Portland Cement type

[Warhorn6]

Hello everyone, New to the forum here. So much information its kind of overwhelming. I'm in the material gathering stage for a homebrew cast oven build. Been reading alot of posts from members here as well as other forums. Oviously everyone uses portland cement for their homebrew refractory but no one ever says what type. Yesterday i went to the local hardware store to get some portland cement but was surprised when i saw that the bags said they were type II / V. After a quick google search i found out there is 5 different types of cement. So my question is would that
type II /V work for homebrew castable refractory? Thank you for any responses they are greatly appreciated.

[UtahBeehiver]

Reach out the David S, he is our resident cast oven expert. You probably will need to start sourcing, melt extract fibers and alkali resistant fibers which are couple items David recommends. Do try and avoid the gym ball route you see on YouTube oven. Also look at Nick JC build out of the UK, he did a nice cast build.

[Warhorn6]

Thank you Utahbeehiver, I have sourced the SS needles and am currently looking for the Polypropylene fibers. As far as the gym ball route goes, it is most likely the way im going to go because our outdoor kitchen build won't start for a couple of months and i want to have the oven ready by then. Idk if using a sand castle will work because im planning on doing the oven in sections as time permits so i don't know if that route will work. I will definitely check out Nick JC's build. Thank you.

[david s]

The drawbacks of using a gym ball are that it requires a lot more time to build the formwork around the bottom of the ball. Also you will need to move a very heavy casting into position. The sandcastle method, apart from being way faster and cheaper, allows you to cast in situ, just remove the sand when done and the dome is already in position.
There are plenty of cast homebrew ovens in the “other ovens” section on this site, which should address many of your questions.
I can’t really help you regarding Portland cement types as the only one available in Australia are either GP general purpose cement or Builders Cement, which includes some fly ash to help the concrete flow more. Either is ok, but I use GP.
Tegarding building the cast in stages does introduce a problem of having a poor bond between layers, however you could possibly make this an advantage if you have to dismantle and move the cast.

[Warhorn6]

Hi David, thank you for the reply. I would do the sand castle method , but since i most likely won't have time to do it all at once idk if the sand drying out would be a problem. Also like i said before, we are building an outdoor kitchen in a couple of months and i would like oven to be cast and dry by then. That's why im hoping to cast it in sections kind of like how they come in the kits. Also how crucial is the addition of the polypropyline fibers?

[david s]

The burnout fibres reduce the risk of steam spalling and that’s why they are included in the mix for proprietary castable refractory. As all potters know, clay wares are vulnerable to steam spalling if not dried sufficiently. I always allowed wares to dry for a week before firing and for sculptural work allow an extra week for every inch of thickness. Obviously humidity, temperature and air movement either increase or decrease these times. Sculptural work which is usually thicker than pots is more vulnerable. An oven cast is probably even thicker so is even more vulnerable than potters sculptural work. Added to this problem is the nature of wood firings which are much harder to control than the easily measured temperature rises of electricity, gas or oil fired methods. Usually the steam spalling occurs around 200-300c (400-600F) so don’t think you’re safe once your over water’s boiling point.
My own feeling is that they are essential, but you might get away without them.

[Warhorn6]

Thank you for the quick reply. I was just wondering since they are a little pricey. But if im already going through the process i'll just buy them , was just trying to cut costs down a little.

[david s]

You require very few. Around a small handful for every 10 L of wet castable, but you need extended mixing time for proper dispersal. They tend to clump. If mixing by hand in a barrow mix about double the time you would use if mixing it without them. Mixing by hand like this allows you to inspect the mix as you go as well as feel the consistency and viscosity of the mix.

[Warhorn6]

Awesome, thanks for the tip.

[Warhorn6]

DavidS are these the fibers im looking for? Thanks in advance

[david s]

Yes. They are actually designed for reducing slump (holding the wet mix together) and early shrinkage crack reduction in concrete, but for our purpose have the additional benefit of a low melting point which creates the mini pipe network that allows better movement of moisture.

[Warhorn6]

Sweet, thanks again.