Old 03-07-2009, 01:51 PM
Join Date: Jan 2009
Location: New Orleans
Posts: 28
Default Mortar Question......

i have been stuck on the start of my dome. i'm excited to be at this stage but..... i am worried about all the negativity towards the portland and fireclay mix.... everyones wants the refractory mortar used. like HEAT STOP 50 and Refrax..... well i am just scared to start because i want to do the best thing for life of the oven. i will probably use it 2 to 3 time a week. 8 to 10 times a month, will that be a problem for the 1:3:1:1 FORNO recommended mortar mix? or i see a guy doing this on traditionalovens.com 10 : 6 : 2 : 3 Sand, Fire Clay, Portland cement, and Lime. will this be stronger ? sorry for being a pain with all these questions i just want to start and i have what i need. i just can't get my hands on the HEATSTOP or Refrax. it's to high for shipping. what do you think ? thanks -Leo
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Old 03-07-2009, 03:37 PM
dmun's Avatar
Join Date: Jul 2005
Location: New Jersey USA
Posts: 4,216
Default Re: Mortar Question......

Portland cement isn't heat resistant, that's the problem with the hombrew mortar. It's been used repeatedly, and successfully, mostly because only a small amount of mortar is exposed between the bricks. There's an alternative, if you're worried about it and don't want to spring for the admittedly expensive refractory mortar: Frances used a mixture of fireclay and sand, without portland or lime, and this old-fashioned method works fine if you keep the oven scrupulously dry. There is also a method of making mortar from waterglass and fly ash, which apparently sets hard and lasts forever, but I don't know if anyone has successfully used it.

Here's the link (Mortar choice issue - need help to decide):

Hey, this sounds great:

Refractory use

Water glass is a useful binder of solids, such as vermiculite and perlite. When blended with the aforementioned lightweight aggregates, water glass can be used to make hard, high-temperature insulation boards used for refractories, passive fire protection and high temperature insulations, such as moulded pipe insulation applications. When mixed with finely divided mineral powders, such as vermiculite dust (which is common scrap from the exfoliation process), one can produce high temperature adhesives. The intumescence disappears in the presence of finely divided mineral dust, whereby the waterglass becomes a mere matrix. Waterglass is inexpensive and abundantly available, which makes its use popular in many refractory applications.
and this:
Concrete treated with a sodium silicate solution helps to significantly reduce porosity in most masonry products such as concrete, stucco, plasters. A chemical reaction occurs with the excess Ca(OH)2 in the concrete that permanently binds the silicates with the surface making them far more wearable and water repellent. It is generally advised to apply only after initial cure has taken place (7 days or so depending on conditions). These coatings are known as silicate mineral paint.
What is the stuff? Where do we get ahold of it?

On Russian fireplace and stove forums I've mentioned opinions that its not suitable for thick seams (only for 1-3 mm) - but all people here are so smart so they never agree with somebody else, so I'm in doubt
Yes there is a danger of the "not invented here" mentality on this forum, but I think we're open to new ideas when they are good ones. The Russians have long experience with wood burning, and I for one am interested in tapping into that knowledge.

As far as mortar thickness, the commercial refractory mortars are specified for thin joints, but people have used them for thick joint pompeii builds successfully.
Sorry if I'm confusing you with more options...
My geodesic oven project:
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Old 03-07-2009, 05:08 PM
Join Date: Jan 2009
Location: New Orleans
Posts: 28
Default Re: Mortar Question......

well thank you for the info..... i just was wondering if anyone has used the mixes i mentioned, and have had any problems using the oven 2-3 times a week ?
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Old 03-08-2009, 06:20 PM
Join Date: Aug 2008
Location: San Diego
Posts: 279
Default Re: Mortar Question......

I used the homemade recipe, which is in the FB plans. Admittedly I just finished so I don't have a long track record, but so far I haven't had any problems. I thought the 1,1,1,3 was basically the equivelant of of the Heatstop, but I don't no for sure.

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