Gypsum/Bentonite Backfill (B1)

B1 Backfill is a mix of calcium bentonite and gypsum produced as per AS2239.
The backfill surrounds the 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 of water, before completing the total backfilling of the excavation.

Zinc and Magnesium anodes are also available pre-packaged in backfill, using a calico bag. Anodes can be packaged according to AS2239, or to specific requirements. AS2239 requires that the cover of backfill needs to be a minimum of 25mm on corners, measured diagonally and a minimum of 50mm on the top, bottom and sides.

Technical Information

Designation B1 Backfill meets the following composition:
(a) Calcium Bentonite …… 50% by mass
(b) Gypsum ………………. 50% by mass

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.

The resistivity of B1 when wet is relatively high – typically 3-6 ohm.m, and it has good water retention