Remove Air Bubbles from Refractory Cement

[Oven_Man]

Hi Guys

I've finally bought my refractory cement and will mix it tomorrow before pouring it into my mold.

I remember reading about Air Bubbles that could form when pouring cement into a mold. The solution was to vibrate the mold structure when pouring cement into it.

My question is what can I use to vibrate my mold. My mold is on a Wooden Pallet. I don't have any special equipment, so whatever suggestions you guys give needs to be something that's easily available amongst stuff I can find at home.

Thanks
Regards
OM

[Tscarborough]

Orbital sander (no sandpaper in it) works best, and light tapping with a rubber mallet.

[robvious]

There's a book I read on building concrete countertops by Fu-Tung Cheng that (if I recall correctly) that also recommends using a sander to vibrate the form to consolidate the bubbles and then tapping to form lightly to drive the bubbles to the surface.

[Dino_Pizza]

Robv...I used my back vibrator (really) and it worked great! ...on the cement mold, my back was fine



Good luck, Dino

[david s]

You could always ask your wife if you could borrow hers.

[Oven_Man]

Hi

Unfortunately I didn't have a orbital sander or any other vibrating equipment on hand so I just used the old fanshioned method of manual labour and vibrated the structure using a hammer but lightly tapping the form and areas around the form.

All went well in that regard but I had another problem. The mixing instructions of my Refractory cement was to add 2.5L water per 25KG bag of cement. I added steel needles to my cement mix as well before adding in the water, and when I added the recommended amount of water, I found that my cement was stil very dry. I was under the impression my cement would be very "runny" but it wasn't so at all. It was such that if I threw a handfull of cement against a wall, it would have crumled upon impact rather than stick and den seep down. When I tried to fill my mold with cement, it didn't run to the bottom of the mold like I expected it to so I vigourously vibrated the structure thinking this may cause it to seep to the bottom of my mold and then I would fill on from there but this didn't happen either so I had to use a thin strip of masonite and push the cement from the top of the mold to the bottom and dis definately seemed a miss to me, so I did the sensible thing and added more water to my mix, a little at a time till I found the cement a bit more moist.

I've got 2 concerns now.
1 is I may have weakened the cement by not following the recommened water measurement specified by the manufacturer. I ended up adding 50% more water thus adding 3.75L per 25KG bag.
My second concern is maybe I didn't add enough water because I made my cement in multiple batches as I was told I hardens quickly and I need to work fast so I thought I'd work with small batches at a time. Some batches were a little moist than the others.

My cement is still in the mold drying out but how do I know if its strong enough or not or whether it would even hold up when in use?

I was thinking maybe the addition of the steel needles could have altered the substance of the cement when using the recommended amount of water but thought I'd check in with you guys if I need to be concerned and what steps can I take to resolve such a problem and also avoid it in the future. By the way, I used a spade and mixed the cement by hand.


And another problem I experiened is I placed cling wrap and tape over my mold so the cast will be easy to remove from the mold but some of the cling got removed when I was trying to force the cement to the bottom of the mold using the masonite. Now I am worried the cast might not be as easy to remove. Any tips on how I can avoid damaging or even destroying the cast when removing it.

Other than that, all went well. Just a little information about my mold, it's 1 quarter of a full dome and I plan on making 4 pieces in total to complete the dome.

I can finally say, I've started after months of contemplation and preparation and hope these teething issues doesn't hinder my progress.

Look foward to hearing from you guys.
Regards
OM

[Tscarborough]

You will have to break it up and start over.

[Tscarborough]

Psyche!

It should be fine, at worst you may have to patch some bugholes here and there.

[david s]

You should be fine. I've always found I have to add more water than the manufacturers say too. you can reduce the amount of water by adding a plasticizer which will reduce the viscosity without having to add more water. Too much water makes it weaker, but not enough tends to leave you with voids against the mould which you have to fill.Use the "ball up" method of correct consistency. That is you throw a ball of the mixture afoot in the air and it should stay together.

[david s]

I tried the orbital sander as a vibrator on the side of my barrow to keep the mix moving, but only ever used it once. It was REALLY noisy and very annoying. I think it would have worked better if I'd anchored it better to the barrow. I did consider driiling hole through the barrow but thought I might end up with a holey barrow and the thing still wouldn't work any better. I also felt that the frequency was way too high. I meant to look out for a sander with variable speed, but forgot about it, should try again. I now just mix smaller quantities that I can handle easily.