The ballast mix contains plaster/pollyfilla, sand, and poweder paint mixed dry. I can't say the preportions as I mix it dry, but it needs to be about 50% plaster to stick properly. The sand came from the nearest beach and is quite coarse, and powder paint colours vary according to the effect required - normally a lot of black, with red, blue, and green mixed in. Here I wanted a greish colour that fitted with the stonework, cliffs, etc.
The dry mix is then applied to the layout with a tea-spoon, often through an old tea-strainer to ensure no lumps of paint or plaster get through, and brushed into place around and between the sleepers with a cheap stiff paintbrush. It is built up around the track and may cover the sleepers a little, but that is the effect required in a quarry. Once in place I use a plant sprayer to spray the whole lot liberally with water that has had a squirt of washing up liquid added, that is to reduce the surface tension rather than make bubbles, if the water forms droplets add more washing up liquid or adjust the spray pattern. It helps to start spraying away from the layout to build up pressure, otherwise the first spray splatters.
Once it is all well wetted more ballast mix can be sprinkled on to build up certain spots, and the process can be repeated as often as necessary. As always be careful around points, especially the tie-bars, and of course the track will need a good clean up afterwards. The rails should be painted before ballasting, but the sleepers get coloured with the ballast so I didn't bother painting them.
The photo below shows the tools required - the teaspoon has some ballast mix in but the jar containing the rest is not in shot. In this case I had mixed more than required when I built the layout, and kept the leftover in a jar, which meant touching up the damaged ballast was just a case of sprinkling it on and spraying.
The next two shots are supposed to show the dry ballast brushed into place, and the same after spraying with water. Not sure that it comes out, but I'm sure you get the idea!
The same technique has many uses. I use the same mix with different preportions of sand and colours to make roadways, gravel or ash yard areas, and even in a smoother brown mix, use it to cover all the open areas of the layout as a "soil" base for grass etc. In that case it can be mixed with water before applying, in effect a textured and coloured plaster mix.