I make my own limestone blocks as there is plenty of material available in Perth at a cheap price. I mix the crushed limestone with creme mortar and make reconstituted blocks. They cost about a dollar each compared to 7 or 8 retail. I make the mix in a full size cement mixer and tip from there into wheelbarrow. The mixer is on a stand which raises it 50 cms above the ground, as the brickies do. If the wheelbarrow is place in the right position there is no splashing with my mix.The moulds are placed on the ground on pieces of fibro or whatever. Your splashing I presume is because you make a very wet mix to fill your silicone moulds. Forget about slowing down the mixer and concentrate on a barrow that catches all the mix and can pour into the mould. If you look at a cement mixer truck, they have a channel/scoop running from the mixer bowl. On this channel they have a stop gate which controls the cement exiting the mixer. Perhapa you should get a large diameter plastic water /sewer pipe and cut it in half lengthwise and flow the mix into moulds. Please send your cheque for this advice to my retirement fund!!!