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Old 02-05-2013, 02:49 PM   #1
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Mortar Color


About a week ago, I repaired a mortar joint between two bricks that had deteriorated. I spent several weeks trying to match the color by creating samples of mortar by mixing a mortar pre-mix with charcoal concrete coloring to get a specific shade of grey. Once I was happy with the color match, I went ahead and mixed up a small batch, but instead of using water, I used an acrylic admixture in place of water to be sure the new mortar bonds well. Well, it's been a week now, and the mortar has yet to lighten up from what currently looks nearly the same color as the wet mix! The mortar is dry and hard to the touch but looks to be nearly the same color as the wet mixture! I've never had this issue before. It is winter time, but it hasn't been freezing down here in the south. The temperature hasn't really gotten over 50 farenheit though so could that be the reason the color is still dark???? Any thoughts are appreciated!

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Old 02-05-2013, 03:48 PM   #2
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Mortar Color


segal -

You did not use the same materials in the final batch as in your test samples - You used a chemical dispersion (acrylic) instead of water that can have a different effect on the pigment (or dye if you are cheap). Dealing with grays, there are several materials to use - carbon black (soot) that is mechanically locked into the cement matrix, but can wear off in time with weathering or black iron oxide, which is a pigment and is much stronger (tinting strength) and more permanent. Black iron oxide is not usually readily available in small quantities.

If you find a color you are happy with, use the same materials in the real batch.

Dick

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Old 02-05-2013, 04:59 PM   #3
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segal -

You did not use the same materials in the final batch as in your test samples - You used a chemical dispersion (acrylic) instead of water that can have a different effect on the pigment (or dye if you are cheap). Dealing with grays, there are several materials to use - carbon black (soot) that is mechanically locked into the cement matrix, but can wear off in time with weathering or black iron oxide, which is a pigment and is much stronger (tinting strength) and more permanent. Black iron oxide is not usually readily available in small quantities.

If you find a color you are happy with, use the same materials in the real batch.

Dick
I used a charcoal black synthetic mineral oxide powder. I have used the acrylic before but recall also mixing in equal parts water. This particular admixture can be used without water and the manufacturer recommends using it straight when mixing mortar for masonry. I'm rather suprised that the final color would be nearly the same as the wet mortar color, which is the reason for my posting. I've dealt with dark gray mortar when wet that turns nearly white when fully-cured, so this is rather strange that the mortar hasn't really lightened at all! I really want to avoid grinding out my repair and having to redo it.
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Old 02-05-2013, 09:41 PM   #4
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Mortar Color


You did use a good pigment - synthetic metallic oxide. Synthetic oxides are very good as opposed to natural oxides because the impurities and salts have been removed from oxides by the high temperatures that are used to get get different colors.

Apparently, the acrylic does something different than water regarding the curing. Pigmented concrete lightens (even with synthetic oxides) because the curing process of the cement always leaves some free lime in the matrix that makes the appearance lighter. A little free lime is not bad since it becomes cementing material over time with moisture and carbon dioxide exposure.

Hopefully, the shorter life of the acrylics are just delaying the curing process.

When you make samples, use the same materials in the real job mix and hopefully the test samples are allowed to cure well to get their final color.

Dick
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Old 02-05-2013, 09:58 PM   #5
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You did use a good pigment - synthetic metallic oxide. Synthetic oxides are very good as opposed to natural oxides because the impurities and salts have been removed from oxides by the high temperatures that are used to get get different colors.

Apparently, the acrylic does something different than water regarding the curing. Pigmented concrete lightens (even with synthetic oxides) because the curing process of the cement always leaves some free lime in the matrix that makes the appearance lighter. A little free lime is not bad since it becomes cementing material over time with moisture and carbon dioxide exposure.

Hopefully, the shorter life of the acrylics are just delaying the curing process.

When you make samples, use the same materials in the real job mix and hopefully the test samples are allowed to cure well to get their final color.

Dick
Thanks man I'm actually glad to know I used a good pigment that will hopefully stand the test of time. Regarding the curing time, it is quite possible that the process is in fact slowed down by the acrylics and the fact that it hasn't been warm lately. I'll give it another week or so before I grind it out and start my sampling process all over again, but this time with the admixture! Thanks again. -Segal
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Old 02-11-2013, 01:22 PM   #6
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Mortar Color


Well, I decided to grind out the repair because I didn't see an end in sight for the mortar color lightening up. I don't think I'll be using the acrylic latex admixture when I perform this repair again. I prepared a new sample with the admixture and another with just water, and in just one night, the mortar with 100% water went 4 shades lighter while the one with 100% admixture didn't even change from the original wet mortar color. Both mortars were hard to the touch, so it didn't seem like a drying time issue. Both were sitting on cardboard, which does seem to accelerate the normal drying time. Very strange IMO.

-Segal
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Old 02-11-2013, 02:13 PM   #7
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Mortar Color


Why would you think it strange?
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Old 02-11-2013, 08:03 PM   #8
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Why would you think it strange?
Well, my experience is that mortar color lightens up some as it dries. For it not to lighten up at all is very odd to me.
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Old 02-11-2013, 08:49 PM   #9
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Mortar Color


Why does it lighten up? The answer is because it loses moisture. Acrylic and latex additives work as an integral sealer to reduce moisture loss, so the mortar retains both color and moisture.

Just as an FYI for those who may read later, the easy way to match mortar is to use a piece of plywood and make thin smears. The plywood sucks the moisture out rapidly, "drying" (though not curing) the mortar.

Do not make samples with small amounts use full batch mixes. See next point.

Start with minimal color and work up to the degree of color depth required. Use an accurate scale, not volumetric measures for the pigments.

As a starting point, wet mortar will be about twice as dark as dry mortar.

Most importantly, new mortar colored to match old mortar will not match it down the road, so make it a shade darker than matching.

When you are finished patching and the mortar has set, get a handful of dirt and rub it into the patch then wash it off and it will match.

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Last edited by Tscarborough; 02-11-2013 at 08:52 PM.
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