White deposits on the exterior of a home are almost certainly efflorescence. They're just salt crystals that are left behind when water evaporates from the masonry surfaces around a house.
The problem starts inside a chimney, retaining and foundation walls, and patio paving stones. The source of the problem also can be what's behind these materials. Soil, brick, mortar, concrete block, concrete and other materials can all contain water-soluble salts. There are many different types of salts, sodium chloride being one of the most common.
Most salts readily dissolve in water. If you take saltwater and allow it to evaporate, the water turns into a gas (water vapor), leaving behind the dissolved salts.
Behind the walls
When rainwater enters a chimney, it dissolves the salts that may be in the mortar mix, the brick or even the sand used in the mortar. The same can be said about all the other masonry surfaces where you see the deposits. The salt can be in the soil, and groundwater is transporting the salt into the masonry surfaces.
Evaporation is the enemy
The water in all of a home's masonry wants to evaporate. That's true of all water, unless the relative humidity of the air above the water or masonry surface happens to be 100 percent -- and that's very rare. This means that you have a constant invisible conveyor belt delivering the unattractive salts to the surface of masonry all the time.
These salts are not harmful to a home but can be unsightly. To stop the efflorescence from occurring, you need to do one of several things.
Try a chimney crown
Stop the water from entering the masonry or soil that's dissolving the salts. This is much easier said than done. If you have a proper chimney crown with great flashing under it, that will help. You also can apply clear water repellents to the sides of the chimney to stop water from entering the masonry.
The back side of a retaining wall could have been treated with an asphalt spray to block water from soaking into the masonry. To solve the problem now, you can excavate all the fill material out from behind the wall and install a perforated drain pipe along the base of the wall. Backfill the wall with clean washed gravel so any water flowing through the soil toward the wall falls down through the gravel and never makes it to the wall. You can try to do the same thing with a foundation wall, but that could be a massive undertaking.
Trying to clean off the salt deposits with water or any other chemical or liquid is a mistake. The water you're using dissolves the salt and transports it immediately back into the masonry. While you're rolling up your hose and putting away your scrub brush, the invisible conveyor belt starts up and brings the salt back to the surface as the water evaporates once more.
Or just brush it off
The best way to deal with efflorescence, if you can't afford the waterproofing remedies, is to just brush off the salt from the surface. Depending on the amount of salt, it can sometimes be done with an old paintbrush or it may require a stiff scrub brush.
The salts in chimneys will eventually deplete themselves and the problem will go away on its own. But it can take years, depending upon where you live and the amount of salt in the masonry materials.