A: Getting new mortar to blend in with old mortar can be tricky — in part because the look of mortar changes as it ages.
Modern mortar is a mixture of sand, cement and water. When it is installed, the cement paste coats the sand grains, so what you see is the color of the cement. Most cement is gray, but white cement also exists. And it’s possible to add pigment, which tints the cement to shades of red, yellow, brown, green or black.
Over time, however, the cement paste on the surface erodes, exposing the sand grains. From that point on, the color of the sand affects the look of the mortar. Depending on what type of sand is available in an area, an experienced mason may be able to look at weathered mortar and know what to select for patching. There’s also a way to reveal the color of the sand clearly, by soaking pieces of the old mortar in muriatic acid to eat away the cement, leaving only the sand.
But at this point, it’s too late to worry about whether color-matched sand or the appropriate color of cement was used for the patches in your walkway. So how can you tidy up the look?
First, recognize that new mortar can never blend in if you leave the old mortar covered with mold, moss and dirt. Give your walkway, especially the joints, a good scrubbing. Use a pressure washer or a long-handled scrub brush like those designed for cleaning decks.
If dark stains from mold remain, wet the bricks and apply a solution of three-fourths of a cup of chlorine bleach per gallon of water. (Protect your skin and eyes, and wear old clothes that you don’t mind spattering with bleach.) Let it sit for five minutes, then scrub. Wait about 15 minutes, then rinse with water from a garden hose, which will dilute any remaining bleach so that it doesn’t hurt nearby plants. Or you can use an oxygen bleach. Follow the instructions on the package for diluting. OxiClean, for example, recommends four scoops per gallon of water.
There is a good chance that this cleaning will make the old and new mortar match fairly well. But if the old mortar still looks much darker, you might try a trick that Jim Huber, my brother and a masonry contractor, used when he was hired to repair stone paving at Wyntoon, the Hearst family’s castle-like retreat in Northern California. The old mortar was almost brown. But when my brother chipped off a piece, he realized that was just a surface stain, undoubtedly a result of dirt being tracked onto the paving over the years. So he color-matched the sand, applied the new mortar and troweled the joints, as with any mortar job. Then, when the mortar stiffened, he sprinkled handfuls of dirt onto the patched areas. A few rainfalls later, the old and new sections were barely discernible. “Just let it get evenly dirty,” he said.
One curious feature of your walkway is that the new mortar is still a pristine white, even though the work was done two years ago. If the picture was taken recently, it’s possible that the new mortar has resisted mold and moss because the repair person mixed the sand and cement with a liquid similar to white glue, rather than plain water. This makes the mortar stronger. It also makes the mortar less absorbent, so it is less hospitable to moss and mold. If you find that the old mortar grows new moss much faster than the new sections, you might be able to even out the rate at which a fresh crop of moss appears by applying a sealer, which will make the older sections less absorbent than they are now.
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