I went to Bauhaus to buy fine concrete as I’m planning to cast a kitchen countertop. While there, I got a tip about a fiber-reinforced variant that was more expensive but easier to work with since no reinforcement would be needed.
However, now that I'm home and reading the bags, it seems like the Bauhaus staff was confused. I bought Combimix Combitherm, which is described like this: "Fiber-reinforced cement-based rendering mortar for plastering on cellular plastic and mineral wool insulation."
Yes, it is possible to use in theory; this type of material is commonly used in the USA. The problems are as follows:
1. It is an a-putsbruk, which is the hardest among rendering mortars but not as hard as concrete. You can solve this problem by adding pure cement powder (construction cement) to the mixture without increasing the amount of water. Add a couple of kilos per bag you mix.
2. Due to its 1mm fraction, the slab will feel very homogeneous after grinding; you cannot grind out a beautiful structure. You can solve this by buying 4-8mm crushed stone and adding it to the mix. Remember to make the mold 5mm larger in all directions to allow for some grinding margin, as sometimes you have to grind quite deep to get a nice surface.
3. 30mm would have been borderline too thin to cast with fine concrete, so this mortar is a better fit. If you don't have a support surface with many support points for the slab, reinforce with galvanized reinforcement bars. If it's for a sink, add more reinforcement around it, preferably 12mm reinforcement bars.
Hello. Late update but anyway. The material WAS wrong. It took about 8 days for this casting to cure, and when I then lifted the slab, it cracked into a thousand pieces. Additionally, the surface was very porous.
Luckily, the mold was already there, so I washed/siliconed/oiled it, poured in regular fine concrete. I vibrated with a vibration table with some kind of uneven scrap iron (årklyka?). Reinforced halfway through. After 3 days: perfect result. See image.
I have filled and sanded a few rounds. Filled with sieved fine concrete. I have rubbed it in with Osmo hard wax oil 4 times.
