Help with homebrew mortar

[Tscarborough]

Re: Help with homebrew mortar

Unfortunately no. If you want to repair it, you have to grind out the joint.

[Nic The Landscaper]

Re: Help with homebrew mortar

One thing that I have noticed that does not get discussed here (Forno Bravo) is how to finish a mortar joint. I think a lot of peoples' issues with cracks, especially the ones before curing fires begin are a result of not tooling the mortar joint. Especially with the high shrinkage rate that Clay Mortars have it is critical that a joint be tooled.

A tool that every one who builds one of these ovens should have is a half round brick jointer (supper cheap tool) and do a quick search for how and why you tool a mortar joint on google. I think this will take care of a lot of cracking issues.

Also as someone who does a considerable volume of masonry work, I am in 100% agreement with Tscar, that the least amount of water that can be applied to a firebrick before mortaring the better. You want that firebrick to draw water, and with the water, cement, clay, and lime from the joint into the brick. This will create a stronger bond, as well as reducing the volume of water that you are going to have to try and bake out of your bricks later which just leads to a higher chance of structural cracking.

[GianniFocaccia]

Re: Help with homebrew mortar

Thanks for the insight Nic. Yours is the first input I've seen citing reasons behind finishing a mortar joint to help reduce cracking. I believe I understand the process of mortar/brick adhesion created by the wicking of the wet mortar ingredients into the brick itself. Unfortunately, in my warm/dry location, the best results I've gotten are from pre-wetting the bricks. I am going to go get a jointing tool, however.

[nissanneill]

Re: Help with homebrew mortar

I tend to agree with some of what you say Nick, but when your joints inside the oven are minimalist or nil, pointing will not prevent the cracking/separation of the mortar from the bricks.
In all my experience and when working with bricklaying lecturers in an Adelaide Tafe Trade School, they maintain that the excessive pulling of water/moisture from the mortar from a dry brick will cause adhesion problems.
Therefore, thoroughly wetting the bricks will reduce this from occurring BUT a drying/curing process MUST be undertaken in order to prevent damage caused by steam generation within the wet bricks.

Neill

[Tscarborough]

Re: Help with homebrew mortar

It is a fine line, and it changes with the weather. So long as they are SSD, you can't go wrong. If the brick have enough water in them for it to be visible as water on the surface, they are too wet, and if a drop or 2 of water instantly disappears they are too dry.

As for tooling the joints, my assumption was that the joints were too small to bother with, but I guess in reality it would be a good idea on anything over 1/4". A spoon will work in a pinch, as will a stick.

[ThermoJax]

Re: Help with homebrew mortar

Could you guys tell me the effect of getting the homebrew recipe wrong. I did the same recipe, except 2 parts sand not 3 parts. Serious trouble? I thought the most important part was the lime, as the portland gets burned out, leaving the lime as a backbone?

Thanks

Tom

[Tscarborough]

Re: Help with homebrew mortar

The actual formula reads like this:

2-1/ to 3 parts aggregate by volume of all cementious ingredients.

I doubt most are doing this, since this includes the lime as a cementious ingredient, as well as the fire clay. So unless you did that you are actually probably closer to a 1 to 1 ratio. Still nothing to worry about, but it will tend to crack a little more than a weaker mix would.

[ThermoJax]

Re: Help with homebrew mortar

Thank you Tscar. Why does more sand in the mix(weaker?) mean less likely to crack? I like the hardness that this mix gave me, at least as far as the cladding for my "shell". Nice and hard to the touch. I would have thought that stronger was better, but I'm a total newbie. Thanks

[Tscarborough]

Re: Help with homebrew mortar

Mortar is not concrete. The important properties of mortar are bond strength and flexural strength. The rule of thumb for mortar is to always use the weakest mortar that will still perform adequately.

High lime mortars are high in bond and flex, high portland mortars are low in flex, high in drying shrinkage.