Alan Scott oven project begins

[david s]

Portland cement is generally considered unsuitable and confirmed in practice to fail at the temperatures we use.

https://www.concreteconstruction.net...temperatures_o

https://www.civil-engg-world.com/200...-concrete.html

Regarding aggregates this is pretty good information
https://link.springer.com/article/10...069-016-0175-2

“When the temperature exceeded 150 °C, Portland cement paste contracted with increasing temperature. The thermal shrinkage of the cement paste and expansion of aggregates have potentially resulted in thermal stresses in the cement pastes causing micro-cracking. “

[david s]

One big drawback of casting your own vermicrete slab is that it takes a very long time to dry it out by firing the oven. Builders have reported that it takes months to eliminate and their ovens continue to improve long after they think it must be dry. Water in insulation reduces its ability to insulate because it is so much more conductive than air. If possible allowing it to remain exposed and allowing sun and wind to do much of the drying for you makes the process easier as does adding weep holes under the insulation for moisture to escape once you do start the drying fires.
I did a little experiment to test the water elimination by natural drying that you might find instructive. Probably too late if you have already built over it, so don't expect the oven performance to be as expected for some time.

Vermicrete insulating slab copy.doc.zip

[astroinfidel]

Hi David,
Yes it was articles like you provided about Portland losing strength that caused me to consider alumina concrete instead. But alumina concrete only appears to be beneficial above 1500F, and I am hoping to keep the temperature of the slab less than 1000F, and ideally not much over 800F. This will require one or more long slow firings, to pump the heat gently into the slab rather than quickly. I used a cement to sand/aggregate ratio of 1 to 3, and poured at between a 75 and 100 mm slump. Combined with the crushed granite only aggregate, this should make for some pretty strong concrete. Granite is not the very best, but is better than most for lack of expansion as it heats. And I poured the slab at 4.5 inches instead of 3.5 inches. This should give me 28% more resistance to flexing. So I am hoping that I can stand to lose 50% or even more of the strength of the slab and still have enough. I am not hearing of people having problems with the slab with this kind of oven, so it must look worse on paper than it works in reality. Or so I am hoping.
Dean

[astroinfidel]

And David the Perlite slab wept water for 2 days. I kept it wet and covered for 7 days, and will let it start slowly drying out now, simply because I want to start working on the walls and will have it uncovered for most of the day. I of course can't dry the Perlite part of the slab without also drying the hearth slab, and I am in no hurry to have the hearth slab lose water any faster than necessary. I will probably try to keep it from drying out completely for a few more weeks to let it gain maximum strength. I can't soak it with the hose anymore though, as the hearth bricks are laid on the fireclay/sand mix, and that hasn't even begun to dry yet either. No rush. Now that I am not keeping it covered, it will start to dry faster. I will cover it before rain though. By the time I am ready to use the oven the Perlite part of the slab will likely be close to dry, and some gentle firings should take care of driving out the remaining moisture.
And unfortunately I wasn't able to open that document you linked to. Likely my own lack of office tool knowledge...
Dean

[astroinfidel]

Walls are laid out, and I will start mortaring them into place tomorrow. I have a rather unconventional plan for doing so that I am hoping is going to let this rookie bricklayer get nice thin joints, and a nice straight wall. More to come...

Dean

[david s]

Try this

Vermicrete insulating slab copy.doc.zip

[astroinfidel]

David it opens as code..gibberish to this code Luddite. I tried notepad and Explorer to open it with the same results.

Here are a few pics of the beginning of the walls this afternoon. The Heat Stop 50 is actually very nice stuff to work with. I laid the first 4 angled bricks horizontally on the top of the foundation wall. I let them set up, then turned them 90 degrees and laid the 4 bricks as one big brick. I was going to use that method for all the walls, to avoid having to try to butter the vertical surfaces. But the Heat Stop 50 sticks so nicely to the vertical brick faces that even a rookie like me has no trouble buttering them. So I will do the rest of the wall bricks one at a time using a line as in the pic.

Dean

[david s]

Apologies, I think it may be that my Microsoft Office is out of date and it won't let me export docs. I'm feeding it money, hope that will fix it. Get back to you in a few days.

While i'm here this thread of an Alan Scott oven should be of interest to you.
#1

[UtahBeehiver]

I "snipped" and converted DavidS vermculite study to a jpeg file and attached.

[astroinfidel]

Got it! Thanks Russell! Wow that is a long time to dry. I will be very careful drying the slab when I eventually fire it up. That is going to be a while yet though. I got a few more bricks in the wall today. Here are a few pics. The Heat Stop 50 is so nice to work with. It spreads like peanut butter and clings like glue to the bricks. It is so expensive that I only mix up enough for a few bricks at a time. I will hopefully finish the back wall tomorrow.
Dean

[astroinfidel]

Here are the latest pics from my oven build. I removed the heath slab forms, and have started the vaulted roof. I'll attach more pics in another message.
Dean

[astroinfidel]

Checking to make sure the peel won't snag a high spot on the hearth bricks. Cleaning excess mortar, and a look at the inside of the first arch. I will finish the second arch tomorrow afternoon.
Dean

[UtahBeehiver]

Nice tight inner joints. Due to the low dome configuration and full soldiers on side wall, are you going to buttress the soldiers. The low dome shape will have quite a bit of horizontal load at the dome and soldier joint.

[astroinfidel]

Russell I was going to brace the soldiers as per the plans, but recently decided not to. I recently saw a build online of a guy that didn't without problems. And I tried to knock a joint loose on the soldiers with a brick set 2 days after laying the bricks, and it wouldn't break after 3 good hits. They soldiers had been up a week or two when I started the walls, and I think they are tied in well enough to not need it. The whole thing is going to be covered in a few inches of concrete soon, and that will brace the walls for the long term.

I got the second arch done today. Here are a few pics. And I am striving for zero mortar on the arch joints inside. Not always achieving it, but it keeps them small when I don't.

Dean

[UtahBeehiver]

Hope you are right, when you add heat to the equation and you start to get expansion and contraction going on there is going to some movement. Right now the oven is at a static ambient temperature. I guess the Alan Scott plans suggested buttressing for a reason.