Heat/fireproof cement

[steve999]

Guys, sorry to be a pain with all the questions.. Ciment Fondue is incredibly expensive. I’ve sourced bricks for my dome. Am I right in thinking I can make heat proof cement by mixing Portland cement, sand and Hydrated lime? Thank you all in advance...

Steve.

[chas]

No problem Steve, but you do seem hell bent on over-speccing. If the bricks are soft reds and you don’t want to go down the potters’ clay route, then lime mortar will do the job. There’s little point in bonding bricks with something harder than they are. Personally I wouldn’t include the OPC. Your not looking for strength, the mortar is just gap-filler.

Chas

[oblertone]

Homebrew = 3 Parts Sand, 1 Part each cement, lime, clay by volume = Jobs a carrot and as fireproof as you'll need..

[essexsussex]

Can the homebrew be used for render as well? And can the clay be omitted if sift builders sand is used?

Final question - if it can be used as render, does it need a waterproofing coating and what would that be

Thanks a million.

[downunderdave]

Mar 30, 2019 21:47:10 GMT essexsussex said:

Can the homebrew be used for render as well? And can the clay be omitted if sift builders sand is used?

Final question - if it can be used as render, does it need a waterproofing coating and what would that be

Thanks a million.

The homebrew contains a high proportion of clay, which leads to shrinkage cracks, making it a poor choice as an external render. A better mix is 4:1:1 by volume, sand (variety of grain size, OPC, hydrated lime. This mix contains no waterproofing. Alternatively  a proprietary cement render can be purchased dry, in bags ready to add water. These have partial waterproofing which you can tell on mixing as the water is a bit difficult to penetrate on initial mixing. You can add acrylic (designed for the job) to the mix, around 50/50 water/acrylic for a stronger bond and easier application.

[essexsussex]

Thanks down under Dave - really helpful. So I will use a mix of soft and sharp sand for the render, to give a variety of grain sizes. Do i need to waterproof? What would the right product be? So very helpful.

Thanks again
Will

[dave36]

Mar 31, 2019 2:15:30 GMT downunderdave said:

Mar 30, 2019 21:47:10 GMT essexsussex said:

Can the homebrew be used for render as well? And can the clay be omitted if sift builders sand is used?

Final question - if it can be used as render, does it need a waterproofing coating and what would that be

Thanks a million.

The homebrew contains a high proportion of clay, which leads to shrinkage cracks, making it a poor choice as an external render. A better mix is 4:1:1 by volume, sand (variety of grain size, OPC, hydrated lime. This mix contains no waterproofing. Alternatively  a proprietary cement render can be purchased dry, in bags ready to add water. These have partial waterproofing which you can tell on mixing as the water is a bit difficult to penetrate on initial mixing. You can add acrylic (designed for the job) to the mix, around 50/50 water/acrylic for a stronger bond and easier application.

Or K-rend. Water repellant, breathable, comes in a variety of colours, slightly flexible (less cracks) and really easy to apply. See London newbie thread under clay oven build for an example. There are other examples too in the forum. D

[downunderdave]

Mar 31, 2019 6:29:59 GMT essexsussex said:

Thanks down under Dave - really helpful. So I will use a mix of soft and sharp sand for the render, to give a variety of grain sizes. Do i need to waterproof? What would the right product be? So very helpful.

Thanks again
Will

Will,
there are a few different approaches re waterproofing ovens. Italian outdoor ovens were generally finished with a lime based render which is not particularly waterproof, but remember that an impervious layer will trap water in as well as keeping it out. A third option is to use a modern proprietary render that is partly waterproof so will be resistant to water entry yet still allow water to escape.i think the weather patterns in your area should dictate which route you want to go.

Dave,
KRend is a brand name and they have a lot of different render products.A 100% acrylic product will be totally waterproof, and expensive. A cementious acrylic modified product partly waterproof and a cement/lime, sand render you make yourself, fairly porous.
The outer shell is usually applied in two or three thin layers 24 hrs apart, reinforced with chicken wire. Because of the time factor I skip the chicken wire reinforcing and add random AR glass fibres instead and do the whole shell in one 12-15 mm layer in one go, sponge finish, wrap in cling wrap for one week to enhance strength. Finally after several cooking fires I apply a 100% acrylic render, thinned with water so it can be applied by brush in around 3 coats to a depth of about 2 mm. This produces a completely waterproof surface.

[downunderdave]

Mar 31, 2019 6:29:59 GMT essexsussex said:

Thanks down under Dave - really helpful. So I will use a mix of soft and sharp sand for the render, to give a variety of grain sizes. Do i need to waterproof? What would the right product be? So very helpful.

Thanks again
Will

Will,
there are a few different approaches re waterproofing ovens. Italian outdoor ovens were generally finished with a lime based render which is not particularly waterproof, but remember that an impervious layer will trap water in as well as keeping it out. A third option is to use a modern proprietary render that is partly waterproof so will be resistant to water entry yet still allow water to escape.i think the weather patterns in your area should dictate which route you want to go.

Dave,
KRend is a brand name and they have a lot of different render products.A 100% acrylic product will be totally waterproof, and expensive. A cementious acrylic modified product partly waterproof and a cement/lime, sand render you make yourself, fairly porous.

The outer shell is usually applied in two or three thin layers 24 hrs apart, reinforced with chicken wire. Because of the time factor I skip the chicken wire reinforcing and add random AR glass fibres instead and do the whole shell in one 12-15 mm layer in one go, sponge finish, wrap in cling wrap for one week to enhance strength. Finally after several cooking fires I apply a 100% acrylic render, thinned with water so it can be applied by brush in around 3 coats to a depth of about 2 mm. This produces a completely waterproof surface.

[essexsussex]

Thanks Dave - infid look at K rend but I couldn’t work out which to buy, if I can work it out I may well go for that’s if easier to work.

But otherwise so pleased to have the home brew recipe.

Waiting for my chimney , hopefully arriving tomorrow , get the arch turned then all that is left is rendering. Oh, then maybe cook with it.

Thanks all

[downunderdave]

From the look of the pic, you’ve not left much room at the sides for insulation plus an outer shell. I hope you’ll be able to squeeze them in.

[essexsussex]

Thanks Dave - I intend to wrap the oven itself and then render the thernalite blocks (which will not be blanketed) as well as the oven. I hope too there will be enough room!

Will