white stuff from wall

[malo]

after 12 months there is a white dusty powder appearing on the wall of my conservatory in the mortar joints what is it, and how can i get rid of it

 

[masona]

It's lime.

Brushed off with a stiff natural bristle brush and collected so that they do not redissolve and enter the wall lower down.

Normally goes after 2/4 years of normal rainfall and can be reduced by keeping the masonry as dry as possible.

 

[Building Contractor]

Actually its efflorescence, a white crystalline or powder, often fluffy/fuzzy deposit on the surface of masonry materials like concrete, brick, clay tile, etc.
It's caused by water seeping through the wall/floor/object. The water dissolves salts inside the object while moving through it, then evaporates leaving the salt on the surface.

It can be relatively easy to remove compared to some other stains. Often these salts are water soluble and, if outside, may disappear of their own accord with normal weathering. This is particularly true of "new-building bloom." The water soluble salts can be removed by dry brushing or with water and a stiff brush. Heavy accumulation or stubborn deposits of white efflorescence salts can usually be removed with a solution of muriatic acid and scrubbing (1 part acid to 12 parts water -- this is a real acid, follow precautions on the label). Wet the surface well before and after the solution is applied.