Dark Streaks on concrete panels

[diazmeyer]

Hello,

I have a dark streak problem on my gray concrete panels.
It occurs from what I gather after rain. It seems that the panel is OK if it does not rain on it shortly after de-moulding.
The dark streaks seem to harden and it becomes almost impossible to remove them effectively.
In my opinion they are un-hydrated cement particles which are washed by rain an deposited as dark streaks which harden after a given time.

Any tips?.

Best Wishes

 

[shin25]

do you have slag in the mix? usually slag has a darker color after stripping but goes away with time.

 

[diazmeyer]

In theory the cement has no slag.
I will however ask the manufacturers if there is any.
Thanks for the reply.

 

[concretemasonry]

diazmeyer -

Are you you dealing with plant cast panels or site cast panels?

If they are plant cast, there are certification requirements (PCI) for plants that require accurate records on materials, batching and curing periods.

What kind or curing and controls over it?

From a "seat of the pants" viewpoint, there could be some problems of improper batching and mixing since the concrete does not sound like it is homogeneous. - Very common for site cast even with pant ready-mix since the placement and finishing could also be causing the problems.

Cement may contain slag as a raw material and still meet specifications. Slag is not necessarily detrimental and offers some real benefits in a proper operation or facility. What were the actual material specifications for the cement and aggregate? Aggregate could also be the source of the discoloration.

Dick

 

[diazmeyer]

These panels are plant cast,
There are no differences in batching or curing between panels with streaking and those with not.
It just affects the surface of the panel, rain seems to wash the surface and cause streaking.
It does not seem to be atmosphere contaminants, we tried tap water and the panels streak in a similar manner.
The paste is homogeneous.
We thought it was un-hydrated cement locked into the de-moulding oil which was free to move with water, causing deposits which hydrated and hardened.
Have a look at the attached photo.

 

[KateP]

It can be caused by content of CaO in cement. Take look at this aspect.

KateP

 

[diazmeyer]

I will have a look at the CaO content,
Thanks

 

[KateP]

One more time about dark streaks problem on the wall. My factory used to CEM I (then we didn't have a problem with it so much), but now we use CEM II/A-LL with more amount of CaO. After the rain I find out this kind of defections. We tried to clean it by water on the pressure, by grinding - but it didn't work.

Have You got some advices?

 

[Ultrafine]

Are you using class C fly ash in the mix at all? There has been a history of class C fly ash staining concrete when the humidity is high. I have seen in the past that putting acid to etch the concrete a little will lighten the stain enough so that it is not noticeable.

 

[KateP]

Ultrafine,

First: It's true with fly ash, but I thought that the problem is with amount of CaO in cement, only - so I was wrong. I infer that we are using class F fly ash (ASTM), not C, (The Europen Standard EN 206-1 doesn't give classes of fly ash, I have only several properties which may group this ingredient).
Amount of CaO was measured: 0,08 (free CaO) and 3,76 (reactive CaO).

Cement II - when we talk about binding process, strength - is similar to CEM I - we cheched it.

Second: I've heart about acid solution, but we don't want to use it. Once solution is painting the wall (but it doesn't mean it is good every time (I mean opinion of investor).