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Comment by ogogmad | Hacker News Reader

People keep saying the word "concrete". I'm going to embarrass myself by asking what concrete is, and what it isn't. I've looked it up and it seems to be a paste which can be made into any shape, and which then sets into stone. So its main properties are that it can be set into any shape, and when it hardens, it hardens into something strong and reliable.

Further questions:

- Is there a difference between cement and concrete? What's their relationship? Cement is also a paste which hardens into rock.

- Is there a difference between mortar and concrete? What's their relationship? Mortar is also a paste which hardens into rock.

- What is the difference between concrete and stone? I guess the closest thing to "setting" stone is chiselling it, and that clearly takes longer than pouring and setting concrete.

So the main feature is that you can shape it without chiselling it.

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You essentially got the concept right, concrete is essentially artificial stone that can be initially poured into a mould and once set has characteristics similar to some types of stone.

Concrete is a mix of elements with different sizes (granulometry), like sand, gravel, (crushed) rock (of various sizes), with one or more binding agents, it is a "conglomerate" (in quite a few languages concrete is properly called the equvalent "cement conglomerate", in English "conglomerate" designs a natural rock that shares the same kind of structure:

https://en.wikipedia.org/wiki/Conglomerate_(geology)

Cement is just the most used binding agent, usually very, very fine.

As a comparison, what is usually called asphalt is another kind of conglomerate where the binding agent is bitumen.

Cement by itself has very good hardness but it is fragile and/or brittle (and it has very relevant shrinking when setting).

Mortar is cement+sand, it is a sort of midway between the two.

Stone (it depends on which kind of stone we are talking) is ultimately a sort of natural concrete where the particles (that may also be monogranular) have been "assembled" by pressure (and heat) over countless time.

Typically stone is quarried from very huge monolithic deposits, or it comes in large slabs, that are cut to make (transportable) bricks or samller slabs, that you then assemble with mortar.

Chiseling is not involved in building with stone (anymore) if not for decoration/aesthetic reasons.

Whether casting concrete or assembling stone elements in - say - a wall is faster or slower it is hard to say, it greatly depends on the structure, on the available tools/equipment and local costs for the one or the other and for the labour, usually concrete is cheaper because the components cost less, the tools and equipments are readily available and there is less labor involved, but it depends on the specific place/country.

In the case of reinforced concrete, the advantage is that you can make much thinner structures with same or better structural strength than masonry.

Concrete is made up of both cement (the CONtinuous part) and aggregate/stone (the disCRETE part), so cement vs concrete is kind of like broth vs soup, and concrete and stone like soup vs vegetables.

> So the main difference is that you can shape it without chiselling it.

I guess, yeah. Both are really good at withstanding compressive forces. Unlike Roman times, concrete today is usually reinforced with steel rebar, which gives it good tensile strength as well. This makes it useful for a lot of things that stone is not.

Disclaimer, take everything here with a grain of salt. Continuous-discrete is not the etymology of concrete. This might help too: https://youtu.be/UOHURuAf5iY

Thanks. This was a helpful comment, especially the CONtinuous/disCRETE thing.

The " CONtinuous/disCRETE" might be useful as a mnemonic device, but you should be aware that there is no relationship between "concrete" and the "-tinuous/dis-" part.

"Con-" from "concrete" is indeed the same prefix as the "con-" from "continuous" and in both cases it means "together".

However, "-crete" from "concrete" is not the same suffix as the "-crete" from "discrete".

"-crete" from "concrete" means "grown" (it is the participle of the Latin verb "crescere", to grow), so "concrete" means "grown together".

"-crete" from "discrete" means "sifted" (it is the participle of the Latin verb "cernere", to pass through a sieve, to separate), so "discrete" means "sifted away", it comes from the same source as the verb "to discern".

Knuth's "Concrete Mathematics" book claims to be a blend of CONtinuous and disCRETE mathematics. Thus, Knuth is using the suffix "-crete" incorrectly as well.

If I were you, I would send an email to Knuth reporting the error. You may receive a reward check from Knuth himself, which will give you eternal bragging rights.

I believe that Knuth knew very well the true meanings of "concrete" and "discrete" and he has just made a word play exploiting the fact that the "-crete" suffixes in the 2 words are homonyms.

However, not all his readers have understood the pun.

Concrete is a human-made agglomerate of a filler (usually sand), a binder (nowadays usually Portland cement, the Romans used lime) and an aggregate (gravel, crushed granite, crushed old concrete, ...) which cures to a solid. It can be poured as a slurry, formed as a paste, dry-pressed and hydrated (concrete roof tiles are made this way), sprayed, injected and more - i.e. it is a versatile material.

The difference between cement and concrete is the difference between boiled linseed oil and paint in that cement is the binder for making concrete while boiled linseed oil is the binder for (traditional) paint. It is what keeps the other ingredients (filler and aggregate for concrete, pigments for paint) together.

Stone is a portmanteau for the material from which rock is made, this can include sedimentary conglomerates like sandstone, metamorphics like soapstone, igneous extrusives like basalt, igneous intrusives like granite and more. In short, it is the hard mineral stuff rocks are made off.

The "setting" you refer to is the time it takes for the binder(s) to react to produce the final agglomerate, not the shape into which it sets. It would be possible to shape basalt by pouring lava (i.e. molten rock) into a heat-resistant form, going from hot to cold to create the final product. It is also possible to sinter ceramics at high temperature in some desired form, going from cold to hot to cold. Since there are no ovens big enough to sinter buildings this tends to be limited to smaller parts, e.g. refractive (heat-resistant) stones for building hearths etc.

Hate to sidetrack in what is otherwise an excellent set of definitions, but…

> Stone is a portmanteau for the material from which rock is made…

I do not think that word means what you think it means. But I also am not sure what word you were looking for to describe the relationship/distinction between the words ‘rock’ and ‘stone’.

But it’s definitely not portmanteau. A portmanteau word is a word that is made by shoving two or more other words together, like ‘brunch’, or ‘hackathon’.

Is it possible that because a portmanteau is also a word for a large suitcase, you are looking for a word that means ‘word that can contain a lot of distinct sub meanings?’

cement is the binder, usually made of lime, which is mined from limestone.

concrete is cement plus other materials such as gravel.

stones are naturally forming solids made from minerals.

this should help get you started. wikipedia is your friend.

I've looked this stuff up on Wikipedia before asking. There's too much information. A bit like your answer: Detail without explanation. It's like wading through descriptions of gear mechanisms before learning what a wristwatch is.

This article got the closest to answering my question: https://www.thespruce.com/difference-between-cement-concrete...

The summary is that cement is a constituent of mortar and concrete, which is never used on its own, but is only used to make the latter two. Mortar is used to "glue" bricks together. Concrete is a substance which can be made into lots of different shapes before hardening. Where is the summary misleading?

[edit: Changed the word "cement" in the 2nd last sentence to "concrete". That was an accident.]

Concrete is cement + sand + stones. It's basically stones held together by mortar.

You need to have a sufficient range of particle sizes to fill in the gaps between the stones.

An individual stone is strong and has no voids. A pile of stones is weak because it has voids, the stones can roll around each other, and don't have much contact area. In concrete (and asphalt, etc.) the voids between the large stones are filled with sand. The gaps between the sand grains are then filled with cement, which is even finer. You end up with a solid mass of material with minimal voids and all the particles fixed in place.

Concrete is stronger than mortar because a larger portion of it is solid material (the interior of the stones is solid, whereas the "mortar" contains voids between the sand grains).

Thanks. Combined with https://news.ycombinator.com/item?id=32116807, this is a very helpful answer.

There's also "aircrete" which is basically foamed cement. Air bubbles take the places of stones and sand.

> Foam concrete, also known as Lightweight Cellular Concrete (LCC), Low Density Cellular Concrete (LDCC), and other terms is defined as a cement-based slurry, with a minimum of 20% (per volume) foam entrained into the plastic mortar.[1] As mostly no coarse aggregate is used for production of foam concrete the correct term would be called mortar instead of concrete

https://en.wikipedia.org/wiki/Aircrete

It's pretty crazy stuff. It's light enough that it floats on water, and it's waterproof. It's also fireproof, as well as bug-proof and mold- and mildew-resistant. It's very durable but you can work it with hand tools, saw it, drill it, etc. It takes nails. It's also a great insulator.

There are folks who figured out how to make backyard-scale foamers for ~$100 of parts from any hardware store, and they make these beautiful aircrete dome homes: https://www.domegaia.com/

I've also seen companies that manufacture large aircrete bricks that are assembled onsite like giant LEGO blocks.

Structural engineer here. Much of what is said in the other responses is correct, but I will try to explain it as structural engineers do and also use industry-standard terms.

There are many things called concrete. I am going to use the word "concrete" to mean "Portland cement concrete". That's what you are describing in your questions.

Concrete is the stuff sidewalks are made of. Some people use the word "cement" when describing sidewalks. But that is akin to calling bread yeast. Sure, there is cement in concrete like there is yeast in bread but concrete is more than cement as bread is more than yeast. Cement and yeast are the active ingredients only.

Hardened concrete is a composite material made up of aggregates and a binding matrix that holds the aggregates together. Aggregates in concrete include sand (fine aggregate) and gravel or crushed stone (course aggregate). The matrix consists of cement particles and minerals that grow out of those particles as the cement reacts with water: a process we call hydration. These minerals or "hydration products" grow around the aggregates and give the concrete its strength.

What we call cement is rightly called Portland cement. It is added to a concrete mix in the form of a very fine powder. The cement is manufactured from limestone (primarily) that has been fired in a kiln before being ground to that extremely fine powder. The kiln treatment is how it becomes hydraulically reactive.

Material that reacts with water similar to Portland cement also occurs naturally around volcanoes. I believe that is where the Romans got the cement for their concrete. But nowadays we manufacture Portland cement it instead.

Hardened concrete is strong in compression (when squeezed) but weak in tension (when pulled apart). When we need tensile strength, we add steel reinforcement. The resulting composite material is referred to as reinforced concrete. Concrete can also be used without steel reinforcement when it is not subject to tensile loads. This we call either plain concrete or unreinforced concrete.

The word "rock" or "stone" encompasses a wide range of naturally-occurring solid materials made up of various minerals. Limestone, granite, marble, and sandstone are common examples. Most of the solid rock you see around your world is much stronger than concrete. Concrete is an imitation of rock. In fact, the name Portland cement was given to it because the concrete resulting from its use supposedly looks similar to natural stone quarried on the Isle of Portland in the United Kingdom.

I hope this helps. In short: Portland cement, water, and aggregate (sand and crushed natural rock or gravel) are mixed or "batched" and harden by a chemical reaction called hydration to produce a solid material similar to natural stone but with many practical advantages over rock.

>Material that reacts with water similar to Portland cement also occurs naturally around volcanoes. I believe that is where the Romans got the cement for their concrete. But nowadays we manufacture Portland cement it instead.

It is called Pozzolan or Pozzolanic cement, JFYI:

https://en.wikipedia.org/wiki/Pozzolan

Modern Portland cement is rarely "pure Portland" and there is a a trend to increase Pozzolan presence in Portland cement.

Good point. In the United States there are two pozzolanic materials frequently used as "admixtures" in concrete: fly ash and silica fume. Fly ash is far more common. It is a beneficial byproduct of coal combustion. Being a byproduct of another industry, it is less expensive than Portland cement.

There are several reasons for adding it to mix designs. It can replace a portion of the Portland cement without sacrificing long-term strength, so it is added to reduce cost. It adds to the strength by reacting with some hydration products that do not contribute to the strength of the hardened concrete, converting those hydration products to those that do increase strength (calcium-silica hydrate for example). It reduces the permeability of the hardened concrete which improves resistance to corrosion in marine environments. It reduces the heat of hydration (which can be damaging), because it reacts less vigorously than Portland cement. There are probably other benefits, but those are the ones that come to mind.

Yes, we (Italy) often use as well flying ashes added in the concrete mix, to reduce permeability and, besides, it acts in the wet mix as a sort of lubricant and makes the pumping of the concrete much easier.

BTW Pozzolanic is way stronger (in the long run), "pure" Portland based concrete is much faster in setting, but then hits a ceiling, so where there is no particular need for fast setting (massive foundations and similar) Pozzolanic is (was) preferred (and also costed a little less).

What is called Pozzolanic cement here is anyway now essentially Portland cement 65-70% and flying ashes 30-35%, while "pure" Portland is between 90 and 95% Portland clinker and the remaining is limestone.

JFYI: https://news.ycombinator.com/item?id=29377799

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