FMF - Friends of Minerals Forum, discussion and message board

[MSrockhound]

Why do high values of cation exhange capacity cause engineering problems? high values in things like montmorillonite would swell less than those of kaolinite, with its low exchange capacity, so why it it such a risk to engineers to work in high CEC clays?

Cheers,
Chris

[Matt_Zukowski]

Many civil engineering problems concerning soil has to do with strength, not CEC. Montmorillonite is a swelling clay that looses strength rapidly with increasing water content. If you are trying to build a road or a foundation, you do not want place your structure on "bad soil," which usually includes any fine grained material like clay, but especially swelling clays.

If your goal as an engineer is to isolate with an impermeable barrier, swelling clays are the material of choice. Different horizons in wells are separated by placing bentonite (mostly montmorillonite) in areas where the designer wants to prevent the vertical movement of water. Landfills are lined with, among other barrier material, layer(s) of swelling clay.

Cation exchange capacity does have some engineering applications, especially as filters, but swelling clays are lousy filters because they become impermeable goo with the addition of water. Zeolites usually make better mineral cation exchange media.

[MSrockhound]

Thank you so much! I was confused looking at diagrams of CEC of kaolinite and montmorillonite but you have have put it in terms much easier for me to relate to a particular problem on my eng geo final exam!

Cheers,
Chris

[Matt_Zukowski]

Fell free to PM me with any other questions you may have. I have a MS in geological engineering and a PE in civil.