Years ago a company i worked with a company that used to repair failed screeds. This was done with an epoxy resin, the difference was it was very thin and just poured over the sandy screed. It soaked in and set hard fixing the problem. Now what you can do is partly similar to Jazz's suggestion. Take some granite chippings (or pea shingle) and mix this with silica sand as your base. Use some epoxy resin and thin it down with automotive thinners and ram it down the tubes. Or fill the tube with the dry mix and add the liquid (do it in small sections). This will give you the result you want at a reasonable cost. Now the reality is you are filling a tube so the type of resin does not really matter and ordinary fibre glass resin will do.