how much cement to 25 kg of ballast?
You'll need a bit more than one 1 ton bags of ballast and around 6 bags of cement I reckon. A 1 ton bag is actually around 900Kg ish, so probably 40-45 bags at 25kg each is my guess. 1:4 ratio is quite strong, 1:6 would be fine - 12 shovels of ballast to 1/2 bag of cement.