Grout Advice for a Custom Marble Mosaic

[leehuds3]

Hi All,

New here but not new to tile installation. We are, however, in unfamiliar territory with a custom marble shower floor we designed, fabricated and are preparing to install. It's been an incredible effort up to this point and much like most perusing this forum, don't want to become another cautionary tail. So, we're seeking some advice.

Mosaic Materials:
-Nero Marquina (black)
-Calacatta Gold (white)
-Limestone (grey-taupe)
*All honed. Pics attached

Grout Specs:
Joint Size: 1mm
Color: Black

We spoke with Mapei a few times over the past week and they arrived at two options - both of which have some downsides, of course.

Keracolor SF
Pros: Superfine unsanded grout that works with narrow joints
Cons: Dark Cementitious grouts contain lots of pigment which could cause picture framing even if the tiles are sealed in advance. We’ve had success applying Miracle 511 to Carrera Marble with light-medium grey grouts, but have real concerns with black grout on the much whiter calacatta marble.

Flexcolor CQ
Pros: No pigment to cause picture-framing.
Cons: This is really designed for joints at 1.5mm, but Mapei seems to think it should still work with a 1mm joint. However, we’ve read that the quartz in Flexcolor CQ can still scratch marble, which seems likely given the extra effort needed to work the grout into such a narrow joint. Also, since this product premixed I suspect we cannot thin it out to ease that effort.

Side note:
We understand the grout specs are not ideal and we can surely debate what other options would be best suited for these materials, but in the interest in preserving the design we'd prefer to accept it for what it is and discuss best practices and how to minimize issues.

Hope y'all have some advice!

-Lee

[cx]

Welcome, Lee.

It sounds as if you are familiar with your stones and their applications, but I wonder if you are fully aware of the issues commonly associated with white marbles in shower, particularly shower floor, applications.

MAPEI's advice about the use of their grout in joints no smaller than 1.5mm would seem to comply with the general ceramic tile industry standards. I have tried their Flexcolor CQ in 1/16th" joints and found it to work quite easily in that joint size, but in a joint much smaller than that, I really can't say if forcing it into the joint would be as successful. I've never tested it on a polished stone surface and can't address the potential for scratching.

I have tested it in their black color and it was black and remained that way when cured.

Perhaps your design could survive with a slightly larger joint?

In any event, I would certainly recommend a test board and application of whatever grout you elect to use before committing to the floor.

My opinion; worth price charged.

[Jason@mapei]

You mentioned sealing the tile, are you referring to six side sealing or just sealing the surface. Our library includes an option for the Multi sided sealing. I have attached a Link here.

https://cdnmedia.mapei.com/docs/libr...sn=dd2bc862_18

[jeffnc]

Flexcolor CQ is "sanded", but it's more of an artificial thing than actual sand, and by design does not have sharp edges so it won't scratch tile. I've never seen it scratch anything including marble, and Mapei says it won't. Where did you read that?

[leehuds3]

CX
Admittedly there's always more to learn, and although we have read through most of the information available in the TCNA handbook regarding issues with natural stone in wet areas and best installation practices... our ongoing quest to find the right grout lead us to an interesting article. (article name and author below since I don’t have url privileges yet

Article: Marble moisture discoloration: don’t blame the stone!
By Pavlo Starykov

Since we will be installing our marble over a bonded membrane, I think the author makes a compelling argument for using epoxy adhesive and epoxy grout to 'provide a highly hydrophobic/water-repellent subsurface, almost waterproof grout joints, and a reduced-to-minimum presence of moisture inside its pores, which enables relatively quick topical water evacuation, evaporation of moisture, and drying of the stone.'... Forgive me for opening another topic in the same thread.

In either case, we'll put a few test boards together and see how Black Flexcolor CQ, Keracolor SF and an epoxy variety perform with white marble. Happy to share the results afterwards.

Jason
Thanks for the link. We had planned to seal the top and sides of the material, but not the bottom surface. I don't see the harm in 6-sided sealing, but do you think it's necessary since only the tile surface and sides come into contact with grout?

Jeff
I initially read it in Mapei Flexcolor CQ tech specs, but on the second read it just sounds like a generic warning "Some types of sensitive tiles such as glass mosaic or marble can be permanently stained, scratched, dulled
or damaged when grouting..."

I do remember reading something about it in another thread, but I can't seem to pinpoint where exactly - It's also entirely possible that I inferred that it could scratch tile if the rep referred to it as a 'sanded' product - although artificial as you pointed out. Thank you for clarifying.

[jeffnc]

I've seen the advice to use white thinset instead of gray for installing marble, but I've never seen a difference. Last year I took some carrara marble and installed it over drywall with gray thinset, drywall with white thinset, Kerdi with gray thinset, and Kerdi with white thinset. The only difference between the drywall and Kerdi is that the marble stayed darker longer because it took longer to dry (it is possible to dry out through drywall.) There was no difference whatsoever in my little test between white and gray thinset.

I had to do this to convince the customer that it was simply dampness that was darkening the marble, and it wasn't permanent. Something the designers at tile stores typically fail to tell customers, among other maintenance issues.

leehuds, that is interesting, reading the tech spec now I can see they tried to cover their ass or something. It says both "Use for glass tile/mosaics as well as porcelain, ceramic and natural-stone tile (such as granite, travertine, slate, marble and polished marble" in one place, then "Some types of sensitive tiles such as glass mosaic or marble can be permanently stained, scratched, dulled or damaged when grouting" in another place. ???

https://cdnmedia.mapei.com/docs/libr...rsn=f7065cad_0