White salt like powder out of my house bricks

[William_H]

Not boaty and I apologise but there are some clever people on this forum.
The discussion on Aluminium oxide in SS to Al bolts and how to dissolve it got me thinking.
The dark red brickwork of my house has a white powder coming from the bricks. I would love to be able to clean it off. I have tried hydrochloric acid as brick cleaner but this while dissolving the concrete mortar does not really shift the salts.
So I tried an alkaline Caustic soda. Again no real solution.
It probably is a local inclusion in the now 40 year old bricks but someone may have an idea.
Thanks in anticipation olewill

 

[Thistle]

Agreed it's probably from the bricks or from the mortar. As a salt it's unlikely to be affected by either acid or alkali (though as you have found, the bricks and mortar may be.) I'd suggest trying a pressure washer but try to aim it at the bricks rather than the mortar. As always, try any new treatment in an inconspicuous area first.

 

[Mistroma]

I'm not an expert in brickwork but have seen this before on plaster and brickwork. It's called efflorescence and I'm certain you'll find lots about it on the web. Normal mechanism is that dissolved salts leech out ot the porous brick and air drying concentrates the inorganic salts on the surface (i.e. white powder/crystals form). Actual chemistry can be more complex but this gives you a general idea of what happens.

You say bricks are 40 years old and it sounds as if the problem is new. That's odd after 40 years. If this is the case then something has changed.

One of several scenarios is that water has recently found its way into concrete that's touching the brickwork somewhere (under floor, near foundations?). Then Calcium Hydroxide in solution passes through the bricks. When it reaches the surface it combines with CO2 from the air to form a pretty insoluble carbonate. Acid washes should slowly dissolve this but probably attack the mortar more quickly.

Of course it could be something else entirely that neutral and pretty insoluble so won't dissolve in acid or alkali.

You could try to scrape some of the "white powder" into a container and add some hydrochloric acid. If has all dissolved by next morning then it probably was Calcium Carbonate after all. But I can't think of an easy way to get rid of it without damaging the wall. If you haven't fixed the cause then it would just return anyway.

Perhaps you'll get more practical advice from someone in the building trade instead of an ex. (very ex.) chemist.

 

[davidwf]

Had that on a block paved drive, no point in washing it, as all it does is go back into the bricks, best way to deal with it is to brush it off. Strange its happening after so many years though.

 

[Momac]

Has it been doing this long term and throughout the property?

 

[Lakesailor]

Quite a good article on here about it and possible causes on older brickwork.

But (an Oz site) warns - Insoluble white deposits appear almost as a milky film on the brickwork. The hard white deposits are insoluble in water and are invisible when wet. Do not confuse these deposits with efflorescence, which is soluble in water.
The regime for removing this scum is more difficult

 

[lw395]

Not boaty and I apologise but there are some clever people on this forum.
The discussion on Aluminium oxide in SS to Al bolts and how to dissolve it got me thinking.
The dark red brickwork of my house has a white powder coming from the bricks. I would love to be able to clean it off. I have tried hydrochloric acid as brick cleaner but this while dissolving the concrete mortar does not really shift the salts.
So I tried an alkaline Caustic soda. Again no real solution.
It probably is a local inclusion in the now 40 year old bricks but someone may have an idea.
Thanks in anticipation olewill

Might a different acid be better?
HCl will produce chlorides which tend to be white salts...
maybe Phosphoric or sulphuric acid?

Using a silicone water-resist might help prevent recurrent problems?

 

[VicS]

Might a different acid be better?
HCl will produce chlorides which tend to be white salts...
maybe Phosphoric or sulphuric acid?

Using a silicone water-resist might help prevent recurrent problems?

The colour of salts is more due to the cation ( the metal of which they are salts) that the anion ( acids of which they are salts)

Chlorides, sulphates nitrates phosphates etc of sodium, potassium, calcium magnesium etc are white

The same salts of other metals such as copper and iron cobalt nickel are coloured. Vanadium , mentioned somewhere in Lakesailors link produces a whole range of coloured salts depending on its oxidation state.

States of hydration can vary the colours too . Copper sulphate crystals are, as everyone knows, blue. Anhydrous copper sulphate is white.

 

[AntarcticPilot]

Might a different acid be better?
HCl will produce chlorides which tend to be white salts...
maybe Phosphoric or sulphuric acid?

Using a silicone water-resist might help prevent recurrent problems?

The point is that the only insoluble chloride that is at all common is silver chloride - and if you're getting that, I think your bricks are worth more than the house! Otherwise, chlorides are (mostly) pretty soluble, so they wash off. Nitrates are even more soluble, but nitric acid is rather nastier stuff than hydrochloric acid.

I knew that A-level qualitative analysis would come in handy one day!

However, there is a very good chance that this is something more complicated. A whole series of minerals are readily formed by the action of water on the components of cement and brick; in geological terms, thee are pretty reactive materials. The white deposit might well be a zeolitic hydrated silicate mineral; getting rid of that could well be difficult as it will be more stable than the underlying brick and cement; it must be as it wouldn't form otherwise. And the only acid I know of that routinely dissolves silicates is hydro-fluoric acid, and you REALLY don't want to touch that outside of a well-equipped chemical laboratory (my office was once the other side of such a lab where they dissolved rock in Hydro-fluoric acid!)

 

[Lakesailor]

According to the problem-specific article I linked to washing will result in the deposit leeching back into the brickwork, to appear all over again.
They also warn against any sealing or oils which make the deposits almost impossible to remove.

 

[Scotty_Tradewind]

Many bricks were fired with a salt glaze process where some of the salt remained near the surface of the brick unglazed.

 

[William_H]

Brick salts

Thank you very much everyone for your replies. The links were particularly interesting.
I suspect it is effluescence.
It was OK when I bought the house 30 years ago but has been getting gradually worse on particularly the eastern side. One area is [particularly susceptible to water from a down pipe.
It is more of a retaining wall than a house wall. The other worrying wall is stained in the middle and I can't imagine it getting wet from inside. Anyway I will get a wire brush and try dry cleaning. Thanks again olewill