Liquid glass / water glass (sodium silicate / Na₂SiO₃) is another choice for making a sand casting mould. The seller told me that after mixing the substances, they will harden the mould by injecting CO₂ to it for industrial use, but in my case he wasn't sure if it would work as my mould is tiny and I haven't got CO₂ and the tools.
I am going to give a try anyway. I didn't inject CO₂ to the first mould either and it worked.
I mixed the silica sand and Na₂SiO₃ in the proportion of 10:1 for the first try. The mixture was soft and had a greasy texture, and little silica sand fell from the mould when I separated the top and bottom rings.
I decided to make another mould and leave it for a while, in order to see if a 'hardened' mould would be a better one. I came back after 15 minutes. It looked quite close to the first mould but harder and crispier, and about the same amount of silica sand fell from the mould.
I made the third mould with silica sand and Na₂SiO₃ in a proportion of 10:2. It was not so different to the previous ones, except the texture is stickier so that the mould was not neat and tidy. CO₂ is absolutely needed.