How strong is the FB refractory mortar recipe?

[Fio]

I FINALLY started building the dome on my oven today. I laid the first two ring courses using sand/fireclay/cement mortar in the 8/2/3 ratio, respectively. After a while I determined that I needed to keep it on the wet side, to make it easy to work with.

Even with fireclay, the mortar is not "sticky" It just does not stick to firebrick after it has been on for a little while. It flakes off. I soaked each firebrick in a bucket of water for a few minutes until they stopped hissing and boiling.

At the end of the day, I noticed a few little gobs of the mortar had dried. I tested to see how strong it was.

It CRUMBLED in my fingers!

Is this supposed to happen? I'm worried that the mortar will be structually inert.

TIA,

- Fio

[Fio]

I think I may have an explanation - is this correct?

The crumbly refractory mortar crumbs I found yesterday had dried out in only a couple hours.

This morning, the still-wet joints were getting firm.

I'll bet that the disparity is due to drying time. Cement needs time to cure. Anything made with cement can be strong or weak depending on how long it cures. You can take a batch of properly-prepared mortar and remove a teaspoon of it and let it dry on a trowel. It will dry in a couple hours and be crumbly.

A properly-spread joint of the same mortar will take a day to harden, and a couple days to fully cure. After a couple days, it will be hard as a rock. The reason is that the cement stayed wet for the long time it needed to cure into hardness.

Am I on the right track?

Thanks.

[Marcel]

Be sure that you're using CEMENT

(M) Fio, you are probably OK but if you misunderstood and bought "mortar" rather than the pure cement you would not have enough adhesive strength.

(M) Cement has no sand or other aggregate in it.

(M) By all means, do wet your dome bricks prior to setting them!

Ciao,

Marcel

[Fio]

Here's what I did

(M) Fio, you are probably OK but if you misunderstood and bought "mortar" rather than the pure cement you would not have enough adhesive strength.

(M) Cement has no sand or other aggregate in it.

(M) By all means, do wet your dome bricks prior to setting them!

Ciao,

Marcel

Here's what I used:

8 parts sand
3 parts portland cement (the pure powder - no aggregates);
2 parts fireclay

It goes on easier when it's the consistency of peanut butter or softened cream cheese.

At the end of laying the two base rings, I had some mortar left over, so I added a bit more water and stuffed it into a grout bag, and went back over and filled in the gaps. It was fun - like frosting a cake.

By the way, I DID mortar the base (upright) ring to the floor, but I used a minimum (just a schmear) of mortar.

I soak the dome bricks in water (they stay in the water for about 5 minutes) before I butter them. When adding bricks to the dome, I use a spritz bottle to wet the bricks already in the dome. They don't wet up as well as the soaked bricks.
I'll post pix when I get a chance. I think I'll actually start my own thread.

[DrakeRemoray]

Grout bag huh? I have not seen one of those. I think I need that.

Drake

[Fio]

Grout bag huh? I have not seen one of those. I think I need that.

Drake

You do. It's like a pastry bag. Make sure your mortar is fairly wet and pliable.

[CanuckJim]

Grout Bags

Drake,

I, too, went to the expense of buying grout bags from a masonry supplier. There are two types: heavy and reusable, plastic and disposable. Went for the disposable type; not cheap. Later, went to a baking trade show. Lo and behold, the HD disposable pastry bags one supplier offered there were exactly the same size and thickness as the "masonry" bags at about an eighth of the cost.

Go figure.

Jim

[Alf]

Use what most bakers use for topping cakes etc, a plastic bag with the corner cut away. You want a small amount of material, small cut, large amount of material, big cut. Learned this whilst building an oven for a baker!

Alf