I would suggest a high sand mortar, without Portland cement. This will degrade at a similar rate as the substrate while a Portland based compound will be harder and more resilient than the sandstone. This will cause severe spalling and fracturing of the stone under the stresses of a freeze- thaw cycle.

The recommended blend is called " type 6" or " type G" mortar. I believe it is 3 parts course sand to 2 parts slaked lime, but its been awhile for me so I would look it up. Perhaps in a CPT (center for preservation technology) journal or NPS bulletin. 

If you want to swing towards the preservation purist side of things you can use a synthetic polymer based stone adhesive. This is done to achieve a clear differentiation between original components and later repairs.

Hope that was helpful;
Sean Doyle
Preservation Specialist
SWCA Environmental Consultants
Broomfield, CO.

"Nancy S.  Dickinson" <[log in to unmask]> wrote:

 
What is the recommended grout for repairing a millstone?  The  millstone, 
early 19th century, has a rough 1" by 8" section that has broken  off an 
outside edge.  All pieces of the fine-grained sandstone are  available for 
mending.  Once repaired, the millstone will be installed in a  park as a 
landscape feature.  Although it will not be subject to weight  and/or pressure, it 
will be exposed to the extremes of a cold-weather  climate.
Thanks,
[log in to unmask]