Backfill is a mix of calcium bentonite and gypsum and is used to surround buried sacrificial anodes to improve the capacity of the anode by reducing electrical resistance.

Backfill can be placed around the anode during installation, the backfill must be well compacted around the anode and dampened down by liberal application.

Backfill should meet or be similar to the following composition:
(a) Calcium Bentonite …… 50% by mass
(b) Gypsum ……………… 50% by mass

(a) Very little swelling (25%) occurs when wetted from dry state, and minor shrinkage occurs during drying from wet state causing the formation of a minimal number of voids and small cracks.of water, before completing the total backfilling of the excavation
(b) The resistivity of the backfill when wet is relatively high – typically 3-6 ohm.m, and it has good water retention