WFO construction mortar cracking Durban South Africa

[WayneH]

Hi... first post here I think after beginning construction on my domed WFO using the FB provided plans. So far everything has gone according to plan ie the planning has been smooth and the herring bone cooking surface is in place.
However, I have hit a snag on the opening archway where I seem to be encountering some cracking in the mortar joints at two points being the left side of the first major curve change an then again in the top half brick adjacent to the top key stone. Unable to get fire clay for the mortar mix I have been using a blend of hydrated lime with Portland cement and building sand in the ration of 0.5 : 0.5 : 3. All other joints are fine and I have ensured that brick are well soaked before applying the mortar.
Any clues as to what could be causing the problem ? Ambient tem in Durban at the moment is 26 - 28 but with about 45% humidity.

thanks

Wayne

[cobblerdave]

Re: WFO construction mortar cracking Durban South Africa

G'day
You can get away with out fireclay its built to take higher temps than our ovens get to anyway.
You can use bricklayer sand which has a clay content to make the mortar stick a lot better and give it the "squish " to make it easy to use.
Failing that powdered clay added to the mix.
What will happen to the clay when fired at temps between 350C and 500C is it will become super dehydrated. Not only the water between the particules will be dried out but also the water chemically bonded in the clay will be driven off . The result is the clay can not be rewet and return to sticky clay again. Yes it hardens.
1 part Portland, 1 part hydrated lime, 1 part clay are the cementious parts, the glue. With only 3 parts sand, the homebrew cement is low on the agragate part. You can increase that to 5 parts and gain a softer mix. You can think of the mortar as a gasket between the bricks rather than a glue, it needs to be softer and more giving.
You might want put a couple of wedges under the arch form so when they are removed the form will drop away and put no pressure on your fresh mortar.
Same with the IT tool a spacer between IT and brick can be removed before swinging the IT away will stop it from dislodging the fresh layed brick .
Why don't you start a build thread, it would be a great resource for those African builds that come after you . Local conditions do vary around the world and there have not been many documented builds from you part of the world of late
Regards dave

[Greenman]

Re: WFO construction mortar cracking Durban South Africa

Welcome Wayne - What Dave posted is good advice. if fireclay is not available you can use the product of the bricks you cut in your mortar. Not sure if you are cutting or splitting bricks but that would work ok.

It will be interesting to follow your build. It is a project well worth the doing and brings a lot of satisfaction both in the build and the great food when you are done.

[david s]

Re: WFO construction mortar cracking Durban South Africa

What will happen to the clay when fired at temps between 350C and 500C is it will become super dehydrated. Not only the water between the particules will be dried out but also the water chemically bonded in the clay will be driven off . The result is the clay can not be rewet and return to sticky clay again.
Regards dave

Yes, this is pretty much exactly what happens, although the clay only becomes permanent when it reaches 573 C, the stage which is called quartz inversion. Anything lower than that and the clay can still return to mud if rewet. This means that only the surface facing the fire, or the bits at the crown of the dome will reach these temps. Do not try to fire your oven really high in an attempt to make all the unfired clay contained in it permanent. The quartz inversion stage must be taken really slowly or the clay will crack. It is not possible with a wood firing to do this in a controlled slow way and damage to the clay will result.