If you can take the weight it would be a good application for ferrocement.
I've seen government programs for making water and grain storage tanks in
"developing" countries that way.

--
------------------------------------------------------------------------------
William R Watt National Capital FreeNet Ottawa's free community network
homepage: www.ncf.ca/~ag384/top.htm
warning: non-FreeNet email must have "notspam" in subject or it's returned