There’s a wide variety of fill material that can go in earthbags – road base, subsoil, gravel, crusher fines, etc. Use what’s low cost and locally available. But also consider durability and climate. Do you live in a very cold climate? Then consider using insulating materials in the bags. Do you live where flooding is common? Then raise the site as much as practical and use water resistant, stabilized fill materials such as lime stabilized subsoil.
This blog post introduces yet another possibility: pumice-lime or scoria-lime in the earthbags. This material is similar to pumice-crete, except cement is substituted with hydraulic lime. The basic process involves mixing some lime with one of these lightweight aggregates to improve wall strength and stability (loose fill materials tend to shift) but yet retain a fairly good level of insulation.
From Building with Pumice, by Klaus Grasser and Gernot Minke:
Building blocks can be made of natural pumice and lime. Indeed, such blocks used to be quite common. However, careful consideration must be given to the characteristics of the lime. In the first place, use only hydraulic -or better -eminently hydraulic lime. Dolomitic or magnesium lime, i.e. lime with a somewhat grey color, is preferable to fat lime, i.e. chalk-colored lime, for making good pumice. Lime blocks, thanks mainly to the fact that the grey types, as the name implies, contain more magnesium, which reacts with the silica fraction to give the finished product superior strength properties. On the other hand, whatever lime is used should contain as little salt as possible, particularly in the form of sulfuric acid, because salt causes efflorescence and detracts
from the blocks’ mechanical strength.
To obtain pumice-lime blocks with strength values exceeding 20 kg/cm²:
– the exact chemical composition of the lime and all pumice materials under consideration should be ascertained by way of careful chemical analysis, and
– sample blocks and compression strength test specimens should be prepared.