Help with Stucco Cracks

[maver]

Re: Help with Stucco Cracks

Neil.....or others
just back on the stucco/render

using a mix of 3:1 (plaster sand, cement).....
what would lime do to the mix eg
3:1:1 (plaster sand, cememt, lime)

reduce cracking?

I bought some with the last lot of gear to stucco/render the outside of my dome (final weatherproof coat)......but I forgot why its been a few pizzas since then

I'm certainly no expert, but I used a scratch coat with portland cement and sand and finished with white sand and lime. I found the lime coat much easier to work with - it was significantly more sticky, and it did not crack at all, which my scratch coat did. It is not likely as weather resistant or durable as the portland, but it looks good, and I have a roof overhang to protect the sides of the oven (the stucco). I may still paint the oven after the move - new house, new colors.

[Balty Knowles]

Re: Help with Stucco Cracks

Damon,

I recall some thread on the forum with reference to a specialist who said that lime has no part in refractory materials, (even though I used it in my mortar for the dome).

Neill

See the thread on refractory mortar question. I was having problems with the formula in the plans & called the chemist at Lumnite. He told me that lime is used as an accelerator for portland cement but calcium aluminate goes off very fast & lime would exacerbate this. Said to omit it all together for his product.

[james]

Re: Help with Stucco Cracks

Hey Balty,
I have moved this thread to Design and Finish. This seems to be the right place for a very valuable posting.
James

[Bacterium]

Re: Help with Stucco Cracks

thanks all....recapping I think this is how I'll do it:

-First (scratch) coat will be a 3:1 mix (plaster sand/portland), slighly drier mix, laid on thick - and I'll scratch up surface once on

- Next coat (on top of first) will be same ratio, thinner coat, slightly wetter. I'll mix in the oxide (by weight) to get the desired colour. I'll do this coat either same/next day to first.....its winter here so I guess it will be 2 days cure at least.

Once its all cured/dried I'll get some clear surface coating to maintain the look......but the actual waterproofness will really be from the 3:1 mix

[nissanneill]

Re: Help with Stucco Cracks

Bacterium,
Now your talking. I wouldn't mix the render too dry, but on the other hand not too wet either.
You need it wet enought to be able to trowel it easily and if you are going to put a "final" waterproofing top coat, a wooden trowel will give you a better surface for the last one to bond to. Steel floats bring the water to the surface and give you a smooth shiny surface which is what you do not want on sub coats. Add enough water for the thinner (rather than a thick layer say 1/2 to 3/4" thick) for the mortar to stick and readily hold together.
Your last or top coat can be at the same wetness consistency but use a steel trowel to provide your smooth water proofed surface.

Neill

[Bacterium]

Re: Help with Stucco Cracks

thanks Neill
.........if anything made me nervous building the oven it was this bit. I'm a sparky (electrician) & I.T. guy not a bricke etc..

Its all fun though.....thanks for starting the post Balty

[Unofornaio]

Re: Help with Stucco Cracks

thanks all....recapping I think this is how I'll do it:

-First (scratch) coat will be a 3:1 mix (plaster sand/portland), slighly drier mix, laid on thick - and I'll scratch up surface once on

- Next coat (on top of first) will be same ratio, thinner coat, slightly wetter. I'll mix in the oxide (by weight) to get the desired colour. I'll do this coat either same/next day to first.....its winter here so I guess it will be 2 days cure at least.

Once its all cured/dried I'll get some clear surface coating to maintain the look......but the actual waterproofness will really be from the 3:1 mix

If we are talking stucco here the "color coat" (very last coat) need only be around 1/8" thick and of a consistency you can almost paint on, professionally it is mixed like soup and put on with a stiff sponge float. It takes A LOT of color to change the color of Portland, unless you are using white cement which I don't think you are.
As far as lime goes it is still a very valuable component of masonry work now and will be in the future.It is not out dated by any means, adding lime to mortar for (lime, cement, sand) is the only way I lay bricks,block or stone it was the way I was taught and it works much better than a chemical plasticize additive. Lime helps the mortar sick, makes it more easily trowlable and keeps its board life up. As far as mixing it with the re-factory stuff defiantly do not.
Thick is something you want to avoid at this step much better to do 3 steps
Scratch,brown, finish (color) too thick a coating will give you cracks every time.
You already have the scratch coat (although its not scratched) get a pint of "concrete bonding adhesive" about 6 or 7 dollars this is a glue (I swear its just Elmer's glue) if you get it smell it and see what you think. we use this all the time for bonding old and new cement it works very,very good.
Paint this on, wait a bit put your brown coat on about 1/2" with a sponge float make it nice and smooth. Its consistency should be a little wetter than the mortar used to lay brick but no where near dry.. brown coats are mixed at 1 part cement to 3 to 5 parts sand.
finish coats are 1 part cement to 1-1/2 to 3 parts sand. If you choose to add lime count it as cement.
J.P.


F.Y.I. you said in another post "Maybe I'll swap the lime for some oxide instead" actually the oxide is a form of lime, Calcium oxide (CaO) this is different that hydrated lime (masons lime) Ca(OH)2

[Bacterium]

Re: Help with Stucco Cracks

Hi Uno,

yeah I'm using Portland cement (not white). When I mean oxide its the stuff we use as a colouring agent. Say for 5kg (11 pounds) of cement you would use approx. 0.5kg(1.1pound) of oxide.
Interesting how oxide is just another form of lime

The original scratch coating on my oven is now covered by a thermal blanket and vermiculite. But I get your point to go for the 3 coat approach.....and changind the sand cement ratio as the coats go on.

Think I'll print this thread and have it on hand