Many cracks in cast dome - Please help!

[cheyne]

Hello,

I am casting a 33" oven with home-brew refractory cement, and am getting an unusual amount of large cracks. I was planning to put the final thickness layer of home-brew on the top of the dome today, but am worrying about structural integrity. There are smaller cracks in the sides, and bigger cracks on top where is thinner. It was 60 degrees here in New Mexico yesterday, and kept it covered with a blanket and tarp, and didn't let it freeze last night. I've tried searching as much as I can on this forum already :/ What am I doing wrong? Any Advice? Thank you!

I Used Paper mache form around an exercise ball to get the shape

Home brew 3, 1, 1, 1
Play sand
Portland cement type 1/2
Type S lime
30 mesh fire clay

[david s]

Hello,

I am casting a 33" oven with home-brew refractory cement, and am getting an unusual amount of large cracks. I was planning to put the final thickness layer of home-brew on the top of the dome today, but am worrying about structural integrity. There are smaller cracks in the sides, and bigger cracks on top where is thinner. It was 60 degrees here in New Mexico yesterday, and kept it covered with a blanket and tarp, and didn't let it freeze last night. I've tried searching as much as I can on this forum already :/ What am I doing wrong? Any Advice? Thank you!

I Used Paper mache form around an exercise ball to get the shape

Home brew 3, 1, 1, 1
Play sand
Portland cement type 1/2
Type S lime
30 mesh fire clay

From the appearance of the surface it seems to me that your mix was too wet. The wetter the mix the greater the shrinkage. Use the "ball up" consistency test. ie if you throw a handful of the mix two feet in the air, you should be able to catch that ball again. Too wet or too dry won't work.Also the clay content imparts shrinkage, and there is quite a lot of it in the homebrew mix. I used to sometimes get shrinkage cracks in the mix when used as a mortar and now modify the brew to 3:1:1:0.6 sand,OPC,lime,clay, so you could try reducing it by around 30%. The fine polypropylene fibres, if well dispersed, apart from providing assistance for safer moisture removal on firing, also hold the mix together to reduce early shrinkage cracking. This is their primary use when used with a standard concrete mix.

[gregoru2]

Hi cheyne I'm wondering if you found a solution? I also have a dome with cracks in it...

[Macrinehart]

Use the "ball up" consistency test. ie if you throw a handful of the mix two feet in the air, you should be able to catch that ball again. Too wet or too dry won't work.

david s tried this tip with my furst batch of home brew and the ball was catchable but flattened when caught, is that still too wet? Thanks!

[david s]

Sounds about right. You need to wriggle each handful against the side of your mould so you don’t get too many voids. Start at the bottom and go all the way around, making a flat ledge on the top to take the next row. Wear rubber gloves.