Gypsum/Bentonite Backfill (B1)

Backfill surrounds buried sacrificial anodes and serves 3 main purposes:

1. Provides a uniform environment around the anode to allow it to corrode evenly and allow effective continuous operation,
2. Lowers the anode-to-earth resistance & improved electrical conductivity,
3. Maintains a moist environment

• 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.

Backfill is available in 3 types:
B1 (for general use)
Calcium Bentonite …………50% by mass
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 Ω.m, and it has good water retention.
B2 (used where there is a permanent water table)
Sodium Bentonite …………60% by mass
Gypsum ………………………40% by mass
Swells greatly (approx 600%) when wetted from dry state & shrinks greatly when drying occurs from wet state causing the formation of extensive cracks and voids.
The resistivity of B2 when wet is relatively low – typically 1-4 Ω.m and it has moderate water retention.
B3 (used when improved water permeability is req’d)
Similar properties to B1 with additional sodium sulphate (typically 5%)