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[Cob] Flyash in Cob foundation

Mahavir Mahedu lonthia at
Sun Oct 5 02:30:44 CDT 2008

Regarding the issue of moisture being wicked through the concrete foundation, I have a suggestion : why not add sufficient "flyash" to the concrete ?

It is well known that flyash reacts with the free lime in the cement concrete and converts it to Calcium silicate hydrate (CSH) which is larger in volume and occupies the voids and pores and thus reduces the permeability to a great extent.

There are two qualities of flyash available : Class C and Class F. One with higher calcium content and other with less. The one with lesser Calcium (Class F) is better at reducing or eliminating permeability of concrete.

I request the concrete experts in the list to comment on this point. Particularly on the extent to which the flyash can reduce the permeability and what proportions of flyash would be required to achieve this effect.

Also, what would be the effect of addition of flyash to cob ? at least to the clay plaster ? It would make the cob wall somewhat impermeable to water and also leave the walls breathing to some extent. Provided there is some lime in the clay to start with. Or we can use the Class C flyash which itself has some proportion of Calcium.


Mahavir Mahedu.

----- Original Message ----
From: Pack McKibben <gakayaker at>
To: coblist at
Sent: Friday, 3 October, 2008 7:20:16 AM
Subject: [Cob] Web Pages

For anyone interested, I've changed my HomePage to:

also, I'd like to add as many links to other natural builders, cobbers, etc. 
as possible.  Please send me your URL so I can add it to my links page.
if you're not already on the links pages...


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