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The type of backfill used in a groundbed depends on whether the cathodic protection system is sacrificial or impressed.
Chemical backfills: The chemical backfill used with galvanic anodes provides an environment which is conducive for anode dissolution. A typical mixture is 75% powdered gypsum (calcium sufate), 20% granular bentonite and 5% sodium sulfate. This mixture has a resistivity of 50 W cm and is suitable for use in high resistivity soils. The function of the bentonite is to absorb water and expand, thus ensuring good contact between anode and soil by lowering groundbed resistance. A 75% bentonite 25 % gypsum mixture (250 W cm) is recommended for low moisture soils.
Carbonaceous backfills: Impressed current anodes are usually surrounded by a carbonaceous backfill. Types of materials use include coke breeze, calcined petroleum coke and natural graphite. The dual purpose of the carbonaceous backfill is to reduce the groundbed resistance by increasing the effective size of the anode and to provide a surface on which oxidation reactions could occur. The latter function prolongs anode life. To ensure good electrical contact, the backfill must be tamped around the anode. Resistivity of carbonaceous backfills are in the order of 50 W cm.
Particle size and shape are also important when specifying a backfill. Both parameters determine the contact area between anode and earth whilst influencing the porosity of the column which is important for gas ventilation. A general purpose coke breeze is for use in shallow horizontal and vertical groundbeds. It has a resistivity of 35 W cm. For deep well applications a special calcined petroleum coke breeze is available. It has a resistivity of 15 W cm and can be pumped.
