Portland cement/sand or plain Portland with fiberglass cloth

[Forsoothe!]

I haven't read this whole thread, but one thing to keep in mind is that concrete is cured by damp heat, ~110° to ~140°F, usually the product of exothermic reaction, the heat produced by the combination of water and Portland cement (in appropriate ratio). In thin sections as you describe in the original post, your part will dry, but not cure, making it very friable. You need to get it up to ~130°F, while it is still wet, and keep it damp through the process, and/or increase your thickness to ~^3/4" or greater, which will still be marginally too thin causing too much heat loss during the curing process of 24 to 48 hours. What I'm telling you is that you need to prototype your process in order to establish how you're going to get the acceptable cure at some desired, or trade-off thickness, before you shoot the moon. There's only so much fiberglass in your inventory, so do a square foot at a time to figure out the details...

One other small detail: Just as all plastics shrink as they cure, all plasters and Portland-type cements expand as they cure. The inclusion of fiberglass may, or may not, induce lineal stress causing cracks if the concrete wants to expand longer than the fiberglass will stretch. (In plastics, if the plastic shrinks more than the fiberglass allows, it causes warping or bowing.) Nothing is easy.

Sorry, I know this is not what you were looking for.

 

[Brian Van Fleet]

I haven't read this whole thread, but one thing to keep in mind is that concrete is cured by damp heat, ~110° to ~140°F, usually the product of exothermic reaction, the heat produced by the combination of water and Portland cement (in appropriate ratio). In thin sections as you describe in the original post, your part will dry, but not cure, making it very friable. You need to get it up to ~130°F, while it is still wet, and keep it damp through the process, and/or increase your thickness to ~^3/4" or greater, which will still be marginally too thin causing too much heat loss during the curing process of 24 to 48 hours. What I'm telling you is that you need to prototype your process in order to establish how you're going to get the acceptable cure at some desired, or trade-off thickness, before you shoot the moon. There's only so much fiberglass in your inventory, so do a square foot at a time to figure out the details...

One other small detail: Just as all plastics shrink as they cure, all plasters and Portland-type cements expand as they cure. The inclusion of fiberglass may, or may not, induce lineal stress causing cracks if the concrete wants to expand longer than the fiberglass will stretch. (In plastics, if the plastic shrinks more than the fiberglass allows, it causes warping or bowing.) Nothing is easy.

Sorry, I know this is not what you were looking for.

It is not necessary to heat-cure concrete, only to keep it above freezing (above 50f is ideal), and it will actually gain more strength over a longer time when allowed to cure at a lower temperature. Also, concrete shrinks as it cures. Later, it can expand in heat, but not during the curing process.

 

[Drew]

Please, remember an amateur's work, just for my amusement...

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These are great, thanks for posting!

 

[my nellie]

Thank you for your kind words, @Drew
One thing that I missed to refer to is about leaching of lime from concrete into the soil. As per the instructions I read I kept those planters submerged into water which I changed every day (or every other day). I had also added some vinegar in the water. Kept them like this for one month or so. After that the pots were left under the weather conditions (sun, rain)

These two are already having their inhabitants
This is infact a tanuki. A piece of deadwood of Morus to which I have attached a seedling of Morus papirifera

And this is a Vitex agnus castus which has completed its first year into bonsai pot and training. I hope this coming season it will grow better...

 

[Joe Dupre']

I finally got around to making a few pots. Portland cement, play sand, fiberglass cloth, old towels and drywall mess tape were all tried and worked to varying degrees. This blue painted pot was the best. I used the cement/sand mixture strengthened with several layers of drywall mesh wound around the sides with an old towel soaked in the thinned cement mixture draped over the plastic form ( pot is built upside down).

I would say that the easiest and fastest method would be placing a suitable sized cheap container on a cement/sand soaked towel and pulling the towel over the sides and tucking it neatly into the inside. It would make a lighter, thinner but more rustic pot.

I figure pots in the 10" long size could be made for less than a dollar to a bit over a dollar if you actually wrap a towel around a Dollar Store plastic container.

 

[Animron]

There are some nice tips how to reduce weight of a concrete, especially suitable for bigger pots:
1. mix one part of Portland (or any other type) cement with two or three parts of perlite + water. Or mix one part of cement with 2 parts of perlite + one part of fine sand (a little heavier variation).
2. mix one part of cement with two or three parts of vermiculite + water.
3. mix equal parts by volume of sphagnum peat moss, perlite, portland cement, and water.
4. you can even mix cement with turface or expanded clay if you find the one with small enough particles.
5. you can do all above with a white cement (it's not that cheap though), if your goal is white or more light tint of your pot.

 

[Leo in N E Illinois]

There are some nice tips how to reduce weight of a concrete, especially suitable for bigger pots:
1. mix one part of Portland (or any other type) cement with two or three parts of perlite + water. Or mix one part of cement with 2 parts of perlite + one part of fine sand (a little heavier variation).
2. mix one part of cement with two or three parts of vermiculite + water.
3. mix equal parts by volume of sphagnum peat moss, perlite, portland cement, and water.
4. you can even mix cement with turface or expanded clay if you find the one with small enough particles.
5. you can do all above with a white cement (it's not that cheap though), if your goal is white or more light tint of your pot.

Mixing vermiculite and or peat moss will make a pot that will break down rapidly with freezing and thawing. This break down can be an attractive part of the process, ''hyper-tuffa'', as it deteriorates, and moss grows on it, the look can be ''cool''. Life span of a hyper tuffa pot in the Chicago area might be 5 years tops. In frost free areas it can last a long time.

Plastic chips have been used as light weight aggregate.

The expanded foam like recycled glass, sponge rock sold by Home Depot is also used commercially as a light weight ''green'' aggregate.

 

[peterbone]

Here's a pot I finished at the weekend. It's papercrete, for which you can find instructions here. It's fairly large at 33cm and seems quite strong. Slightly lighter than a ceramic pot. More photos here.

 

[Joe Dupre']

That's a really nice pot. I love the texture and the color. What did you use for a form?

 

[Coppersdad]

Here's a pot I finished at the weekend. It's papercrete, for which you can find instructions here. It's fairly large at 33cm and seems quite strong. Slightly lighter than a ceramic pot. More photos here.

@peterbone, thanks for the update. It will be very informative to learn how these creations stand up to use over time. The combination of sun, rain, frost and the pressure of the bonsai roots on the pot over the years should tell us a great deal.

 

[peterbone]

That's a really nice pot. I love the texture and the color. What did you use for a form?

Thanks. If you click the last link you can see other photos of the process. I couldn't find a suitable oval container as a mould so I made one out of soil (my garden soil is mostly clay).

 

[William N. Valavanis]

Try Ciment Fondue