Mixing hard ballast

   I need hard ballast for two purposes. First, I need a single large block with positive buoyancy to make the ROV neutrally buoyant at depth. Second, I need something to make the tether neutrally buoyant all the way from surface to depth. So the ROV ballast could theoretically be compressible, because it's only working at one depth at a time. The tether ballast, on the other hand, has to be incompressible. I tried wrapping the tether wire in polypropylene rope, but it looks like the rope isn't light enough to float the tether on its own. I would need so much rope that it would defeat the purpose of having a single-pair tether. I'm going to try wrapping the wire in a single thin layer of polypropylene and also adding on hard floats every x feet. So I'll have distributed and lumped.
   I can't afford syntactic foam. At least I don't think I can. I've emailed several companies that make it but none of them have responded. Also, I'm pretty sure it's the type of thing you buy by the truckload, so that's not really what I need. The idea I came up with instead is to create a very low-density concrete. I'm trying to add in different light aggregates to create a mixture that is significantly lighter than water. Basically it's the same thing as syntactic foam, but with cement instead of metal.
   I went and talked to some people at UT's civil engineering department and they were happy to help. A few days later they gave me 40 pounds of expanded shale. That's about 40 times as much as I need. I can work with that. The first problem is that shale is stored wet. I had to bake it for two days in order to dry it out. The second problem is that after I dried it, I weighed it and found out that it was 95% the density of water. That's before I added in the cement or polymer resin to bind it together. So shale isn't going to work by itself.
   The other idea I had initially was concrete mixed with styrofoam. The foam provides the buoyancy and the concrete provides the strength. I was using fiberglass resin instead of portland cement for two reasons. One, the resin has half the density of portland cement. Two, it doesn't absorb water. But as soon as I fixed those problems, another one came along.

   It turns out polystyrene dissolves completely when it's mixed with fiberglass resin. So that won't work. When it dissolves, however, it produces gas bubbles, some of which stay in the resin. I might be able to use that. The sample I made with no shale, just resin and dissolved polystyrene, was almost neutrally buoyant. Instead of having polystyrene bubbles, I can have air bubbles. So now I'm looking for a cheap and easy way to aerate the resin.
   The other option is to go back to portland cement. I got some of that and I should be able to mix up some samples this weekend.

Unfortunately, you can't buy cement in small packages. It's only available in 90-lb bags. At least it's cheap. The main problem with this is that it's really dense. I don't know if I can add enough polystyrene or air to make it float. We'll see.