Sand is not bad actually, not as good as 'insulation' like rockwool or fiberglass of course, but it's thermal conductivity is lower than portland cement and having some thermal mass is a good thing to keep temps stable. Plus, it's very easy to work with.
Are you talking about using the cement dry or mixing with water? Getting the moisture out of a 2" layer between 2 steel drums would be an issue where the sand is going to be very low in moisture content to begin with.
Just thinking outloud and looking at other options, thanks.