How to Cast a Concrete Geometric Sphere (Basic & Advanced Techniques)

Advanced Concrete Casting: Tools and Ingredients

Now, if you want to take your concrete casting further and get even better results, then there’s some additional ingredients, tools, and techniques you can use. These include using a high strength concrete blend, a mixer or heavy duty drill with a stirring rod, in a 5 gallon pail, a concrete plasticizer water reducer additive, a digital scale, finishing wax and a wet stone polisher and diamond pads or diamond pad attachment for a drill.

Advanced Mold Preparation: Reusable Silicone Gaskets

Here’s how the concrete casting process changes using the advanced methods:

I apply a thin coat of finishing wax paste to the inside cavity of the upper and lower halves of the mold. When the wax dries, I buff off the haze with a soft dry cloth to bring up a shine. This helps the concrete casting release from the mold easier. As before, apply the latex caulk and secure the halves with the fasteners.

You can also make a repeat use watertight seal using pure silicone caulking. You first need to apply a thin coat of petroleum jelly to the flange of one of the geo-sphere cavities. Here I’m spreading it with a small brush.

Now I’ll apply a small continuous bead of silicone to the other half of the mold. I’ll gently mate the two halves together and secure them with a few fasteners. I just want the two halves together without squeezing that silicone bead very much so I won’t tighten the screws. I’ll set that aside for it to cure overnight.

The next day I pulled the halves apart to check this newly formed gasket. The silicone stuck to one half but, not the side with the petroleum jelly. I’ll clean up the excess jelly with a paper towel.

Now I have a gasket that will last a few castings. I’ll bolt the halves together and finger tighten the screws and bolts. This will compress the cured silicone and make the seal watertight.

So there’s an alternative to using the latex based caulking.

Mixing High-Strength Concrete with a Water Reducer

I like the 5000 psi or 6000 psi high strength concrete mixes as I have been getting a better finish on my castings. And it’s only a few more dollars a bag. Using my scale I will weigh out 27 pounds of the dry mix in a pail. I will also weigh the water I need. I have found 2.2 pounds of water is about right. You may need to do some tests on your own to get the concrete mix water and ratio just right.

Next I’ll add a water reducer to my pail. There’s a few water reducers (or plasticizer) on the market so again you may need to do some experimenting here. I like the ADVA Cast 555 and I add 1 ounce to the water. I have also used a powdered water reducer called Supercizer 7 in the past.

These additives allow you to get a concrete that mixes and flows well using less water. And less water creates a stronger concrete. As these castings are decorative and not structural, strength is not an issue. But, mixing it this way has given me a better appearance and durability to my castings.

I’ll add about half the water mixture to the 5 gallon pail and half the dry concrete blend. I’ll use the mixer and attachment to blend it well. I have found that mixing my concrete this way results in less trapped air in the mix and fewer bug holes in the final casting. And by bug holes I mean air pockets on the surface.

I’ll now add the rest of the dry mix. Blend it. Then add the remainder of the water mixture. I’ll blend this for a bit then stop and give the pail a stir by hand. This will allow me to see if there’s any dry clumps in the bottom. Looks good so, will blend it just a bit more.

Using a mixer in a pail appears to make a thicker and better incorporated concrete mix. Compared to mixing with a shovel in a wheelbarrow, the pail method creates a stickier mix more like yogurt. Even with the regular concrete mix and no plasticizer. So, if you can, adopt the pail and mixer method for the best casting results. Regardless of the concrete blend you choose.

Pouring and Unmolding the Advanced Mix

I’ll bring the pail into the shop and give it another hand stir before transferring some to a smaller pail. I’ll give the funnel a light mist of water to help the flow.

Filling the mold is done the same way as before. And as with the first demonstration, I only settle the mix with a slight shake of the mold. I don’t vibrate the mix at all. Doing so will bring up bubbles that will get trapped along the inside of the mold near the fill hole.

This batch flowed well and I didn’t need to help it along with the wire. I slightly over filled the mold then left it for 10 minutes to settle. Then came back to remove any extra and trowel it smooth.

Unmolding after 3 days is also done the same: remove the fasteners, apply heat, and lift off the mold halves.

This casting came out very well. There were only a few air pockets. So few that I didn’t bother to fill them. I just wrapped it in plastic to cure.

Wet Polishing and Final Thoughts

After curing for a few weeks I can polish this geosphere as before. But, this time I’ll use a wet polisher.

The polisher I bought comes with an assortment of discs from coarse to very fine. I used the 50 grit one to start to remove any excess concrete at the parting line and it worked great. Again, a polisher is not required unless you plan to get into more concrete projects. This polisher I connected to my garden hose and that will supply a steady stream of water.

And there is the final finished geo-sphere.

I might do a follow-up video that goes into depth of the challenges I faced designing this mold and casting my first geo-spheres. Leave me a comment if that’s something you’d be interested in.

So, there’s the basic and advanced options for castings these geo-spheres. Thanks for reading! We’ll see ya next time.

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