Remove Air Bubbles from Refractory Cement

[Oven_Man]

Re: Remove Air Bubbles from Refractory Cement

Thanks david

I just hope that certain parts of my cast don't get stuck to my form and my attempts to remove it doesn't damage my form or the cast for that matter but when the time comes, I'll only know.

I'll try your method for casts to follow and maybe even try using oil to loosen the current cast from the form should it get stuck.

Guys, please feel free to provide any other tips.

I'll let you guys know how it goes

Take Care
Regards
OM

[david s]

Re: Remove Air Bubbles from Refractory Cement

You may need to drive some wedges between the mould and the casting to assist removal. Be patient the first one is always the hardest to remove. Sorry have I already said that?

[countryboy]

Re: Remove Air Bubbles from Refractory Cement

You've already poured the 'crete, but another option, for future readers is a hammer drill. I chucked a bolt into the hammer drill so the head was exposed and then placed that against the form and pulled the trigger. WOW!! It would create huge vibrations pulling the 'crete into the bottom of the form!

[Tscarborough]

Re: Remove Air Bubbles from Refractory Cement

You have to be somewhat careful. There is an ideal frequency that will remove air bubbles, consolidate the material but NOT lead to aggregate segregation. Hand orbital sanders are as close as you can get without spending a good chunk of change to rent or buy an actual immersed or form vibrator.

[Oven_Man]

Re: Remove Air Bubbles from Refractory Cement

Hi Guys

Thanks for all the replies. You guys have been undoubtably helpfull.

Today I reached my friends house to remove the cast from the mold and got a surprise of my life.
I just felt like I've wasted a load of money and time but I know someone here probably has a solution.

Never the less, here's the problem.
My mold is in 2 pieces. First I have a 1/4 sand dome ontop of which I built a form from wood. The plan was to place the form over my sand dome and pour the cement in since I heard the cement would be very runny. Anyways now when I opened the top mold being my wood form, I got a shock. My cast was completely filled with voids and holes. The perfect example to compare with is a Bee Hive. The cast was filled completely with holes and looked just like Hive. Sorry I know I'm repeating myself but I'm just so frustrated and upset right now.

Anyways, I've taken pictures of the Cast to better explain the situation I am in.

The first picture shows the bottom left hand side of my cast. This side was where I initially started my cast. This was the first or possibly second batch of cement that I made and I was still learning about refractory cement in particular and how to use it.

The second picture again was the bottom of the cast and also is the first few batches of cement when I realised I needed more water and the consistancy of the cement just wasn't right.

The third picture is that of the top most part of my cast which was the last part of my cast I completed. This was after few tries with this cement that I found to get the right consistancy, I needed to increase the water contact by 50%. So instead of the recommended 100ml of water per KG of cement, I needed 150ml of water per KG Cement.

I also noticed the steel fibres are just pulling loose from the voids and holes in my cast.

Right now, I've removed the wooden form, but my cast is still sitting over a sand dome made from concrete and I can already see issues releasing the cast from this concrete.

I thought, before proceeding further, let me first sort out the voids and then I can try to remove the cast entirely.

Guys, I'm at a loss here and in great panic right now. I just don't know what to do or try to sort this out.

Really looking forward to hearing from everyone.

Take Care
Regards
OM

[Tscarborough]

Re: Remove Air Bubbles from Refractory Cement

No big deal, just mix up some of the castable in trowelable form, dampen the cast, and "rub" the bugholes with the mix. For the larger ones, fill and then rub.

[Oven_Man]

Re: Remove Air Bubbles from Refractory Cement

Guys, sorry, I'm having a bit of trouble uploading the photos from my mobile.

I'll try and attach them as soon as I am at my PC.

In the interim, if you guys have any advice, please feel free to post them.

Thanks
Regards
OM

[Oven_Man]

Re: Remove Air Bubbles from Refractory Cement

Hi Tscarborough

Thanks for responding so fast.

I will surely give that a try. I was wondering whether I would need to add steel fibres when mixing this cement to do the patch up.
I found that with the fibres, the cement is more "lumpy" and "Coarse" rather dan smooth and runny.

Regards
OM

[Tscarborough]

Re: Remove Air Bubbles from Refractory Cement

I wouldn't use the fibers unless they are very short.

[david s]

Re: Remove Air Bubbles from Refractory Cement

If I have any voids to fill I find it better to sieve out the coarse aggregate. This makes the mix much easier to apply and also gives you a richer blend.

[Oven_Man]

Re: Remove Air Bubbles from Refractory Cement

Hi

I don't think I'll use the fibres. They are about +-5mm long but seems though it really alters the consistancy of the cement.

I'll give your advice a try and see what happens.
Thanks

Regards
OM

[david s]

Re: Remove Air Bubbles from Refractory Cement

no don't use the fibres for filling the voids.z

[Oven_Man]

Re: Remove Air Bubbles from Refractory Cement

Hi Guys

I've just removed my Cast and it wasn't too difficult after all.

Before filling my mold with cement, I lined both my mold pieces with plastic and then filled my molds with cement. Only a small portion of my cast got stuck to my mold because the plastic on that portion lifted and the cement made contact with the mold. I started removal of the cast from that part first and it just lifted off from the mold like a breeze.

One problem I've got though. Plastic wrap has now gotton stuck to the cast but should be easily removable or worst case, I could burn the plastic. A bigger problem I'm afraid of is some plastic wrap got mixed into the cast and cannot be seen or removed easily. This wrap was probably loose in my mold and got mixed with the refractory cement when I was filling it in. Should there be anything to be worried about and would this plastic cause the cast to crack when in use since the plastic will melt with high heat and the part of the cast that the plastic fills will now be an empty space. Any tips on what I can do to sort out this problem?

I hope the above makes sense but if you guys need any further clarity, let me know.

Thanks
Regards
OM

[david s]

Re: Remove Air Bubbles from Refractory Cement

I think you'll be OK but you won't make the same mistake a second time. Try filling what you can and let the fire burn out what's left. Or if you think it's too bad you can cast an extra one anyway.

[Oven_Man]

Re: Remove Air Bubbles from Refractory Cement

I would just like to recap some of the mistakes I made and what steps to take in future to avoid them.

1) a) Biggest problem I had
was the consistency of
my cement.I now know I
need 50% or more water
to get the right
consistancy.

b) I also noticed on the
fact sheet of the
cement regarding the
grain size of my cement
is 6mm. Physically the
size is quite large and
when mixed together
with water, I'm left
with lumps of cement,
not a smooth runny mix
which I was expecting.
I tried to crush one
batch of cement with a
hammer before mixing,
but it didn't make much
difference. What
suggestions do you guys
have to ensure the
cement is fine enough
before mixing in the
water?

c) Lastly, on the point of
cement, I would still
continue adding the.
steel fibres as I've
been told to do so to
streghnten my structure
as this is a mobile
oven and will be moved
around from place to
place. In my next post,
I will move this thread
to the original thread
that was created months
back with specific
regard to building a
mobile oven and my
initial plans which
david s was so
instrumental in that as
well (Thanks David and
everyone else
especially Tscarborough
who has been a lot of
help in this thread and
I know all you guys are
going to be a lot of
assistance to the
future progress of this
oven)

2) The plastic wrap that got mixed into the cast could pose as a major problem but I will only know when the oven is in use. I will try my best to avoid this but it is difficult when lining my mold with plastic. I found that lining my mold made the removal of the cast very easy. Any other tips on how to avoid my plastic wrap becoming a hazard to my Cast.

3) Lastly, and I was wondering if this could also have been a contributing factor to the problems experienced on my first cast. I mixed my cement in batches, mixing approx 8KG at a time. By the time I mixed the next batch, the first batch which was already filled into the mold was more or less dry. When I filled in the next batch, I would vibrate the structure and use a stick to ensure the cement finds its way to the bottom of my mold. I was wondering whether this could have caused any problems with the batches that was already in the mold?

David, as for recasting an extra piece and disposing of this one, I'm trying my best to avoid doing so and wanna salvage this piece as far as possible but if its not saveable, then I would have to do so. I was hoping to cut down on cost but I could just consider it as a payment for a learning experience

That's all I can think of for now and once there's more progress, I will be sure to inform you guys.

Thanks
Regards
OM