How to stop the stones around the edge of the pond from deteriorating any further?

[mvdct]

Hi forum,

We moved into a new home last year. The garden has a pond, which has a pond liner in it - which seems to be in tact and some fish.

From what we can tell, at some point in the pond's past, a new row of sandstone stones was added to the top of the pond. These seem to be breaking down. Presumably, through freeze-thaw-weathering. See pictures below.

Not long after moving in, I added a thin layer of mortar on the top and side of the new row to try and protect the stones. The mortar layer was fine in the late summer/autumn. But during the winter, when the rain increased and there were frosts, it cracked and blew.

So, how can we fix this? Can it be done with mortar? Would the layer need to be thicker? If so, how thick?

Might it be useful to add lime to the mix, to give the layer more flexibility and breathability?

Are there any other solutions?

The pond has fish in it, so doing anything with paint or sealant with chemicals that risk leaching into the water is probably not something we can risk. - unless you know better.

All suggestions gratefully received. Thanks in advance for your help.

 

[Odds]

Replace with granite, slate or suitable river stones. Riprap could be used but make sure they're well bedded-in. Putting more mortar on sandstone is just kicking the can down the road.