Re: [GBlist] two questions-rainwater and concrete

[David Seth Melchert <dmelchert@earthlink.net>]

Danny Sagan wrote:

>
>
> 2- I have just heard about a concrete made with volcanic ash that makes the
> concrete imperveous to water penetration. Is there any truth to this? How
> will this affect our choices for concrete?
>

Pozzolans react with free lime (CaOH) in the concrete to form Calcium Aluminate
Hydrate. In normal concrete the free lime is highly soluable and water can
penetrate easily. In freeze-thaw cycles, or near salty sea water, the results
are disastrous (as Canadians are now finding). In high-volume flyash concrete
there is very little water penetration, as you have been told.

I have never seen this prescribed for the purposes of water repelling. It does
have significant implications for durability, (and thus greenbuilding). Ever
wonder why Roman cement lasted so long? However the water to cement ratio is
critical to the efficacy of this material. Proper curing is also essential.

Seth


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