Very nicely done.
For others, one of the keys to
@moke 's success is the use of a vinyl-cementitious patching compound. Regular concrete is not intended for applications less than 4 inches (10 cm) in smallest dimension. The vinyl, in the cement matrix gives the flexibility needed to use in thin applications. Mortar and regular portland cement mixes do not do well in thin applications.
There are dozens of different cementitious-polymer blends, some use vinyl, some use acrylic, some use other polymers. My preference is for the vinyl-cementitious blend, though the others may work equally as well.
Ciment Fondu - a high alumina cementitious material (non-Portland cement) is a different creature than Portland cement. THe spelling "Ciment" is from French as the process was originally patented by LaFarge, in France. The spelling today indicates any of the high calcium aluminate cement materials. It is commonly used in foundry applications as it is very heat resistant. Concrete made with Portland cement will decompose at high, foundry temperatures. It is often used in "art projects" because it is much slower setting than Portland cement based products. It is also used more often in Europe, because it was invented and is still primarily marketed by the French company, LaFarge. Erik K uses Ciment Fondu for his works.
Personally I think The vinyl-cementitious patching products are MUCH easier to use than Ciment Fondu. I think the vinyl in the matrix makes them more durable, crack resistant than any other product.
I am retired from the concrete industry, so while I have not made any concrete bonsai containers, I have worked in the lab with concrete chemistry, for over 35 years. I retired 10 years ago, and have forgotten many details, so I don't claim to "know it all". And my area was admixtures for concrete, rather than other areas of concrete applications.
So for what it is worth
this is the type of stuff to use