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Discussion Starter · #1 ·
Hello
I need advice on how to treat efflorescence on plastered interior walls.
I am building the future house of my family in Mozambique and some time (I can’t tell you how long) after the bricklayers had plastered the walls white stuff appears on it. It is only on the interior part of the south wall (the one which doesn’t get much sun and that is most exposed to winds and rains) where this happens, except for one other small place on the exterior wall just next to an area on the patio, where rainwater accumulates. (Will fix this).

The primer paint on the exterior part of the south wall had an unusually big number of hairline cracks, which I treated with crack filler before I painted it with final paint a couple of months ago. The whole exterior part of the house got painted with primer about a year after it was plastered and was then left for almost two years before it was painted with final paint.

None of the interior walls have been painted yet, apart one room. In this room, the paint on the wall with the efflorescence (the south wall) begun to peal off pretty much immediately. I don’t know if the painter cleaned off the efflorescence from the wall before he worked on it, I suspect he only sanded it. First, he applied gypsum paste and then a primer and finally the final paint. The gypsum is also coming off from these areas. These walls where painted during the rain period, 5 months before I treated and painted the outside wall.

On the exterior wall no paint has come off so far (painted in May this year).

Also want to mention that the concrete slab roof of the house was without waterproofing for a year or so, with rainwater accumulating on it that caused the parapet to crack in many places so that the water could get out. The roof is now waterproofed with an EPDM liner.

The efflorescence appears in streaks all over the interior part of the south wall, on the ground floor and on the first floor, the worst area being in the stairs tower leading up to the roof terrace (but still only on the south wall and a bit on the south-east). In the same places where there are white streaks there are also dark areas on the wall. These areas where humid during the rain period, before I treated and painted the exterior wall. Right now we are in dry season with the rain period expected to begin in November.

I can’t tell you whether the affected areas have grown over time or if they have stayed the same, since I never gave this phenomenon any thoughts. Didn’t know that it was a problem before the painting started to peal off in the 1st room.

The house is designed in a way that the south and north walls are very long (about 35 m each) with only 4 m wide east and west walls. This means that the south wall and tower wall are about 200 m2 all in all.

The house is built with a steel reinforced concrete structure with walls of burned hollow clay blocks. Big and small construction companies in the area use these blocks and I have never heard of them causing similar problems. All interior and exterior walls are plastered with a mix of cement and fine sand. I have seen some other constructions in the area with efflorescence, probably due to salt in the sand or water used for plastering. In theses cases the efflorescence appears mainly half a meter off from the ground. Not like me, all over the wall.

Please advice me how to treat this problem.
 

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retired painter
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Efflorescence is caused by moisture passing thru masonry. If the exterior walls are adequately sealed the efflorescence won't grow.


Here in the states we don't normally plaster over block/brick on the inside so the only thing I've ever done is wire brush off the block what I could of the efflorescence before painting. Never had any issues painting over efflorescence that way. I suspect your issue stems from moisture coming thru and degrading the gypsum coat. When it fails so does any coating applied to it.
 

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Discussion Starter · #3 ·
Thanks a lot for taking your time to reply! The exterior walls are now painted with primer and two coatings of exterior wall paint. The hair cracks where treated with a outdoor gypsum. Is this enough sealant?
 

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I'm not sure what outdoor gypsum is. The gypsum products we use are water soluble. When moisture migrating thru the exterior wall is expected to be an issue I like to patch the cracks and then coat the exterior wall with an elastomeric paint.
 

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Discussion Starter · #5 ·
Ok it't not gypsum. It's a portuguese product called Alltek. It's an aqueous paste for filling and regularization of exterior surfaces, with good elasticity and hardness. Hm, to paint the whole exterior wall with an elastomeric paint, don't know if that is a good idea. It would have to be the whole house in that case.
 

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While you would need to entirely paint the affected wall, elastomeric paints can be tinted to match most other paints [not the real deep tints] One wall painted with elastomeric will look different but many won't even notice that difference.
 

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Discussion Starter · #7 ·
I am not worried about the aesthetics. However, I have understood that if I use elastomeric paint I will have to paint the whole house with it, otherwise humidity might leak from the other walls in behind the elastomeric paint and cause bubbles. Since the elastomeric paint will constitute a barrier.
 

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Not sure you can purchase this product in your location but look for XYPEX this is a dry powder that you mix with water and brush on it will stop the moisture migration of the moisture on a concrete wall or ceiling it can also be used as an admixture. As for the roof slab being left unprotected and allowing water to enter the clay block cells could very well have caused your effervescence problem as there is now salts in the cell area and any moisture will cause it to migrate thru the wall. You indicated that other houses in your location have the effervescence problem at ground level look very close and see if the effervescence looks like it has hills & valleys if it does it is caused buy ground water wicking up from the ground thru the clay blocks. You also stated that it is only the South wall is this wall the hottest area during the day ? & is the interior air conditioned ?
To answer your question on the use of an Elastomeric coating yes any moisture that is trapped behind the coating will cause Blistering. Ask the manufacture of the coating what happens if moisture ( water ) get on the back side and see what answer you get .
 

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Discussion Starter · #14 ·
Not sure you can purchase this product in your location but look for XYPEX this is a dry powder that you mix with water and brush on it will stop the moisture migration of the moisture on a concrete wall or ceiling it can also be used as an admixture.
Thanks! Will look for it. Does it also work on plaster?

As for the roof slab being left unprotected and allowing water to enter the clay block cells could very well have caused your effervescence problem as there is now salts in the cell area and any moisture will cause it to migrate thru the wall. [/QUOTE] Hm. If that is the case, wouldn't the efflorescence only be on the upper parts of the wall, or at least only on the 1st floor? It is all over the south wall, ground and 1st floor.


You indicated that other houses in your location have the effervescence problem at ground level look very close and see if the effervescence looks like it has hills & valleys if it does it is caused buy ground water wicking up from the ground thru the clay blocks. You also stated that it is only the South wall is this wall the hottest area during the day ? & is the interior air conditioned ? The South wall is the wall that get least sunshine and most wind and rain. No A/C yet.

To answer your question on the use of an Elastomeric coating yes any moisture that is trapped behind the coating will cause Blistering. Ask the manufacture of the coating what happens if moisture ( water ) get on the back side and see what answer you get .
 

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You stated that the structure is metal frame with clay block walls.
If the interior floor is supported by the steel structure & the floor terminates on the interior side of the clay block walls water can enter at the top & exit at ground level. Also if the roof deck is the same design water can enter where the roof slab meets the clay block wall. Most clay blocks are used as infills & are not structure supporting. As for the use of Xypex it is for Cement only I am not aware of it being used over Gypsum products. If your walls are all Cement Plaster you can use it on the walls. You stated that the South wall is the coolest & dampest wall. You may have a dew point problem with the dew point occurring within the wall area which could cause efflorescence on both sides of the wall. You are going to have to make sure that all points for water to enter is made as water tight as possible. Check all Flashing , Caulking joints & cracks on the exterior to insure all are correctly sealed. Than water proof the exterior.
After the above is completed you should weight one month of time for each inch of thickness of the wall so if your wall is a total of say 6 inches the drt out time will be 6 months.
 

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Discussion Starter · #17 ·
You stated that the structure is metal frame with clay block walls.
If the interior floor is supported by the steel structure & the floor terminates on the interior side of the clay block walls water can enter at the top & exit at ground level. Also if the roof deck is the same design water can enter where the roof slab meets the clay block wall. Most clay blocks are used as infills & are not structure supporting.
The structure is not metal frame. It is a regular steel reinforced concrete structure. the interior (1st floor) floor slab is made of prefabricated concrete beams with cement blocs put in between, put on top of the cast steel reinforced concrete beams and then cast concrete on top. The slab doesn't stop before the concrete beams. It was cast on top of them.

As for the use of Xypex it is for Cement only I am not aware of it being used over Gypsum products. If your walls are all Cement Plaster you can use it on the walls. You stated that the South wall is the coolest & dampest wall. You may have a dew point problem with the dew point occurring within the wall area which could cause efflorescence on both sides of the wall. You are going to have to make sure that all points for water to enter is made as water tight as possible. Check all Flashing , Caulking joints & cracks on the exterior to insure all are correctly sealed. Than water proof the exterior.
The roof is very waterproofed, with an EPDM liner. No problems there. But I have been explained that a cement facade should not be waterproofed as the cement needs to breath. It I were to waterproof the facade, how would that be done?
 

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OK it does sound like your problem is caused by the water entering the wall as for the concrete breathing that is correct.
You should now be looking for a coating that becomes part of that wall system.
Look into using a Potassium Silicate product like you will find manufactured by Keim look at both the Façade coatings & Concrete Coatings. All of these products will become an integral part of your wall system.
As for Xypex it will work on the exterior of a wall it only comes in a grey color.
 
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