Better minds can respond to this query ,but lime mortar absorbs water then  
at a certain point sheds it , 
salts are crystals that " bloom"with the wet dry process 
prior to Portland ,fort work was "capped " with Natural Cement (see  
conference with Mike Edison) this according to documentation found in the Army  Corps 
of Engineers 1898
Why? because of its strength and ability to shed water 
 
 

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