Carbonation of lime mortars: Photos

[johnandrew]

Just for fun I thought I would post the following photos of the lime mortar sample I made up a few months ago. I split it this morning and applied some phenolpthalein to the newly exposed surfaces. Unfortunately, I forgot to include a ruler in the photo. The sample are about 35mm across. Maybe, I'll try again in six months time and see how much smaller the pink area is.

 

[Flyfisher]

Maybe, I'll try again in six months time and see how much smaller the pink area is.

Ooh yes, please do. It would be very interesting to see how long it takes for complete carbonation.

It would also be interesting to have a number of standardised samples 'on the go' at the same time, but with each sample stored in a slightly different ambient environment. My understanding is that keeping the mortar damp aids carbonation, but it would be interesting to (literally) see how much effect the moisture level really does have.

Thanks for an interesting post.

 

[JoceAndChris]

Eh? Could someone please kindly explain, in girl language, why John is dyeing rocks pink?

 

[johnandrew]

Joce

Pheonolpthalein is a clear liquid that turns pink when it come into contact with a substance with a pH greater than 9 - such as lime (calcium hydroxide)

The photos show how the sample has a varying pH through its section giving an indication of how deep into the sample the lime has carbonated.

John

 

[JoceAndChris]

Hmm. Thank you for trying, off I go to the dictionary. :lol:

 

[worms]

Could someone please kindly explain, in girl language

If it was a cake and you were testing it with a skewer to see if it was done, the skewer wouldn't come out clean.

 

[Flyfisher]

Eh? Could someone please kindly explain, in girl language, why John is dyeing rocks pink?

I thought you girls like pretty pink things.

OK, how about:

Lime mortar 'cures' by reacting with CO2 (carbon dioxide) from the atmosphere. This is why lime putty can be kept almost indefinitely if it is kept covered with water and, thus, out of contact with air.

When exposed to the air, the lime starts to react with CO2 to 'set'. This 'carbonation' process happens fairly slowly and it's normally impossible to tell when the process is complete - we just assume it will take a fair while and let things take their course.

Johnandrew's magic pink liquid (a form of 'indicator' that is a staple of analytical chemistry labs) conveniently turns pink when it is in contact with "unset" lime mortar (calcium hydroxide) but remains clear when in contact with "set" lime mortar (calcium carbonate).

Thus, the photos of a section through a lump of lime mortar show the outer layer of the lump that has "set" (i.e. not pink) because of its exposure to the air, whereas the inner part of the lump is pink because it has not yet had time to react with the air, because the air takes time to permeate through to the centre of the nugget.

Given enough time, the pink section of the nugget will become clear, which would indicate that the entire nugget has fully reacted with the air and is then fully "set".

Hmm. I was hoping that would be clearer but I'm now worrying that it's merely long-winded.

 

[Feltwell]

Hmm. I was hoping that would be clearer but I'm now worrying that it's merely long-winded.

No - sounds very clear to me. O Level Chemistry comes flooding back!

 

[Gareth Hughes]

Now the nugget's been broken, won't carbonation take place far more quickly, thus giving a misleading result in another six months as to what would have happened had it been left intact for a year? (unless, of course, there's an infinite supply of samples to keep us all on tenterhooks for years to come, in which case I might consider opening a book on the de-pinkification over time. Anyone want to start me at 20 years to depinkify? Winner takes the pot)

 

[biffvernon]

I think a great deal depends on dampness. Carbonation will take place quicker if there is a film of water in which the CO2 can dissolve.

 

[johnandrew]

Now the nugget's been broken, won't carbonation take place far more quickly, thus giving a misleading result in another six months as to what would have happened had it been left intact for a year?

Yes, I think that's right. The now exposed surface can no longer be used to give an indication of carbonation throughout the sample. I do have some more of the sample left so could use that in six months time. I'll have a tenner on 10 years to depinkify a sample of the size shown in the photo.

I also think Biff is right in that if I kept spraying the mortar with water on a regular basis the carbonation process would be quicker. In fact I think Biff mentioned in a post many years ago that the carbon dioxide dissolves in water to make carbonic acid (I think) and that is key to good carbonation.

 

[robgil]

Would a sample twice as large take twice as long to carbonate to the same degree?

 

[Flyfisher]

There are lots of variables to play around with; sample size, temperature, humidity, material moisture content, etc, so a full study could take a very long time indeed.

For all practical DIY purposes I suspect "a long time" is probably all we really need to know. :wink:

 

[johnandrew]

Would a sample twice as large take twice as long to carbonate to the same degree?

I think it might take more than twice as long. The relationship probably has something to do with the ratio of surface area to volume - and all the variables FF mentions above.

For anyone really interested some chap called Robert Lawrence did a Ph.D on the carbonation of non-hydraulic lime mortars at Bath University in 2006, which is available to view at http://opus.bath.ac.uk/394/. That has some good photos of carbonation over time (a year, I think).

 

[AMc]

Would a sample twice as large take twice as long to carbonate to the same degree?

If both samples were spherical then I would expect the carbonation to get progressively slower as you go 'into' the ball there is less opportunity to expose the mortar to CO2?

 

[robgil]

Would a sample twice as large take twice as long to carbonate to the same degree?

If both samples were spherical then I would expect the carbonation to get progressively slower as you go 'into' the ball there is less opportunity to expose the mortar to CO2?

Thats how I see it as well , so we could assume that lime mortar will be softer in the middle and get harder towards the outer layers.

 

[Flyfisher]

Only for a period of time. As I understand things, all the mortar will eventually harden to the same degree once carbonation has occurred throughout the mortar mix.

 

[Zebra]

Please may I ask whether these samples were made with hydraulic or hydrated lime or putty? I'm still trying to understand the difference, and then I remembered this thread.

 

[MdB]

It doesn't matter, they would all react the same. All start off with uncarbonated lime and slowly carbonate using the carbon dioxide from the air.

Hydraulic lime also has clay in it which causes it to set using a different chemical process but my understanding is that this doesn't change the carbonation process.