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Concrete turning to dust

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This is the ceiling of my pool pump house, where you can see the ceiling is literally turning to dust. It happens only at two locations, which is the far left and far right side.

The top of the ceiling is clad with slate stone.

Any suggestions on why this is happening, and how to repair and prevent it continuing?

IMG_20260223_082734_880@1790058834.jpgIMG_20260223_082817_995@179488829.jpgIMG_20260223_082743_795@-1748224429.jpg

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It's because they didn't mix the concrete properly.

Replace roof with properly mixed concrete, or some other roofing material.

  • Author

Replacing the roof is a no-go, as it would require to remove pumps and everything else inside, so I'm looking for solutions to fix and prevent.

Note the pump house is 15 years old and the deteriorating only started about 2 years ago

The discolouration shown on the photos might indicate concrete cancer. Some info on this link:

https://www.southernremedial.com.au/what-is-concrete-cancer

Not sure this is a massive help to you (not my intention to be 'smart' on this), but hopefully might help understanding of the issue.

  • Author
3 minutes ago, Watawattana said:

Google AI has some suggestions that may or may not be relevant to you. The discolouration might indicate concrete cancer. Some info on this link:

https://www.southernremedial.com.au/what-is-concrete-cancer

Not sure this is a massive help to you (not my intention to be 'smart' on this), but hopefully might help understanding of the issue.

Thanks I had a quick read of that link.

I suspect water ingress, as I actually have 2 pump houses with the same symptoms.

Both have their roof clad with slate stone, and I suspect water get under it. There is no issue with the walls of the pump houses, only the ceiling.

I am considering to cover the roof with a skim coat mixed with waterproof admixture, then repair the ceiling inside, and maybe cover it with waterproof paint.

https://down-th.img.susercontent.com/file/th-11134207-7ras9-m93wmadjq4pb63https://down-th.img.susercontent.com/file/th-11134207-7r992-lthjigk19xmnd4https://down-th.img.susercontent.com/file/th-11134207-7r992-lu7ltjlziegx7b

Another suggestion which may give you a longer-life repair (but more expensive) is to apply an epoxy resin bonding agent (I used Sikadur 32TH which I got from Sang Chai) and then a skim coat of Lanko 731 fiber reinforced repair mortar (which I also got from Sang Chai).

You need to use a wire brush to remove all the loose material, then vacuum to remove dust, apply the bonding agent with a brush in a circular motion (not side to side), let that cure and then apply the structural mortar.

  • Author
5 minutes ago, banyanman said:

Another suggestion which may give you a longer-life repair (but more expensive) is to apply an epoxy resin bonding agent

I researched that also the other day, and found sources which advised against that on poorly cured/soft concrete, as it would not stick for long.

A product like the Lanko 731 is what i will use on the ceiling.

1 hour ago, CallumWK said:

it

1 hour ago, CallumWK said:

then repair the ceiling inside, and maybe cover it with waterproof paint.

Do not apply anything from inside. You need to seal it from top.

On 2/23/2026 at 9:27 AM, CallumWK said:

Replacing the roof is a no-go, as it would require to remove pumps and everything else inside, so I'm looking for solutions to fix and prevent.

Note the pump house is 15 years old and the deteriorating only started about 2 years ago

Where are you at, and what kind of sand did they use ? If it is sand with salt, you can experience this, or just poor mix as someone elses says. Rebuilt is the best solution

2 hours ago, Watawattana said:

The discolouration shown on the photos might indicate concrete cancer. Some info on this link:

https://www.southernremedial.com.au/what-is-concrete-cancer

Not sure this is a massive help to you (not my intention to be 'smart' on this), but hopefully might help understanding of the issue.

Never heard the term concrete cancer, but contamination as described is most likely what it is. Seen it many places in Norway where they used seawater or sand from the shore. Seawater is not that bad as sand, but it will make your binding rust and expand

Just verify my poor explanation with AI

Concrete contamination

from using improper aggregate (sand from the shore) or seawater in mixing is a severe issue in Norway, leading to accelerated steel reinforcement corrosion, expansion, and cracking. The salt content causes high porosity, reducing strength, while chlorides destroy the protective layer on reinforcement, inducing rust and cracking the concrete

On 2/23/2026 at 9:27 AM, CallumWK said:

Replacing the roof is a no-go, as it would require to remove pumps and everything else inside, so I'm looking for solutions to fix and prevent.

Note the pump house is 15 years old and the deteriorating only started about 2 years ago

On 2/23/2026 at 8:37 AM, CallumWK said:

This is the ceiling of my pool pump house, where you can see the ceiling is literally turning to dust. It happens only at two locations, which is the far left and far right side.

The top of the ceiling is clad with slate stone.

Any suggestions on why this is happening, and how to repair and prevent it continuing?

IMG_20260223_082734_880@1790058834.jpgIMG_20260223_082817_995@179488829.jpgIMG_20260223_082743_795@-1748224429.jpg

Water ingress, look for leaks, reseal top

A way I in Denmark used to cure bad concrete quality – not mixed well or not enough cement in the original mix to save costs – was to take some pure cement and dissolve it in water to a paste, I've used it on old farm buildings, where some were build during WWII and there was likely used to little cement in the concrete mix. The cement-water mix must be liquid enough to be brushed on. Then brush it on the bad concrete in a thiock layer, and let the bad concrete suck the cement paste. Eventual, repeat the process once or twice. It will produce a hard crust on the bad cement. If the concrete deck you mention is like 10 centimetre thick, it might only be part of it that is bad – otherwise you will see the same on the opposite side of the deck – then you may solve the problem by incapsulatre the bad part(s).

The method is also fine to closed cracks on a cencrete deck by poring a fairly thin cement-paste out and brush it into the cracks.

Looks like long term water leak to me. If you can make it water-tight, then give time to dry, then treat the ceiling to improve the appearance.

  • Author
1 hour ago, Pdavies99 said:

Water ingress, look for leaks, reseal top

Yes that is also what I suspect, so will put a skim coat mixed with waterproofing solution on the top, and use repair cement to repair the ceiling.

Thanks everyone for the suggestions.

Nitrate (NO₃⁻) is an inorganic compound that can be present in soil, groundwater, and rainwater. In construction, nitrates can cause problems, particularly due to their hygroscopic nature and their potential to react with building materials. They can seep into masonry via moisture and crystallize upon evaporation, causing physical damage to mortar and bricks due to volume expansion. Nitrate can also contribute to the corrosion of reinforcing steel in concrete. While calcium nitrate is sometimes added to concrete as an accelerator and may inhibit corrosion, other nitrate compounds can be harmful to concrete. The presence of nitrates in construction can have various causes, including contaminated groundwater, contact with wastewater or fertilizers, or even the use of contaminated aggregates or mixing water.

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