Mixing sand with Refrax mortar

[stidik]

We've built two-thirds of my Pompeii oven dome from Refrax mortar from Forno Bravo, which we have mistakingly mixed with sand. When removing the plywood form from the entry arch, cracks developed in two mortar joints. I plan to use Refrax without sand for the remaining dome construction, but I worry that the mortar mixed with sand may not be strong enough and may develop problems in the future. At this time, except for the entry arch cracks, the dome seems to be sound. Any advice on how to proceed?

[Frances]

Re: Mixing sand with Refrax mortar

How much sand did you mix in with the refrax and how far have you got with building the dome? If you could post some pics of the cracks it'd probably help, too.

I hate to say this, but maybe you should start thinking about a complete rebuild as the best solution...

[stidik]

Re: Mixing sand with Refrax mortar

Thank you for your reply. The sand was mixed in by hand. It was mixed to form a pasty consistency, approximately 50/50. The crack is a hair-line crack. I'm working on sending some photos.

[stidik]

Re: Mixing sand with Refrax mortar

I am enclosing some photos of the pizza oven with hair-line crack.

[wlively]

Re: Mixing sand with Refrax mortar

I saw your photo. From what I can I would not worry too much about the arch crack. You did a nice job cutting the bricks to be a tight fit. Since it cracked right away after form removal, I would say the mortar is very brittle or it is not holding the bricks at all.

When I built my dome, I was suspicious of the durability over time of the mortar joints as well as my lack of direct experience. So, the big thing to keep in mind is that the proper mortar will not fail in compression, but can in shear and tension. I built my oven with this in mind, I assumed that the mortar would not hold. I pulled the form on my arch before the mortar was dry, to test the build. It did not move. Each ring is tight, in the plane of the ring.

I would worry about the mortar being brittle. You will need to test some of your incorrect mortar, left over dropping or test piece, for compression stability. Can you crush or crumble it? If so, then that would be a bad sign. As long as you have cut the bricks to a nice tight fit for each ring, so that even if the mortar let go, the bricks will have no where to go and the incorrect mortar passes the crush test, then I would say build on. Another remedy may be to give the bricks a good "coating" or cladding of correctly mixed REFMIX to reinforce them.

One more thing to consider, is how the sand will effect expansion at heat. That much sand I would think would change the rate of expansion of the mortar, which is supposed to be the same or similar as the firebrick. I don't have a table handy to tell the difference, there may not be much or any, but it is also something to think about. Hopefully, James will read this one and can get you the REFMIX makers opinion on the effects of your error.

Any way you look at it, you have a tough call to make.

[dmun]

Re: Mixing sand with Refrax mortar

My call? Rebuild the front arch, which is the highest stress piece, and let everything else be, for a years worth of cooking. I think it will be fine, after all just plain fireclay and sand will do the job.

I wouldn't build a million dollar high grade enclosure until you were sure that the diluted mortar will hold.

[Dino_Pizza]

Re: Mixing sand with Refrax mortar

I have no comment about your mortar but the pictures you uploaded of the dome and your brick work are great. You've done a nice job, I think It'll cook fine for you. Maybe put some cladding, a good 1/4" around the whole thing and it will firm it all up. Overall, nice job. -Dino

[stidik]

Re: Mixing sand with Refrax mortar

Thank you so much for the above replies. The mortar and sand mix seems to quite hard and noncompressable. I see no other cracks. So I have continued to build on, completing the ninth ring and the entry arch, landing area. The arch which cracked is the inner most portion of the entry, which is reinforced from behind by the dome and in front by the rest of the entry arch. I will add a layer of cladding at the end.
Thank again,
Steve
I have up loaded photos to the photo gallery