Before pouring the concrete into the plaster mould I had made, I experimented with different mixes. I began by doing 4 different mixes; a 1:1 ratio of cement and concrete ballast (a mixture of sand, fine aggregates and coarse aggregates), 1:2, 1:3 and 1:4. Below are the images of how that turned out.
They are all quite similar, however the 1:4 mixes has more air bubble than the rest and the 1:1 mix is the hardest but is also quite brittle. I want the surface of the the form to be quite rough as the surface of mercury is heavily cratered, see below for a image of Mercury’s surface. I thought the 1:2 mix looked the best with no air gaps, cracks or bubbles and a little rougher than the 1:1 mix.
I did my first mix and poured it into the mould which was coated in Vaseline to create a barrier between the plaster and concrete. The first mix went wrong as the aggregates in the concrete were too big stopping the mould actually closing. So I have brought some sharp sand which I am going to mix with the concrete ballast, getting rid of largest of the stones in the mix by hand to give a smoother mix. The mix can’t be too fine however, as the aggregated give it strength. The mix also needs to be quite wet to allow it to run into the mould and settle on the surfaces of the form, but again, it can’t be too wet as this will weaken it. So I need to get a good ratio of water, small aggregates and sharp sand to get a mix will will run into the mould well filling all the gaps! The mix I will be using is 2:2:2 of cement, small aggregates and sharp sand.