Mix fly ash with white cement? 1

[structuresguy]

I am working with an architect on a project that will have lots of exposed structural concrete. It is a high end project, and the architect is very concerned about the quality of the concrete finish. I have spec'd a grout cleaned finish for exposed areas, to fill any voids and provide a nice smooth surface. Ultimately, the concrete will be painted white. But the architect wants us to use white cement in our concrete, as he is worried about not getting a nice, bright, uniform white finish over grey concrete. Can't say I really blame him there, as I have seen the various grey tones show through paint before.

My question is, can you add fly ash (or slag) to a mix with white cement and still have a nice "white" concrete? When i have used it before, which has been faily limited, we did not have fly ash or slag in the mix.

 

[MiketheEngineer]

Or paint it twice

 

[concretemasonry]

The architect is a little to optimistic and not technically grounded.

White Portland cement has a low iron content for color purposes. Anytime you use a waste product(typically fly ash),in the concrete you are adding materials that have an effect on the strength and the visual appearance that is technically beyond the normal technology of concrete.

If you are dealing with pigmented (not just colored) concrete, iron oxides are the strongest and most permanent pigments available, but there is a limit on the amount used (usually related to the % of of cement).

If he is talking about "paint" he is a decorator and not concerned with concrete quality and durability. Anyone can go to Home Depot and pick out colors of paint.

There are some very good products that are not "paint" but are cement based materials used for years in the repair, renovation of significant concrete structures.

Dick

 

[hokie66]

Well, paint is the word most people, even engineers, use to describe a coating. For concrete, there is no reason why a uniformly white surface cannot be achieved using three coats of a 100% acrylic coating. This won't be a gloss finish, and a gloss finish is undesirable as it would make the wall look irregular. I see no point in the white cement, but no reason not to use it either, except the cost. Omit the fly ash, just use the Portland cement. Got to keep those architects happy.

 

[Zambo]

I agree with Hokie66. The white finish can be achieved with paint (or coating if that sounds more technical). I think the money spent on the white cement would be better spent on the coating system.

 

[cntw1953]

White cemment mixed with carefully selected sand can be trowelled on hardened concrete element to achieve desired looking. Back to home, it usually applied to a stand out element, such as a lobby wall, to draw attention. Would be quite expansive for large application.

 

[concretemasonry]

If you want a white coating, the best materials would be compatible with the base material from a coverage and durability standpoint.

Adding fly ash to concrete does not necessarily make it more uniform and lighter, but it can provide other properties to the concrete, but usually not for architectural purposes, especially if color lightness/control and uniformity are required.

cntw1953 has a very good point or solution. The use of a cement-based product is compatible IF a coating is acceptable to cover the variations caused by the tampering with the basic concrete mix. There are several cement-based coatings that have been used for many years for repairs and reconstruction of concrete. Some are available in a white color using white cement, finely ground silica and some chemicals for bond, densification and ability to easily be coated/bonded later. The use of the concept has been historical for repairing civil structure or restoring architectural concrete. I had my first encounter when using ThoroPlug (really just hydraulic cement) and then coating with the plain gray Thoroseal about 40 years ago on a dam repair and it is still doing the job.

While I hate to promote a single product, the Thoro line based on Thoroseal (available in white or gray) which is the leader and the basis for the bond and compatibility. It should not be confused with retail products (mainly "paints" or coatings) like the retail "waterproofing" materials sold in big box stores that are used in basements. There are also other compatible architectural coatings in the same Thoro product line, but the basic concept of white cement, finely ground silica sand and polymers/admixtures has worked for many years in coatings and architectural restorations.

There are now many similar products available in many countries, but some are just "knock-offs" that show be used with caution.

Dick

 

[dik]

You should do a test panel using white cement powder and no flyash taking care of the aggregate source and blend. Else, you may have problems with the colours.

Dik

 

[Ron]

The final color of concrete depends on the cement, the aggregate and any additives. Adding fly ash will bring back a gray cast to the color with little black flecks and maybe some iron staining.

Coat the concrete if you want uniform color.

 

[Zambo]

But Ron do you think "paint" is ok? There is a feeling that paint is cheap and temporary but coatings are better. But I think that suggestions of cement based products earlier in this thread may not be superior to "paints".

For paints there are 2 main options. Either you go for a construction chemical company such as Fosroc or Sika, or you go for a traditional paint company such as Akzo Nobel/International. The main difference being the primers.

 

[cntw1953]

Architects usually dictates the coating material. There are many products he/she may choose from to achieve the look he/she wants. I guess the architect in this case wanted the natural feel of the concrete - in white. Maybe shall check with architectural precast panel producers see how they played the color games.

 

[InDepth]

If you are looking for a white pozzolanic material, I would suggest using slag instead of flyash. You also need to be very careful about the quality and source of your slag/pozzolans. When performing a mockup, the same source and materials should be used.

Now, a more risky operation is to actually spray the surface of the concrete with chlorine to bleach it....and flusing the chlorine off as well.

Oh, look into TxActiv, good stuff.

http://www.italcementigroup.com/ENG/Media+and+Communication/News/Corporate+events/20060228.htm

 

[structuresguy]

Thanks all for the comments and suggestions. I was already thinking that adding fly ash might not be best is using white cement. I should also add that when I said paint, I doubt very much that they literally plan on buying a few gallons of glidden from home depot. However, the coating that the architect talked about did sound thin. He also said it was not some thick text-coat product that we are used to using over tilt-up panels, that would hide any small surface imperfection.

The reason we want fly ash or slag added is the project is a LEED building, and we need it for recycled content.

So I guess I will go back to the architect and try to convince him that using white cement is not the best idea. This was already my gut feel, and basically you guys confirmed this for me. He is just used to specing white cement in precast stone copings and such, and that is the finish we wanted for all our cast in place concrete. It seemed crazy to me to make main shear walls, slabs, etc, using white cement.

 

[concretemasonry]

LEED is just a bureaucratic way or term of of political correctness and it does not relate to structural engineering. - It is a political selling term.

There are sources of gray cement that use waste tires as a fuel for 40% of the fuel/energy for production, but apparently few architects have enough ambition and knowledge of basic building materials and are more concerned with the first "WOW" and visual experience.

There are more effective and practical methods that have been used by good architects for centuries. There are also some great white pozzolanic materials, but if the guy specifying and pulling the strings want it, there is little choice since he is elling appearance.

Enough said for todays politics!!!

Dick

 

[RHTPE]

Architectural concrete is best accomplished with an "as-cast" finish. No "grout-cleaning" or "bagging". And no painting. Just strip the forms and knock off the fins, and maybe fill the tie holes. Would not the LEED points be better achieved without the use of a coating?

If it is possible in your area, I would strongly recommend the use of a self-consolidating concrete (SCC) mix, combined with very tight formwork, to achieve the surface finish the architect desires. It may not be the "bright" white he desires, but the finish will be permanent and not subject to future reapplication. Many precasters have gone to producing SCC for this very reason. While SCC for cast-in-place strikes fear in the hearts of many contractors, once its idiosyncracies are understood, the end product far exceeds any other. It may very well be possible to produce an SCC mix using white cement and the appropriate aggregates, but that will require some trial batches by the local concrete supplier.

Just my NTBH opinion.


Ralph
Structures Consulting
Northeast USA

 

[structuresguy]

We thought of SCC, but this is a small town area, and the last time (about a year ago) we asked the local ready mix supplier about SCC, their mix designer didn't even know what it was! Scary.

I would love to use SCC, so maybe once this things get going in construction, the GC can work with the concrete supplier to go that route.

 

[archeng59]

If there is going to be a coating on the concrete, why worry about the cement color. Using regular cement with fly ash might result in cost savings by using more standard and more readily available materials in the concrete mix. The architect can specify the appropriate paints/coatings to get the final aesthetics desired.

 

[concretemasonry]

The architect just has to stop straddling the fence and make decision, like good, practical architects with materials knowledge have done for centuries.

White cement in ready mix is costly, not just is terms of material cost, but the operational costs of cleaning out mixers and truck drums plus the anticipated expectations and the effects of the forms and stripping agents.

There are many good coatings (not paint) that perform well on repairs and restoration and are generally cement-based for compatibility and durability.

A "paint" will give a quick "wow" factor initially, but the architect may want a white cement based structure (if it is in the budget)in case the paint fails early.

Dick

 

[structuresguy]

Well, I have convinced the architect to go with regular grey cement. He is going to make sure his coating will hide the grey concrete properly.

 

[fannanxx]

Paint? Did it come to mind that when painting ther is something called primer? or undercoat? or sealant? All the above will isolate the grey concrete core from the whitest of the white final paint coat. If I was the architect I will worry about the white paint greying from exposure to the elements and leave what is inside the conscrete to the professionals.