If I remember my old cement pond making days correctly, when the
cement has apparently 'cured' you can brush a solution of water and
sodium silicate over all surfaces, when that soaks in and dries it
seals all surfaces considerably

Regards, andy
http://www.members.aol.com/abdavisnc/swglist.html


While dry, carbon dioxide in the atmosphere will react with the calcium
hydroxide making calcium carbonate, limestone, in the outer pores, improving
the water tightness of the concrete. Then turn on the waterfall and monitor
pH. If the pH starts to increase, turn off the waterfall and allow to dry
again.
--
RichToyBox
http://www.geocities.com/richtoybox/pondintro.html