Which sealers can I use on a concrete floor prior to tiling

26 May 2023
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United Kingdom
I'm about to tile a 1950's kitchen concrete floor using Mapei Super flexible adhesive. I'm laying ProWarm boards first using the same adhesive and then the tiles.
I'm got some Bostik Concrete Sealer but I can't establish if it's suitable as a primer or not. Can anyone confirm?
I want to prime tonight so I can make a start with the boards tomorrow, so can anyone recommend a toolstation/screwfix product that is suitable if the Bostik product is not?
I was thinking the following Mapei Primer but not clear which surfaces it's for and which products it can be used with: https://www.screwfix.com/p/mapei-primer-g-5kg/5309p

Reading the back of the Mapei adhesive, there is no mention of a requirement to seal concrete floors other than it saying "very porous surfaces may be primed with Mapei Plaster Primer....".
Not sure if I would describe my concrete floor as very porous, nor would I describe it as plaster so I'm at a bit of a loss.

Thanks for any input.
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If you're going to use the ProWarm boards, the Bostik Concrete Sealer might not be necessary since the boards themselves will provide a good substrate for the Mapei adhesive and the tiles.

However, if the concrete floor is very porous or powdery, then a primer might be beneficial to ensure a stable, solid surface for the boards to adhere to. Mapei Primer G is a water-based synthetic resin primer and sealer suitable for use on absorbent substrates like concrete before the application of adhesives, cementitious levelling compounds, and mortars. It's designed to reduce the absorption of the substrate, improve the adhesion of the adhesive and prevent rapid drying.

The concrete sealer you have might serve a similar purpose, but it's typically used to protect the concrete surface from moisture, stains, or damage, rather than to improve the adhesion of tile adhesives. You'd need to consult the specific product information or contact Bostik for a definitive answer on whether it's suitable as a primer.

As a tiler, you should understand the condition of your substrate before proceeding. Is your concrete floor dusty or crumbly? Then you should definitely use a primer like Mapei Primer G before installing your ProWarm boards.

However, if the concrete floor is solid and non-dusty, you might be able to install the ProWarm boards directly using the Mapei Super flexible adhesive. This would certainly simplify the installation process.

Regardless, I would recommend contacting Mapei directly for advice on whether you need to use a primer with their adhesive, or whether the Bostik Concrete Sealer would be suitable. Every situation can be different, and it's best to ensure you're using the products correctly to guarantee a long-lasting installation.
Thanks for that. Very helpful.
I actually got a response from Mapei shortly after posting. I emailed them this morning, but didn't expect a response so quickly so credit to them.

Their reply stated:
"In answer to your query you can bond directly to the concrete floor without sealing it.
Provided that the concrete is a minimum of 6 weeks old, it will need only vacuuming first to remove all loose dust and debris prior to tiling."

I didn't tell them the condition of the concrete, but after lifting the laminate this morning I can confirm it's solid.

So based on condition of the concrete, your response, and that of Mapei it looks like I'm good to crack on without primer.
Might have a beer tonight instead of priming.

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