Homebrew amounts

[frank1985]

Greetings folks

My dome will have a 30" inner diameter with 2" thick walls.

I've calculated the shell volume to be 53 liters.

My proposed mix is the standard 3:1:1:1 homebrew.

Sand (3 parts) -- 33 L
Fireclay (1 part) -- 11 L
Hydrated lime (1 part) -- 11 L
Portland cement (1 part) -- 11 L

I've determined that 66L will be required to allow for compaction + the addition of water. After wet mixing this will probably bring the volume down to around 58l...a bit more than required but best to have a little spare just incase.

1% of wet mix polypropylene fiber = 0.6l

Does the 66 L dry --- 58 L wet--- 53 L placed shrinkage chain look reasonable in practice? Would there be a further in reduction in dry to wet?

Also regarding the alkali resistant fibers, does the 2% addition assume a wet mix also?

[david s]

Your calculation is pretty accurate considering the slight reduction of shrinkage volume from dry to wet castable. Also the volume lost for the door opening is usually (depending on your design) equivalent to the extra volume required around the mouth, so the complete volume of a hemisphere works out to be the amount required.
The recommendation for clay addition to mortars is that 7% by volume should not be exceeded. The reason is that clay induces shrinkage during the hydration process as the material hardens. Because the 3:1:1:1 recipe exceeds the 7% also confirmed by my experience, for both mortar and if using the homebrew as a castable, I halve the clay addition. Both the pp and the AR fibres do control early shrinkage, but casting over a hump mould leaves nowhere for the casting to shrink. Early careful removal of the sand mould is also advisable (at around 12-24 hrs depending on temperature) to reduce shrinkage stress.

[frank1985]

Thank you....actually that works out better as I'm short on fireclay anyway without halving. I'm using a gym ball, but yes, I will be sure to deflate that at around 12-24 hours.

I have a few more questions before proceeding...

1) Any 'rules' regarding how much water to add as a percentage of the dry mix? I'm guessing 8-10 liters would be adequate...until the mix holds shape anyway.

2) I understand that the poly needs to be broken up and added to the wet mix to aid with dispersal. Should I add the AR fibers together with the poly - or is it better to disperse one before the other?

3) What is the typical working time for the mix before it becomes unworkable, or strength is compromised? Actually one thing I'm considering is casting in halves, not only to mitigate this, but also to make it easier to carry onto the plinth later (can't cast in situ due to several reasons).

4) How would I then rejoin these two halves - is it a case of using more homebrew over the joins, or just dry fitting them before adding insulation and render?

[david s]

1. Water addition can vary considerably because sand is almost always damp. Better to just judge that by feel of the consistency. Some folk use the “ball up” method to test consistency, but trial and error is the best. Make sure you wriggle the mix against the mould which also integrates each handful to the previously placed one. The mix should be a little stiffer at the base where you are asking it to stand vertical.
2. Add the AR fibres into the wet mix before the pp one’s, they will disperse readily and you will be able to see them. Then add the pp fibres pulling them apart as much as possible, then mixing, by hand for longer than you’d normally require. Look for any clumping.
3/4. Hard to answer, because setting is very temperature dependant. I always use chilled water in the summer months to extend working time.
casting over a gym ball is far harder than a sand mould because it requires formwork around the bottom half and removal and replacing the casting. Multi pieces are then a better solution. Stepped joints are better than butt ones but much harder to create especially for a one off. Fill joints from the outside only, you wouldn’t want any bits of mortar falling into your food during cooking.

[frank1985]

Thanks alot dave. I already have formwork built for the ball https://i.postimg.cc/W1pqpj6q/IMG-20260417-WA0000.jpg:) I'm thinking of casting in three pieces - back, front, and skull cap.

Neighbour has a lot of surplus builders sand ... Can I use that for the HB or is it recommended I stick to coarse sand?

[david s]

Just as concrete strength benefits from a range of aggregate size, I think any castable also does. Commercial castable that I’ve used have a range of 3mm and down. I’d expect a homebrew castable would benefit from graded sand (a mix of grain size) rather than using fine sand. I don’t think it really matters that much, any sand should work ok. You can take a sample and sieve it with course and fine sieves to check the proportion and size of grains. Either brickies, concrete or silica sand should be ok.

Contrary to my comment about chilled water as mentioned in my previous post, you shouldn't need to add chilled water to home-brew as it reacts much slower than calcium aluminate based castables

[frank1985]

david s

Do you advise mortaring between the faces of each join (as with bricks), or just mortaring from the outside in order to cover over the join-line? I was thinking that actually having them not fixed firmly together would be advantageous in terms of providing less resistance to thermal expansion. Do you think it could be a problem if they're more or less just loosely butted up against eachother?

I finished casting all three parts yesterday - front, back and skull cap. It went well I think, but if after removing from the mould I find there are voids from where the mix failed to adhere to the ball (edit; there are two or three minor voids) would it be ok do you think to use bonding agent (SBR) before adding the homebrew, or is there an alternative? I have used SBR before but I don't know for sure if it's suited for high temp exposure. I guess the best best thing for a better bond would be to mix a little cement with water and dab that into the voids before adding the homebrew.

[david s]

First question re mortar between cast sections: because you are relocating the sections you will find they won’t sit back exactly into position, leaving bigger gaps than you’d prefer. This is where casting in situ has a big advantage apart from not having to lift the heavy weight of each section, the casting sits exactly where it has been cast. Stepped joints do make the joins come together better, but are more difficult to design, particularly for a one off casting. You may have done butt joints so just squish your mortar in hard from the outside and try to avoid any mortar penetrating all the way through that may fall out, particularly for the top half of the dome. Clean up any mortar from the inside.

Regarding your second question, the sooner you can fill any voids on the inside, the better your bond will be and a moist cast is better for this operation than a dry one. Using SBR, or any bonding agent could be likely to burn away so is probably not of much use, I’m not sure on this. Likewise a cement rich slurry applied first is also likely to be of little use as Portland cement begins to fail over 300C. Calcium aluminate cement slurry is the usual technique applied before any filling with castable refractory, but because you are using homebrew the voids are better filled with the same material. Just sieve out any course sand to make the mix easier to apply especially in small voids. Got any more pics of current progress?

[frank1985]

First question re mortar between cast sections: because you are relocating the sections you will find they won’t sit back exactly into position, leaving bigger gaps than you’d prefer. This is where casting in situ has a big advantage apart from not having to lift the heavy weight of each section, the casting sits exactly where it has been cast. Stepped joints do make the joins come together better, but are more difficult to design, particularly for a one off casting. You may have done butt joints so just squish your mortar in hard from the outside and try to avoid any mortar penetrating all the way through that may fall out, particularly for the top half of the dome. Clean up any mortar from the inside.

Regarding your second question, the sooner you can fill any voids on the inside, the better your bond will be and a moist cast is better for this operation than a dry one. Using SBR, or any bonding agent could be likely to burn away so is probably not of much use, I’m not sure on this. Likewise a cement rich slurry applied first is also likely to be of little use as Portland cement begins to fail over 300C. Calcium aluminate cement slurry is the usual technique applied before any filling with castable refractory, but because you are using homebrew the voids are better filled with the same material. Just sieve out any course sand to make the mix easier to apply especially in small voids. Got any more pics of current progress?

Yes if I could start over I would have cast in situ. Maybe for my next build.

Turns out sbr bond isn't such a great idea as it's maximum operating temp is 100 degrees c lol.

The part where the void is located was cast a week ago so I think maybe I should just leave as is? See attached pics. Id say it's about 2cm wide, 30 cm long and at its deepest point about 5-7mm deep. The wall itself is about 2.5". So maybe the extra half inch over the minimum that's usually recommended may have worked in my favour? I'm thinking leave as is...what say you? (Incidentally the newspaper over that part is proving a bastard to remove anyway, even after going over it with hot water on a sponge). Is there a chance they could pose a problem later down the line or is the issue mainly cosmetic?

I've attached a few other pics of current progress...nothing too visually exciting as of yet. Got a nice solid plinth up, along with six weep holes. Just need some wire to stick in the holes to keep critters out.

Just ordering calcium silicate boards and bricks now. I was going to go for a vermicrete slab but I think the CS is a lot simpler to implement despite the extra expense. I do have vermiculite at hand though to cast around the flue using the galvanised wire and aluminium method you very helpfully suggested in another thread. I will be casting the gallery over the next few days hopefully. Very grateful for all your pointers.

Image gallery; https://postimg.cc/gallery/0NRfGnn

A few more clearer pics of the void: https://postimg.cc/gallery/XYXrkBB
It's not as a bad as it looks tbh

[david s]

If it were mine I’d still be filling that void, but as it’s a week since casting, get a wet sponge and jam it into the void. You may need to use a stick to hold it in place, then keep wetting the sponge so moisture goes into the area surrounding it. After half a day of this fill it with the homebrew, sieved to remove any course sand.
Don’t worry about any newspaper that won’t come off, fires will do the trick there.
If you don’t do anything the voids won’t get bigger, they’re largely cosmetic.

[frank1985]

If it were mine I’d still be filling that void, but as it’s a week since casting, get a wet sponge and jam it into the void. You may need to use a stick to hold it in place, then keep wetting the sponge so moisture goes into the area surrounding it. After half a day of this fill it with the homebrew, sieved to remove any course sand.
Don’t worry about any newspaper that won’t come off, fires will do the trick there.

Ok I will do that. Can these voids worsen over time if left as is or even after having mortared them post-set? Do you reckon I've buggered it up?

[david s]

No, I think you’ll be fine, but as I previously posted, this operation, as is for any bonding, better done asap. Whilst the material is still reacting and setting and still contains plenty of moisture. But the surface to bond onto should not be wet, just moist, otherwise a film of water will interfere with the chemical bond.