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Author Topic: Why Does Concrete Create Heat Whilst Setting ?  (Read 23345 times)

Offline neilep

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Why Does Concrete Create Heat Whilst Setting ?
« on: 12/02/2008 20:27:23 »
Dear Concreteologists ,


See this palette of concrete ?






Nice eh ?...notice how stagnant and non heat producing it looks !!........

Y'see.....I recently heard that when concrete gets hard it generates heat !!!.....what's that all about ?....why's that then ?


What else produces heat as it gets hard ? *norty chortle*

Bad joke...but I ceMent it !!! *lol....even worse joke*






 

another_someone

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Why Does Concrete Create Heat Whilst Setting ?
« Reply #1 on: 12/02/2008 21:47:19 »
In general, anything you do, if it is normally irreversible, will mean that the end product is energetically more stable than what you started out with, and that energetic stability will cause heat to be given off in the process.  How much heat can depend on many things, but even just dissolving salt into water will give off some slight heat.
 

Offline lightarrow

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Why Does Concrete Create Heat Whilst Setting ?
« Reply #2 on: 13/02/2008 08:01:15 »
In general, anything you do, if it is normally irreversible, will mean that the end product is energetically more stable than what you started out with, and that energetic stability will cause heat to be given off in the process.  How much heat can depend on many things, but even just dissolving salt into water will give off some slight heat.

No, the example was bad! Dissolving salts (almost always) absorbs heat. The "energetically more stable" end product must be meant in this way: Gibbs free energy G is less for the end product; that is, ΔG < 0 for a spontaneous (= happens) reaction. But ΔG is not only heat, it's also variation of entropy; a reaction can happen also because the end products has greater entropy than the initials, without the need to release heat; on the contrary, the reaction could absorb heat (as in the case of dissolving most salts in water).

 ΔG = ΔH - TΔS

ΔH is the enthalpy variation, that corresponds to heat if the reaction happens at constant pressure.
 

another_someone

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Why Does Concrete Create Heat Whilst Setting ?
« Reply #3 on: 13/02/2008 08:16:40 »
I would pick one of the few cases where the generality did not work  [:I]
 

Offline that mad man

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Why Does Concrete Create Heat Whilst Setting ?
« Reply #4 on: 13/02/2008 16:48:53 »
The Portland cement that is used in making concrete is very alkaline and contains a number of hydroxides including calcium and sodium hydroxide.

When this is mixed with water the reaction gives off heat as it sets. The setting times can also vary (quick setting) depending on extra included chemicals such as ammonium sulphate?

The quick setting stuff also gives off more heat as it sets.

PS. Its important to wear gloves etc when mixing and using cement as prolonged exposure to skin can also cause caustic burns.
 

Offline lightarrow

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Why Does Concrete Create Heat Whilst Setting ?
« Reply #5 on: 13/02/2008 19:19:54 »
The Portland cement that is used in making concrete is very alkaline and contains a number of hydroxides including calcium and sodium hydroxide.

When this is mixed with water the reaction gives off heat as it sets. The setting times can also vary (quick setting) depending on extra included chemicals such as ammonium sulphate?

The quick setting stuff also gives off more heat as it sets.

PS. Its important to wear gloves etc when mixing and using cement as prolonged exposure to skin can also cause caustic burns.
hmmm...In case, I'd say heat is produced during the acid-base reaction between calcium hydroxide and silica to form calcium silicates.
 

Offline lightarrow

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Why Does Concrete Create Heat Whilst Setting ?
« Reply #6 on: 13/02/2008 19:24:18 »
What else produces heat as it gets hard ?
Some kinds of glue, especially those with two components (when the mix begins to heat up, you have just a few seconds to place it, befor it hardens!).
 

Offline that mad man

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Why Does Concrete Create Heat Whilst Setting ?
« Reply #7 on: 13/02/2008 20:04:42 »
You may be right lightarrow, when it comes to chemistry I'm dumbish.

I know sodium hydroxide can create heat when mixed with other substances, was not sure about the calcium.

The quick setting stuff gives off a strong smell of ammonia but I'm not sure what the ammonia does exactly except evaporate quicker and smell bad. The floor levelling stuff can be hard in less than 15 minutes!

 

Offline lightarrow

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Why Does Concrete Create Heat Whilst Setting ?
« Reply #8 on: 14/02/2008 08:30:06 »
You may be right lightarrow, when it comes to chemistry I'm dumbish.

I know sodium hydroxide can create heat when mixed with other substances, was not sure about the calcium.

The quick setting stuff gives off a strong smell of ammonia but I'm not sure what the ammonia does exactly except evaporate quicker and smell bad. The floor levelling stuff can be hard in less than 15 minutes!


Don't know the composition of Portland cement, but if it also contains ammonium sulphate, than certainly calcium hydroxyde also reacts with it exothermically to form calcium sulphate and releasing ammonia. I was referring to the standard composition, with calcium hydroxyde and silica only (or the main composition).
 

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Why Does Concrete Create Heat Whilst Setting ?
« Reply #8 on: 14/02/2008 08:30:06 »

 

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