Naive lime question

[BMcD]

If it were me, I'd not use hydrated lime to make putty in the first place (especially for ceilings) as you can never be certain that the lime within the bag, even though still a powder, has not partially carbonated and will therefore result in a weak set. I always use well matured putty made from slaking with an excess of water, that way you know that the lime hasn't carbonated. I appreciate that many have had successful results from using hydrated lime but for me it's all about control and minimizing variables and unpredictability.

 

[MdB]

... but for me it's all about control and minimizing variables and unpredictability.

and not cost....

 

[Nemesis]

Fresh lime in a plastic lined bag is far less likely to have carbonated.

 

[BMcD]

MdB wrote:

and not cost....

Since ultimately I'm not the one paying for it, then, no...
It is only right and proper though that when people are paying me to do a job, I use the best materials available to me and use them to the best of my ability as I'm sure anyone who is paying for work to be done would agree. After all who wants to pay again because the materials turned out to be duff.

Nemesis wrote:

Fresh lime in a plastic lined bag is far less likely to have carbonated.

Far less likely but not as unlikely as putty kept in water.

 

[Mark]

... but for me it's all about control and minimizing variables and unpredictability.

and not cost....

I'm sure the ancients measured it out carefully and didnt let any nasty impurities get into their lime

I do think that putty is better for being left a week rather than used immediately but I honestly dont find anything better about the stuff I've bought as putty rather than made. I've also found that I dont need to do a huge amount of stirring when puttin gthe dry in.

Handy to get the tubs though. I find one sack of lime makes two tubs of putty (in those white bucket/tub things). I cant carry a dustbin full.

 

[MdB]

I'm sure the ancients measured it out carefully and didnt let any nasty impurities get into their lime

But they would have gone from quick lime to putty directly (I assume - I am sure someone will correct me on this). They wouldn't have left it as hydrated lime in a bag at the local builders merchant to carbinate when it gets damp.

BMcD has a point that you don't want to use bagged lime that has got damp. I've also not found anywhere so far that sells it in plastic bags, they are all paper which seems silly to me.

 

[Nemesis]

Look inside the paper - mostly they are plastic lined nowadays.

 

[BMcD]

Mark wrote:

I'm sure the ancients measured it out carefully and didnt let any nasty impurities get into their lime

Their used to exist in mediaeval times, teams of highly skilled itinerant workers travelling around Europe whose sole job was to make the best mortars possible using the local materials

Nemesis wrote:

Look inside the paper - mostly they are plastic lined nowadays

But that only makes them moisture resistant and not damp proof. If they've been stored in damp conditions then the plastic liner may not keep the dampness out .

Besides which it's best practice to use putty made when slaked

 

[Nemesis]

Well, I know all the arguments and used to make them myself, but I have to say (although we have not used it in great quantity as yet, but I know others have) bagged lime mixed and kept seems to make a very usable putty.

 

[BMcD]

As I said earlier, I appreciate people are getting very good results from hydrated lime but for me I want to take that element of risk out of my work and know that the materials I use are of a more predictable quality. I'm not going to offer my customers what I consider to be an inferior product. And I'm going to make no apology for adopting best practice in my work, surely that's what people want from their craftsmen

 

[Nemesis]

Sure - just if you are DIYing and cash is tight, it's worth trying.

 

[MdB]

Look inside the paper - mostly they are plastic lined nowadays.

Not the ones I've bought. I had a look the last time this issue was raised.

 

[Mark]

personally I think people get way too uptight about it. I've never bought the 'reassuringly expensive' argument. Either now or in the middle ages, sure people earned a living making mortar and I'm sure they claimed it was the best.

I buy bagged lime from a local builders merchant. Its in plastic lined bags, stored in the dry and gives perfectly good putty.

I rather think that a bag would have to be a long way gone before there was a problem and that would be easy to see. So what if there is some carbonation? Whats the consequence? some CaC03 in the mix - which is what it will set to anyway. So think of it as just upping the aggregate level a bit.

Whats the actual consequence? Mortars dont need to be very strong and you can easily push the ratios out to 5:1 or greater and still get acceptable results. That equates to a 40% or so spoil rate on a standard 3:1 mix.

 

[Penners]

Speaking as a complete ignoramus on this (and so many other) subject, can someone explain to me exactly what "carbonation" is? I presume it's the process that takes place when the putty hardens (is that right?) but, chemically, exactly what's going on?

And while we're at it, cement will harden underwater. Why won't lime?

 

[MdB]

lime + heat --> quick lime + carbon dioxide
CaCO3 + heat --> CaO + CO2

quick lime + right amount of water --> hydrated lime + heat
CaO + H2O --> Ca(OH)2 + heat

hydrated lime + extra water --> lime putty
Ca(OH)2 + H2O --> Ca(OH)2 + H2O (i.e. no change chemically)

lime putty - water + carbon dioxide --> lime
Ca(OH)2 - H2O + CO2 --> CaCO3

The last stage is carbonation, i.e. the carbon is put back into the mix by extracting carbon dioxide from the air. While the putty is wet, the carbon dioxide can't reach the mix so carbonation doesn't occur. I'm not sure why it doesn't occur during the hydrate stage though.

 

[Nemesis]

Lime will harden - just it needs to be hydraulic to harden under water.
http://www.buildingconservation.com/articles/limebasic/limebasic.htm

Carbonation is the process by which non-hydraulic limes 'set' - out in the air. if under water, they don't.


Hydraulic limes harden by being mixed with water, a chemical set.

Yes of course it's all much more complex than that.

http://www.hullcc.gov.uk/pls/portal/docs/PAGE/HOME/PLANNING/CONSERVATION/LISTED%20BUILDINGS/LIME_POINTING.PDF

 

[biffvernon]

While the putty is wet, the carbon dioxide can't reach the mix so carbonation doesn't occur. I'm not sure why it doesn't occur during the hydrate stage though.

It does, but very slowly. The bag (even better if plastic lined) stops the CO2-laden air wafting past. But the reaction is slow when dry. It goes much faster if there is a film of water on the hydrated lime into which the CO2 can dissolve (hence keep the bagged lime dry).

After you've done you brick-laying/plastering or whatever, it is best to keep the lime damp for as long as possible. Carbonation proceeds more quickly in the aqueous environment and furthermore the crystals of calcite (CaCO3) have a chance to grow larger, increasing the strength of the resulting material.

 

[Penners]

Even as I was typing my question, above, I had this nagging feeling that the answer would be... er... challenging.

The strange thing is that, looking at MdB's explanation of the chemical cycle, it almost looks as though you're going through a long, complex process, but ending up back where you started.

As you can tell, chemistry is not my strongest suit.

 

[worms]

ending up back where you started

Precisely! Except that now instead of a limestone rock formation, you have a wall!

 

[biffvernon]

And that's why it's called the lime cycle - it just goes round and round.