There are two main types of materials used for mortaring or grouting stones
on waterfalls and streams that seem to work. One is a foam called Great
Stuff, which is injected between the stones after they have been dry
stacked. It expands and will bubble out of the joint area, but it is easily
cut with a knife to get the shape that you would like. It is a yellowish
color, but can be darkened by burning the surface with a torch, smearing
with soil, or applying dyes. They make a black foam also, but it is much
more expensive. These foams are flexible and allow some movement of the
soil without losing integrity.

The other is a Portland cement mortar or grout. I would stay away from
masonry mortar, since it has slaked lime added to make it more buttery, but
the lime has a pH of about 13, so it will affect the pH of the pond
significantly. A regular Portland cement mortar will also generate some
hydrated lime as a byproduct of the chemical reaction that makes it harden.
Allow the mortar to cure for about a month, and if there is only small
amounts in contact with the water, you shouldn't have to worry. If there is
a large amount of mortar present, you need to monitor the pH and possibly do
water changes due to the pH climbing too high. Portland cement mortars are
rigid and will crack due to shifting of the soil, or freeze thaw action.
--
RichToyBox
http://www.geocities.com/richtoybox/pondintro.html


"Norm Courage" wrote in message
ble.rogers.com...

We are planning to build a new waterfall on our 1600 gallon pond using
flagstone and we are looking for some tips as to what types of mortar are
safe for fish,plants etc, also would like any help in what type of
construction anyone has used when working with flagstone. Thanks in
advance
--
The Courages