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SAN-EARTH M5C vs. Bentonite

False Equivalents: SAN-EARTH vs. Bentonite

Customers looking for ways to achieve low resistance grounds sometimes think that SAN-EARTH M5C conductive cement and Bentonite will provide similar results. Nothing could be further from the truth.

  • SAN-EARTH M5C is the original conductive cement developed by Sankosha Corporation in Japan almost 40 years ago.
  • Bentonite is very inexpensive but it is a poor conductor whose performance is completely dependent on its water content.

Bentonite (sometimes brand named as Lynconite II, Wyo-Ben or Electro-Fill), is good for many purposes. Its absorbent qualities make it perfect for kitty litter. It is used as drilling mud in the oil drilling business. Face creams, mud packs and face powders sometimes contain bentonite. It is used in paper manufacture, ceramics, metal foundry molds, water treatment, pharmaceuticals and animal feed. And while it may be well suited for all of these applications, it is not a good conductive grout.

In its dry state, Bentonite is an insulator. It must be wet to be conductive. If it is installed as a dry powder, it will be an insulator, not a conductor. When it is wet, it expands to many times its normal size. When it dries out, Bentonite shrinks back to its initial size. If bentonite is mixed with water and installed as a slurry, it will shrink and pull away from both the conductor and the surrounding soil unless a large volume of water is continuously present. In the meantime, its resistivity will also increase to 20,000 ohm-m or higher.

The following graphs show that wet, damp or dry, SAN-EARTH M5C has a resistivity of between 1 and 4 ohm-m. In a dry state, Bentonite’s resistivity ranges between 20,000 and 60,000 ohm-m.


After storage in a 98% relative humidity chamber for four days, Bentonite’s resistivity is between 250 and 500 ohm-m. SAN-EARTH M5C’s resistivity, under the same conditions, ranges from 0.5 to 3.7 ohm-m.


Even when mixed with water according to typical grounding application instructions (13 gallons of water per each 50 pound bag), the resistivity level of the Bentonite is only reduced to 12-16 ohm-m.

You cannot depend upon Bentonite to provide the low resistance results the electric power companies and telecom system operators demand for their systems.

Wet or dry, SAN-EARTH M5C, with nearly forty years of consistent performance, will provide a low resistance connection to earth and we encourage anyone designing a grounding system to recognize the significant differences between these two products.

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