How to clean this outdoor granite wall?

[micjai]

Problem 1:
Some white powders appear on the stone surface after installation; the white powders still exist after cleaning with water. (photo1)

Problem 2:
Some stones turned darker after 2~3 weeks' installation (photo2)


How to clean it? Thanks!!

[Sponsored Links]

[Stoneguy]

It looks like you have 2 problems, efflorescence & mineral leaching. Both problems are a sign of water intrusion into the structure. There are many efflorescence cleaners on the market, one is linked here:

http://www.aldonchem.com/pr-efflorescence-treatment.htm

Simply cleaning the surface, unfortunately, will not solve the problem. Anytime water can get into a masonry structure, it is likely you will get these deposits at the point where most water concentrates, then gradually evaporates. As it evaporates, it brings dissolved minerals & salts to the surface.
In you second pic, the darkest areas are concentrated right by that caulk joint. Seems to me that caulk doing a better job of holding water in, than keeping it out.

Tell us more about this wall. Is there a solid cap? Is it a retaining wall with earth behind it? Are there any weep holes? Gravel backing? CMU behind?

[Davestone]

I would agree, and recommend capping the wall,and making sure there is drainage,maybe even removing the effloresence(although it looks like you also have mildew),then using a sealer called dry treat,but you must eliminate the efflorence first,try brushing it off dry,the less water the better.