Ask the Experts – January 2017

QUESTION

Attached are pictures of an exterior grout leaching problem we are having on concrete, waterproofed with a membrane with a latex-modified thinset mortar and sanded grout with additive. Latex leaching (or efflorescence?) came through the grout not long after installation.
We removed all the grout three weeks ago and covered with plastic as you can see. It never got wet for those three weeks and we had fans on the tile. Yet when a penknife was pulled through the joints, the material was still a little damp and the latex is still coming through.
We feel it is in the mortar we used. A manufacturer rep is supposed to look at it. The manufacturer said for us to use unsanded grout with an additive. I do not feel like that will work at all – the latex is leaching through, even with no grout in the joints.
We felt like an epoxy grout would be the answer to fix this. What is your professional opinion on this?

ANSWER

My suspicion is that this is latex migration coming from the mortar.
This does not necessarily mean it is a problem with the mortar itself. I suspect the latex in the mortar may not have been allowed to fully coalesce and may continue to be an issue. Have you lifted a tile to examine the coverage and condition of the bond coat?
It is good that you have asked the manufacturer for a review. They will be able to assist you in determining whether this is efflorescence or latex migration, and its source. If the residue is powdery and salty it is efflorescence. If it is hard and more difficult to remove it is likely latex migration.
It is important to solve the problem then select the grout. Trying to lock in the migration with epoxy grout is not necessarily a cure for the issue. The source of the efflorescence or latex migration must be determined then remedied to ensure a long term successful solution.
If epoxy is eventually selected as a grout, ensure it is rated for UV exposure on an exterior installation.
– Mark Heinlein
CTI #1112,
NTCA Technical Trainer/Presenter