Crack in newly NHL 3.5 plastered wall

[skier-hughes]

Is there a good site which explains the differences between hydraulic and non-hydraulic and why you shouldn't use hydraulic for internal walls?

 

[Gervase]

Mike Wye's site, the IHBC 'Context' archive and the Building Conservation fact sheets have some info, but I don't think they explain exactly why.
Essentially it's because hydraulic lime has a chemical set, whereas fat lime relies on carbonation.
The chemical set results in a harder but more brittle crystaline structure. The slower carbonation allows for a quantity of 'free lime' in the mortar or plaster which gives the material its ability to 'self heal' to accommodate cracks and movement.
Fat lime, because of its slower set, is easier to work for longer, allowing the plasterer to compact the material and close up any shrinkage cracks over time.
Hydraulic lime scores where you need structural strength - an NHL5 mortar after 28 days has up to ten times the compressive strength of a fat lime mortar. And, of course, it sets when wet, making it ideal for exposed conditions. And, as it is less permeable, it is better for exposed surfaces subject to penetrating damp when used in renders or roughcast.

 

[skier-hughes]

Cheers,

Will search and read.

Graham

 

[ajbaker]

Now that Christmas has truly finished I get to return to the laborious labour of love that is my house! What better way to start than by replying to all of the posts I have let slide recently!

Despite there being little progress on either my lounge or bedroom I though I should reply to this thread. First to thank everyone who has offered advice and to provide a brief update.

Thanks Nemesis for your comments; regarding the windows - were you referring to the actual windows (in which case, yes, they are lovely) or the bricked in space that used to be a window in the bedroom?

Gervase. I am not quite sure how you spotted the vertical line of bricks being set-back from the normal course in those pictures - but you are absolutely right, and something I neglected to mention. The plaster in this area is probably over an inch thick. This must therefore be the reason for the cracking, especially given the time between coats (a day or two!) Plasterers yet to come and review the problem (my fault, not theirs).

Updated pictures...

 

[bournonite]

How warm was the room when it was applied... ? This stuff needs plenty of time, and it will crack like nothing if you dry it out too quickly. You need to keep it cool and damp to allow carbonation to start - if it was applied in a working house with heating on - and dried out within say, a week or two, then you've only yourself to blame - it needs at least a month or so with no heat and to be allowed to carbonate naturally....

Cheers

Pete

 

[andrew gayton]

carbonation? - chemical set surely?...... hence the hydraulic specification.

 

[Gervase]

carbonation? - chemical set surely?...... hence the hydraulic specification.

Yup - that was what seemed so odd about the wall in the first place.
Although...
Even in an NHL5 there is a significant amount of ordinary lime among the magnesium compounds and the silicates that induce the catalysed chemical set.
As it understand it, that 'set' is a result of crystal formation within the mortar/plaster, but the insterstices between those crystals are filled with uncatalysed lime, which will eventually carbonate in its own sweet way and add its strength to the crystal lattice, hence the propensity of hydraulic slabs to get stronger over time as the crystals are 'locked' together.
I may be completely wrong, however!

 

[biffvernon]

Spot on, Gervase. Below is a spec for NHL3.5 Singleton Birch lime that I happen to have lying about. After the various active ingredients have done their bit to produce the 'hydraulic set' there will still be a large part of the hydrated lime available for carbonation.


Chemical Analysis Typical % Mass

Calcium Hydroxide Ca(OH)2 75.0
Calcium Carbonate CaCO3 8.0
Silica SiO2 6.25
Aluminium Oxide Al2O3 0.79
Ferric Oxide Fe2O3 0.9

 

[andrew gayton]

although the hydraulic set itself impacts on the carbonation process. so, strictly speaking the carboation process differs from a fat lime carbonation.

 

[ajbaker]

Pete

Thanks for your response, and I'm guessing you didn't intend it to sound as harsh as I may have interpreted it.

As professionals it would have been the responsibility of the plasterers to inform me the heating should be off etc. As it happens I know lime requires time, moisture and no extremes. Also, if NHL required this as much as non-hydraulic they certainly wouldn't have completed three coats in five days!

As regards the environment, the room had no heating. Also, non-hydraulic lime mortar was applied to the walls and ceiling in the room above. One coat was not even starting to cure after 5 days because temperatures we around 5, so we had to introduce a little background heat (10- 15 degrees). These walls have shown little sign of major cracking.

Ever more inclined to think NHL was the wrong choice, and the depth of the plaster cannot help.

 

[Penners]

This is all very interesting (and that's not said with irony!)

I just wish I understood it.

 

[Moo]

I feel exactly the same regarding the thread about re-wiring costs.

 

[Penners]

Actually, Moo, nobody else understands that thread either - not even the people writing it. They've forgotten that all you need for TV is a big H-shaped aerial on your chimney, with a bit of wire running to the TV. You'll see Sylvia Peters and Kenneth Baker as clear as a bell.

 

[bournonite]

Certainly didnt want to sound harsh...!

Think you've hit the nail as it were - the thicker the coat, the longer the set - carbonation takes place in exposure to air - so thick = time. You CAN introduce an NHL component to try and make it a quicker job - but I dont think the purists would recommend it. The second coat needs to go on when the base is just slightly damp - in ordinary conditions about 3 to 5 days - and top coat probably the same I guess... Bottom line - cracks in lime = setting / drying too quickly... for whatever reason..

If it helps, we've just done a big lime plastering job, 3 coat, and have experienced a bit of cracking on a wall where there is a radiator on the other side - in the heated side of the house - so it happens to us all!


Cheers!

Pete

 

[Flyfisher]

This is all very interesting (and that's not said with irony!)

I just wish I understood it.

Ditto. I'm still low down on the lime learning curve and it doesn't help when 'natural hydraulic lime' and 'non-hydraulic lime' have the same initials :roll:

As for the re-wiring thread, that's easy-peasy compared with all this black art building stuff :wink:

 

[Penners]

As for the re-wiring thread, that's easy-peasy compared with all this black art building stuff

Everything is easy-peasy when you know how. :?