If your going to fill with filler then pour, the epoxy will be perfect. The recent interest in river tables and epoxy casting has resulted in non thixotropic resins becoming available... The CSA grout will be lower cost and heavier and stiffer. I have found swarfs do not pack well if you are aiming at high filler ratios. But if they are free then all good, they do need washing/degreasing. Don't tell your other half that you put it through the washing machine....

Humidity is not a problem for CSA. Its the recommended concrete for marine environs over portland... Class C CSA's expand about 0.6% so gets squished into the form and mechanically keys so macro/micro cracking not a problem and not expected. Epoxy shrinks 0.5% so it depends on its great adhesion to the form to stay coupled. I'd go with CSA as its stiffer and denser, both on your wish list. If you use CSA wrap in plastic to stop water evaporating from the surface or keep the surface wet with a towel or similar for several hours. Haven't used it yet so new territory for me. Keep us informed!! Do a little test paperweight first and report back....I won't have any CSA here until new year. Regards Peter


https://www.ctscement.com/product/ultraflow-4000-8 this maybe better. The local chemist talked more about grouts then cements