While researching for my azulejos project i came across the existence of hypertufa. This is basically a sort of concrete made with peat moss, portland cement and perlite. The interesting thing about it is that it is light weighted so you can make a large object with it that still would be moveable. it is quite porous so perfect for drought-tolerant plants.
Hypertufa has the looks of natural tufa rock and gets an aged patina when leaving it outdoors. Moreover it should be winter proof so that we can leave it carefree outside.
All these properties make it a very interesting material. So i decided to do some experiments with this stuff. The most obvious choice is to make some planters. This is easy to do without massive investments. Everyone seems to make them these days...
The first workpieces are made by modeling the morter inside a mold. You could also work in a reversed way, model the morter at the outside of the mold. I am going to try both ways to see what the results are.
For the hypertufa we need:
- portland cement (white or grey)
- peat moss
These materials are sold in large packs so you have to plan a considerable production. You can't keep cement for a long time, it will harden and becomes unusable.
The first time I decided to use white portland cement, i hope that it becomes more beautyfull than grey.
For the mold we can make use of a simple plastic foodcontainers or a sturdy cardboard box with plastic foil on the inside.
There are many variations possible in the composition and the proportions of the material. You could replace a part of the peat moss by riversand and/or use vermiculite instead of perlite.
As to the mixing ratio there is a lot to be found on the internet. Normal concrete mixture is cement/sand/pebble 1:2:3, it is not correct to assume that the result will be stronger by adding more cement. Many hypertufa recipes prescribe a mixture ratio 1:1:1, others 3:3:2 and so on.
With the mixing ratio of concrete in mind, my first experiment was made with a composition of 2 parts peat moss, 2 parts perlite and one part portland cement. This gave a satisfactory result but I don't know if other ratios are better.
It is important to mix the dry materials very thoroughly. Tisi is best done when still dry. The larger particles in the peat moss must be crumbled because they may cause weak spots in the result. After the dry mixing i added water, bit by bit, to end up with a crumbly mud. Too much water would have a negative effect on the strength of the material. When you make a ball of a well made morter you can only squeeze a few drops out of it. in this consistency it is easiest to model and gives the strongest result.
In general it is important that the layer of morter is tightly compressed to make the material as dense and firm as possible. This works better if you model inside a firm mold. But the more sturdy the mold is, the harder it becomes to unmold. Here we have to find a compromise
The mold properties depend on the method of modeling. When modeling inside the mold you can use a fairly weak plastic object. When modelling on the outside, the mold should be so rigid that it cannot be compressed.
I started with modeling inside the mold so i took plastic food containers. You can grease them somewhat with non-stick foodspray for easy un-molding but i didn't do that the first time. I started wit pressing a layer of the morter on the bottom, some 2 cm thick, and than i formed the upright sides. I tried to make the thickness even for each side so that the result will be fairly symetric. I did not make the side as high as the mold as this would make the un-molding more difficult.
When the modeling is done, moving the fresh workpiece should be avoided. When you bend the week plastic mold, a crack may occur in the workpiece.
Next i carefully packed the objects in a plastic bag to prevent them from drying. They will be left alone for at least 24 hours to get an initial cure.
After 24 hours i tested the strenght of the workpiece with a fingernail. When strong enough you can't scratch it. Now you can carefully unmold it. This is the moment to do some post-processing. With a stanley knive i was able to cut the edges to round them off. The workpiece has a smooth surface at the outside, with many gaps in it. I decided to give one of the pots a dry rub with pure portlandcement. Curious what this will do at the end.
There are many ways to finish the surface, you can scratch figures in it with a knive, you can brush it with a wire brush, use a diamond file to file the edges, etc.
The other workpieces didn't get the dry rub. I packed them in plastic after spraying them with water. One day later i placed the pots under water as i had read somewhere that this would be better for the curing process. I stacked some of the pots in a bucket with aluminiumfoil between to prevent them to grow together. After a weekend i found that the aluminium completely had dissapeared and a smooth slimy haze was formed on my pots. The pots in the water without aluminium foil didn't have this haze. This seemed interesting to me, what is the composition of this haze and how will it influence the end result? So I placed some of the pots in another bucket with aluminiumfoil between. See what will happen. Now patience is required, 28 days seems to be be a good period to leave them in the water. This would give optimal strength to the pots.
The preliminary results are encouraging, the workpieces feel solid and look good. The workpieces out of white portland are light colored, not white though. With grey portland i get a color like the picture below.
I am very curious about the final looks of the pieces with the aluminum foil. On the internet i found what reaction is taking place and that this is not conductive to the strength of the pots.
I couldn't wait for the result so i started to make some other objects. This time i modeled the objects in the reversed way. I took some stainles steel kichen bowls because these molds have to be rigid. Now the mold is at the inside so that the outside is the hand modeled side. I liked the inside texture of the other pots so now i have that on the outside. It is much more difficult to work in this way, when you compress the morther to get a firm layer, it will extrude. When i doesn't fit around the mold anymore, pressure on the model may couse cracks.
After the initial cure i saw many weak spots in the pieces so i had to repair them. Also i rubbed the inside with portland as such that there are no pores anymore. Hopefully this can contribute to the final strenght. Maybe even seal the bowl so that it can be used as a birdbath. I think i have to let these bowls cure long in water.
What i learned with this reversed mold method is that you have to work very thorough and patience because it is difficult to form a sturdy and dense layer of uniform thickness around the mold.
I filled the cured pots with water, this would help to get rid of some harmfull salts (like lime) from the hypertufa. The pot that i rubbed on the in en outside looks watertight, so this seems defenitly a way to seal the hypertufa. Hopefully this also applies to the single-sided rubbed pots.
With the leftovers of the morter i modelled some mushroom hats. The stems i made by tapping the morter in plastic cylindershape packing material. After 24 hrs of curing i could cut these cylinders into club-shaped stems. I joined them after some weeks of curing.