What causes Mortar to become 'sandy' and not adhere?


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Old 11-25-12, 11:03 AM
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What causes Mortar to become 'sandy' and not adhere?

We mortared over some of our fireplace bricks - prepping/filling so we can apply thinset & tiles. Some of the mortar is "sandy" - meaning that it brushes off when you rub your finger over it. Other mortar that we applied in the same area does not do this - very solid.

What causes that?

Thanks
 
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Old 11-25-12, 12:11 PM
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Mortar is not meant to be used as a skim coat. I suspect, too much sand in the areas that flaked. Brush off with a wire brush any loose mortar. Apply a cement based patching/embossing filling compound and smooth for your tile install. You might have to apply several coats to achieve desired smoothness. Speedfinish is one such compound:

SpeedFinish 10 lb. Patching and Finishing Compound-SF10 at The Home Depot
 
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Old 11-25-12, 12:22 PM
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Thanks. Although that product makes it sound like that is for floors.
 
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Old 11-25-12, 12:26 PM
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The product is mainly used in the flooring industry, but would work the same to fill in and level an uneven brick surface.
 
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Old 11-25-12, 12:32 PM
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PS - you said " I suspect, too much sand in the areas that flaked."

Why would that occur? I mean all we did was pour dry mortar into a bucket and mixed with water until it was like "peanut butter" texture. Would that happen if it was too dry or too wet? strange.

and thanks ... we'll go to Home Depot.
 
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Old 11-25-12, 02:24 PM
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Some of the mortar is "sandy" - meaning that it brushes off when you rub your finger over it.
Sounds like uneven drying.
Did you wet down the bricks (spray bottle) before you put the mortar on?

I'll bet the brick wall had a pattern? Some bricks were "darker" than others?
Bricks are basically a masonry sponge, and will pull the water out of the mortar
But, a brick with the fired (blackened vitreous) side out is "sealed" and won't pull the moisture out.


Hal
 

Last edited by Hal_S; 11-25-12 at 05:08 PM.
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Old 11-26-12, 08:19 AM
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Ah, that totally makes sense. No we had not moistened the areas first. And the ones that "sucked" in the water were the ones we had chipped the front off (to level them with the others). So they were bright red vs. the older "cured" ones. Thanks for the update!
 
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Old 12-18-12, 06:01 AM
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Well, shouldn't be a problem as long as you brushed the sandy mortar off before applying tiles.

And, the standard tiles-mounted-on-netting, should spread the the load so that IF there is a
weak bond over one brick, the netting should spread the load around.
 
 

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