I will recuse myself from the conversation about whether this deserves to be in any list or collection. However, on the topic of whether it belongs on LW at all, I'll just note that I was specifically invited by LW admins to cross-post my blog here.

Thanks! I’m obviously not saying I want to remove this post, I enjoyed it. I’m mostly wondering how we want to norm-set going forwards.

I think this comment is convincing to me that the post should NOT be curated.

I upvoted primarily for H1 because I enjoyed reading it, and partly for H2.

I think reading more gears-level descriptions of things from day to day life is helpful for keeping an accurate reductionist picture of reality. In particular, I want to reinforce in myself the idea that mundane inventions (1) have a long history with many steps (2) solve specific problems, and (3) are part of an ongoing process that contains problems yet to be solved.

That makes this post nice for me to read day to day, but it makes it definitively NOT a post that I care about revisiting or that I think expands the type of thinking that the curation is trying to build.

This was Awesome. Thanks for sharing.

Concrete is certainly cool, but Wooden structures can be built many stories high. So the implied history of 'mud walls to concrete' is a bit off here. In some places wood was used for spans long after they had concrete: I'll give my own country, the UK 🇬🇧 as an example. Cut and shape a few trees is still a good option in many places. But of course it burns or rots eventually so there is little evidence of very old structures. Composite materials are cool (steel and concrete) but its actually a very old technique: hair and straw reinforced bricks, even sod/turf and as other comments have noted, there are pitfalls to certain kinds of steel, that other mixes may overcome. 😁

Good point, I skipped wood here. This is an old post, I mention wood in my more recent treatments of this topic.

And good point about reinforcing being an old technique! Another thing I learned about after I wrote this post is wattle & daub.

This is so lovely! Pure happy infodump energy. Reveling in the wonder of reality.

It's very close to a zetetic explanation, which I massively approve of. You might even say it's a concrete example.

(Side note: Rebar reinforced concrete was a mistake. It rusts in place and this fucks up so much modern architecture.)

I'd love to nominate to basically everything Jason writes. Heck, I'd totally buy a book of posts from roots of progress. But of those which showed up on LW in 2019, this is one of the two which were most roots-of-progress-y.

I thought this was a great post?

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.

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.

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.

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.

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.”

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?

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.

I'm all for such things existing and a book entirely composed of such things seems like it should exist, but I don't know what it would be doing in this particular book. 

The combination of the two previous reviews, by hamnox and fiddler, seem to summarize: It's a pure happy infodump that doesn't add much, that gets you a lot of upvotes, and that says more about the voting system than about what is valuable.