# Why exactly does cement work? **URL:** \ **Category:** Factual Questions\ **Created:** [July 22, 2010, 12:53am UTC](https://boards.straightdope.com/t/why-exactly-does-cement-work/547537 "2010-07-22T00:53:15Z")\ **Posts on this page:** 18\ **Page:** 1
**Author:** ![Jinx](https://avatars.discourse-cdn.com/v4/letter/j/c6cbf5/32.png) [@Jinx](https://boards.straightdope.com/u/Jinx)\ **Post date:** [July 22, 2010, 12:53am UTC](https://boards.straightdope.com/t/why-exactly-does-cement-work/547537/1 "2010-07-22T00:53:15Z")
Why does cement work at all? Why doesn’t the mix just make soup or a muddy mess? Also, when cementing something in place, does it act like a super glue…or does it add so much weight, the cemented object cannot move (under reasonable conditions)??? ---
**Author:** ![freckafree](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/freckafree/32/3050_2.png) [@freckafree](https://boards.straightdope.com/u/freckafree)\ **Post date:** [July 22, 2010, 1:06am UTC](https://boards.straightdope.com/t/why-exactly-does-cement-work/547537/2 "2010-07-22T01:06:08Z")
First of all, don’t confuse cement with concrete. The terms tend to be used interchangeably (especially in reference to capping the BP well), but they are different substances. Concrete is made from cement and aggregate. Cement hardens or cures because of a chemical reaction. It sort of acts like super glue, in that it binds things together, like the aggregate particles in the concrete. ---
**Author:** ![Dag\_Otto](https://avatars.discourse-cdn.com/v4/letter/d/439d5e/32.png) [@Dag\_Otto](https://boards.straightdope.com/u/Dag_Otto)\ **Post date:** [July 22, 2010, 5:09am UTC](https://boards.straightdope.com/t/why-exactly-does-cement-work/547537/3 "2010-07-22T05:09:01Z")
It doesn’t work at all. ---
**Author:** ![johnpost](https://avatars.discourse-cdn.com/v4/letter/j/f17d59/32.png) [@johnpost](https://boards.straightdope.com/u/johnpost)\ **Post date:** [July 22, 2010, 2:28pm UTC](https://boards.straightdope.com/t/why-exactly-does-cement-work/547537/4 "2010-07-22T14:28:01Z")
water becomes part of a molecule containing the mineral atoms ---
**Author:** ![moldybread](https://avatars.discourse-cdn.com/v4/letter/m/7ea924/32.png) [@moldybread](https://boards.straightdope.com/u/moldybread)\ **Post date:** [July 22, 2010, 2:35pm UTC](https://boards.straightdope.com/t/why-exactly-does-cement-work/547537/5 "2010-07-22T14:35:29Z")
I know theres a chemical reaction involved as freckafree stated. I watched a guy break a bone and decided to make himself a makeshift cast out of concrete, it caused some pretty serious chemical burns on his arm. Im sorry i don’t have a reason why it works for you ☹ ---
**Author:** ![purplehorseshoe](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/purplehorseshoe/32/2996_2.png) [@purplehorseshoe](https://boards.straightdope.com/u/purplehorseshoe)\ **Post date:** [July 22, 2010, 2:37pm UTC](https://boards.straightdope.com/t/why-exactly-does-cement-work/547537/6 "2010-07-22T14:37:23Z")
> [@Jinx](#): > > Why does cement work at all? Not sure what you mean by “why” but if you meant “how” \*\*Freck \*\*answered pretty well. It doesn’t “dry” the way mud dries into clay, which would be water evaporating out - it’s an actual chemical reaction. > [@Jinx](#): > > Why doesn’t the mix just make soup or a muddy mess? Well, it does. At first. 😃 Then it cures, and hardens. > [@Jinx](#): > > Also, when cementing something in place, does it act like a super glue…or does it add so much weight, the cemented object cannot move (under reasonable conditions)??? OK, this just has me all 😕. What did you have in mind when you typed “something” ?? It can act like super glue, and it is heavy. So … both? I guess? ---
**Author:** ![Arkcon](https://avatars.discourse-cdn.com/v4/letter/a/96bed5/32.png) [@Arkcon](https://boards.straightdope.com/u/Arkcon)\ **Post date:** [July 22, 2010, 2:46pm UTC](https://boards.straightdope.com/t/why-exactly-does-cement-work/547537/7 "2010-07-22T14:46:15Z")
You can find better specifics on wikipedia than I’m about to give you. But to help you understand, what I hope you’ll eventually read … cement starts out as a rock, we roast it in a kiln, driving off water and carbon dioxide gas. We ship the powder in bags. We add water and shape it, and while wet, it absorbs carbon dioxide from the air. It becomes rock again, in the shape we gave it. Well, first off, I’m ignoring the effects of silicates in the rocks that are added, and changing composition along the way. But basically, was rock, roasted to powder, becomes rock again is a good model to hold in your head as you read more. The great thing is, you don’t need to use limestone, to roast to get lime, to make cement. Even before ancient Rome, people have known, you can roast seashells, to get lime, to make cement. And you get to have dinner first. > **[Cement](https://en.wikipedia.org/wiki/Cement#The_setting_of_cement)** > > A cement is a binder, a chemical substance used for construction that sets, hardens, and adheres to other materials to bind them together. Cement is seldom used on its own, but rather to bind sand and gravel (aggregate) together. Cement mixed with fine aggregate produces mortar for masonry, or with sand and gravel, produces concrete. Concrete is the most widely used material in existence and is behind only water as the planet's most-consumed resource. > Cements used in construction are usually i... ---
**Author:** ![Wallenstein](https://avatars.discourse-cdn.com/v4/letter/w/b5a626/32.png) [@Wallenstein](https://boards.straightdope.com/u/Wallenstein)\ **Post date:** [July 22, 2010, 2:51pm UTC](https://boards.straightdope.com/t/why-exactly-does-cement-work/547537/8 "2010-07-22T14:51:22Z")
> [@moldybread](#): > > I know theres a chemical reaction involved as freckafree stated. I watched a guy break a bone and decided to make himself a makeshift cast out of concrete, it caused some pretty serious chemical burns on his arm. Im sorry i don’t have a reason why it works for you ☹ Plaster of Paris can have the same effect: > [@](#): > > A school was ordered to pay £19,000 […] after a 16-year-old girl lost most of her fingers when she put her hands in a bucket of plaster of Paris during a school art lesson. > > The teenager was attempting to make a sculpture of her own hands during a lesson in January 2007 when the horrific accident happened, Boston magistrates court in Lincolnshire was told. > > The plaster set around her hands and neither staff nor paramedics could get it off during the lesson at Giles school, in Boston. > > The court was told that temperatures up to 60C can be generated in large quantities of plaster and the girl, who was referred to in court only as student X, suffered terrible burns. > > [Pupil lost most of her fingers in school art accident, court told | UK news | The Guardian](http://www.guardian.co.uk/uk/2009/oct/12/school-fined-art-accident) ---
**Author:** ![Chefguy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chefguy/32/138_2.png) [@Chefguy](https://boards.straightdope.com/u/Chefguy)\ **Post date:** [July 22, 2010, 2:57pm UTC](https://boards.straightdope.com/t/why-exactly-does-cement-work/547537/9 "2010-07-22T14:57:35Z")
> [@purplehorseshoe](#): > > OK, this just has me all 😕. What did you have in mind when you typed “something” ?? It can act like super glue, and it is heavy. So … both? I guess? I think the OP was referring to something like a steel signpost or stanchion. ---
**Author:** ![FluffyBob](https://avatars.discourse-cdn.com/v4/letter/f/ecb155/32.png) [@FluffyBob](https://boards.straightdope.com/u/FluffyBob)\ **Post date:** [July 22, 2010, 3:10pm UTC](https://boards.straightdope.com/t/why-exactly-does-cement-work/547537/10 "2010-07-22T15:10:55Z")
Cement (Portland Cement) does function as an adhesive when used in mortars for masonry purposes or to adhere stucco or tiles to a surface. In concrete it is essentially the binding agent for the aggregate. It is almost always used in combination with aggregates to add body and other additives for various purposes such as flexibility, chemical resistance, temperature. Concrete is very strong in compression but weak in tension and shear, so is usually reinforced with steel rebar, mesh and some times other materials such as nylon or glass fiber. When used as a foundation material in construction, concrete serves to add weight to the lowest part of a structure, as a platform to spread forces and has the added advantage that it fills voids as it is poured. Concrete does not ‘glue’ a post or building into the ground, it works more like an anchor, using its weight and its form to provide a stable foundation. Architects sometimes describe building foundations as ‘floating’ in the ground. ---
**Author:** ![tallcoldone](https://avatars.discourse-cdn.com/v4/letter/t/e9a140/32.png) [@tallcoldone](https://boards.straightdope.com/u/tallcoldone)\ **Post date:** [July 22, 2010, 4:19pm UTC](https://boards.straightdope.com/t/why-exactly-does-cement-work/547537/11 "2010-07-22T16:19:03Z")
Cement/concrete can also damage human tissues due to the corrosive nature of the materials - they are relatively high in pH and can irritate skin etc. when in contact with them for an extended period. ---
**Author:** ![Quercus](https://avatars.discourse-cdn.com/v4/letter/q/7ab992/32.png) [@Quercus](https://boards.straightdope.com/u/Quercus)\ **Post date:** [July 22, 2010, 4:33pm UTC](https://boards.straightdope.com/t/why-exactly-does-cement-work/547537/12 "2010-07-22T16:33:36Z")
> [@freckafree](#): > > First of all, don’t confuse cement with concrete. The terms tend to be used interchangeably (especially in reference to capping the BP well), but they are different substances. Concrete is made from cement and aggregate. In case it’s not clear, in this context ‘aggregate’ means small (crushed) rocks. Probably more or less gravel-sized for concrete. ---
**Author:** ![flight](https://avatars.discourse-cdn.com/v4/letter/f/bbce88/32.png) [@flight](https://boards.straightdope.com/u/flight)\ **Post date:** [July 22, 2010, 6:36pm UTC](https://boards.straightdope.com/t/why-exactly-does-cement-work/547537/13 "2010-07-22T18:36:49Z")
> [@moldybread](#): > > I know theres a chemical reaction involved as freckafree stated. I watched a guy break a bone and decided to make himself a makeshift cast out of concrete, it caused some pretty serious chemical burns on his arm. Im sorry i don’t have a reason why it works for you ☹ Note that part of it might have been chemical burns, but much of it likely was just regular old burns. Concrete curing is exothermic, and it can get quite hot. ---
**Author:** ![Ludovic](https://avatars.discourse-cdn.com/v4/letter/l/7ab992/32.png) [@Ludovic](https://boards.straightdope.com/u/Ludovic)\ **Post date:** [July 22, 2010, 6:54pm UTC](https://boards.straightdope.com/t/why-exactly-does-cement-work/547537/14 "2010-07-22T18:54:13Z")
> [@Dag\_Otto](#): > > It doesn’t work at all. That’s the beauty of it! ---
**Author:** ![mascaroni](https://avatars.discourse-cdn.com/v4/letter/m/dc4da7/32.png) [@mascaroni](https://boards.straightdope.com/u/mascaroni)\ **Post date:** [July 22, 2010, 7:17pm UTC](https://boards.straightdope.com/t/why-exactly-does-cement-work/547537/15 "2010-07-22T19:17:30Z")
> [@flight](#): > > Note that part of it might have been chemical burns, but much of it likely was just regular old burns. Concrete curing is exothermic, and it can get quite hot. No cite, but having dealt with cementitious materials for half my working life I find it hard to believe an amount the size of a plaster cast would create a sufficient temperature to be noticable, let alone cause heat burns. OTOH, it only takes a small amount (a hole in a rubber glove for instance) to cause “lime burn”, which (cite - my right index finger at the moment) you don’t notice until it hurts. My finger will recover, but people have had legs amputated as a result of kneeling unprotected in concrete for hours at a time. (Health & Safety Executive poster on a site canteen wall c.2006). ---
**Author:** ![PsyXe](https://avatars.discourse-cdn.com/v4/letter/p/54ee81/32.png) [@PsyXe](https://boards.straightdope.com/u/PsyXe)\ **Post date:** [July 22, 2010, 10:12pm UTC](https://boards.straightdope.com/t/why-exactly-does-cement-work/547537/16 "2010-07-22T22:12:56Z")
If I recall correctly the curing process, if one could see it, would involve a lot of tiny fibres (long, thin crystalline structures, perhaps?) extruding in starlike patterns and getting tangled up / interlocking with each other. ---
**Author:** ![Wallenstein](https://avatars.discourse-cdn.com/v4/letter/w/b5a626/32.png) [@Wallenstein](https://boards.straightdope.com/u/Wallenstein)\ **Post date:** [July 23, 2010, 7:51am UTC](https://boards.straightdope.com/t/why-exactly-does-cement-work/547537/17 "2010-07-23T07:51:27Z")
When the Hoover Dam was constructed the workmen had to pour the concrete in small sections - it was calculated that if it was poured all at once they would have to wait 125yrs for the concerete to cool to ambient temperatures. ---
**Author:** ![chorpler](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chorpler/32/2888_2.png) [@chorpler](https://boards.straightdope.com/u/chorpler)\ **Post date:** [July 24, 2010, 1:58am UTC](https://boards.straightdope.com/t/why-exactly-does-cement-work/547537/18 "2010-07-24T01:58:42Z")
> [@Wallenstein](#): > > When the Hoover Dam was constructed the workmen had to pour the concrete in small sections - it was calculated that if it was poured all at once they would have to wait 125yrs for the concerete to cool to ambient temperatures. And there were chiller pipes circulating water laid in the concrete, to help it set faster. Or so they tell us every time we go on “the dam tour.”