I am building a retaining wall and was wondering if there are any special requirements for the mortar. Presumably it would have to be stronger than normal mortar and have more mositure resistant attributes?
When there is a chance of sulphate attack its a good idea to use a sulphate resisting cement. Using sulphate resisting mortar below ground a mix of 3 to 1 is recommended. This strong mix reduces flexibility so where possible a thicker wall of two skins tied together would serve best.