cyberzed - 29-11-2006 at 00:19
hi, was wondering if anyone know the answer to this one ...
I live in a newbuild home and got a white rash on the walls in the carport, first i was thinking it was chalk so i tried to wash it off with a dilute
solution of sodium bicarbonate, but it wouldnt come off all that well so i'm guessing this is not chalk.
Anyone has an idea what this white substance could be?
The idea would be to be able to get it off easily without corroding the bricks.
Blind Angel - 29-11-2006 at 01:19
If it's chalk, isn't it Calcium and Magnesium Carbonate too with some oxyde in it? Then wouldn't be a dilute solution of hydrochloric acid or simple
household vinegar be a better solution?
not_important - 29-11-2006 at 02:45
While acids will remove calcium carbonate, they're not the best thing to apply to masonary. You could do a small test, even vinegar would work;
fizzing pretty much means carbonates.
In pottery-land a white coating appearing on unfired greenware usually is calcium sulfate leached to the surface by the water evaporating from the
clay. Treating that with bicarbonate isn't going to do much, unless you really flood the surface.
If the bricks are fired clay, then there may have been sulfur compounds in the clay, and/or the fuel used to fire the bricks. If concrete then
perhaps there was sulfate in the water used when mixing it.
No matter what the actual material, the reason for it appearing is very likely due to moisture contacting the wall and being wicked through the bricks
until it reaches a surface where it can evaporate. Any dissolved salts it picked up, even slightly soluble ones such as calcium sulfate, from either
the wall materials or from before it reached the wall, will be left on the surface. Water high in CO2 can pick up some calcium carbonate by forming
the bicarbonated, which changes back to the carbonate as the water evaporates; that's how stalactites and stalagmites are formed. I'd worry about the
water source, and how much is moving through the walls, more than the salts left behind.
woelen - 29-11-2006 at 03:29
I know the white powder. We also moved to a newly built home 4 years ago and the white powder appears on the concrete and the bricks. I analysed some
of this powder and it only slightly fizzes in acid. I think it is mainly calcium sulfate, with only a small amount of calcium carbonate.
I removed it with a broom. It does not disappear completely, but repeated brushing every month or so makes the white spots disappear. Last year we
hardly had to brush anymore, it almost completely is gone. It looks as if the walls are 'sweating', moisture slowly being expelled from the walls and
calcium sulfate being leached out. Calcium sulfate is soluble, but only in minute quantities, but if it is leached out all the time and the
transporting water evaporates, then in a months time or so, it can leave behind quite some white powder.
cyberzed - 29-11-2006 at 04:41
Hey thanks for the reply's, that trick of the broom i was already aware of, because my wife stuffs this broom into my hands too many, and i's also
pretty big room to go brushing many times.
I wanted to wash the walls once so that i would only have to do this once.
Isn't there any particular solvent that i could use to remove this rash faster then with water?
I didn't try any cleaning product yet so ..
[Edited on 29-11-06 by cyberzed]
12AX7 - 29-11-2006 at 07:32
Nope, not really. Best you can hope for is applying water on the outside (not very easy if it's painted 
) and soaking it out to the inside so you can brush it all off at once. That's not very practical, so you might as
well live with it (or paint it ).
Tim