I am very glad to have found the explanation and the history of the
classic mortar and I am curious about that formula 1.5 to 1 mentioned
above. Is it by volume or weight.
Sencere thanks Zoran
Mortar Mixtures for Tuckpointing
Summary: If you think mixing mortar for your repointing job is going to mimic what you see on residential construction sites today, forget it! The mortar mix you see in those bags is probably not going to match your existing mortar in color and strength. This is especially true if your house was built prior to the 1950's. Houses built prior to the 1950's used site-mixed mortars rich in lime. When mixing your own mortar, first mix a small batch of 7 to 8 ounces and keep track of your proportions.
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A Blending Experiment
The mortar mix you see used by brick layers today contains two ingredients: lime and Portland cement. In fact, the Portland cement ratio is quite high. Old brick layers used to mix their own mortar on the site by blending lime with Portland cement. Old houses, say over 100 years, rarely had any Portland cement in the mortar. The bricklayers just used lime and sand.
Lime and Portland cement are available at building supply houses that supply bricklayers. If you look in the Yellow Pages under "Brick - Supplies" you will eventually find a company that sells bags of lime. It is inexpensive and it is white - pure white!
Mortar Strength / Self-Healing Properties
Brick walls are constantly on the move. The heat of the sun causes them to expand and contract. If the mortar between the brick is too hard, the brick will crack. Portland cement makes mortar hard. Older brick tend to be softer than today's brick. Thus, old mortar had much less Portland cement than today's mortar. The lime content of the old mortar was very high.
Lime adds another benefit, one we could use in today's mortars. Lime is somewhat water soluble and reacts with carbon dioxide and water. As time goes on, the lime in the brick will actually resolidify if a small crack develops. The crack allows water and carbon dioxide to enter deep into the mortar. The lime reacts with the water and gas and heals the crack.
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Proportions
The first step in mixing new mortar is to blend Portland cement and lime together. If the mortar is very soft and old, you may choose to blend six parts lime to one part Portland cement. If your mortar is very light in color, be sure to purchase white Portland cement. If the mortar is darker, you may get by with gray Portland cement. Mix a very small batch to begin. I would mix no more than 7 to 8 ounces (by volume) to begin. You then mix two parts sand to one part of the blended lime and cement. Be sure to write down your proportions.
You blend the sand, lime and cement together dry. Then add just enough water to make it wet enough to form into a ball. The color of this mixture should match very closely the color of the existing mortar when it is wet. If not, you need to start over with your lime and cement proportions. Pay attention to the sand proportions too. If the existing mortar has not too much sand visible, you may have to mix 1.5 parts sand to 1 part of the blended lime and cement.
To get a perfect match, you should let the mortar dry for several weeks. Hold the sample next to the existing mortar to see how close you came.
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Comments:
Zoran Nedich 30 Jul 2009, 23:25
Marshall Brown 15 Nov 2009, 12:35
Tim,
One of the problems with old lime and sand mortar for basement walls is the fact that in some locals moisture migration through the walls will remove most of the lime. My house is 110 years old and you can rub the mortar out of joints by rubbing it with your hand. In my case the walls are rough ashlars but when more rounded river rock is used this can become unstable. I have use water glass, sodium silicate, to replace the water soluble salts in the mortar with insoluble salts and this greatly restores the strength without the need to re point if there has not been too much loss. This was commonly done inside cisterns in day gone by because it works and it's nontoxic.
Michael Stutz 22 Nov 2009, 17:59
Tim,
I have a 3 year old cement driveway that has some fairly significant surface "damage" . . . the top has flaked in areas giving functionally irrelevant, but aesthetically unpleasing "mini" pot holes. Generally around one quarter inch deep, and ranging from small pits to 4 or 5 inch in diameter pits. I looked into coating the driveway with those fancy coatings & the materials run $800-1,200 alone, and it apparently takes some skill to apply. The driveway is about 25 feet long by 40 feet wide. I live in Spokane, Washington, so the winters can run below 20F for weeks on end, followed by summers peaking in the 100+F range. I'm looking to sell the house next spring and would like to fix that "first impression" . . . what can I do myself that is relatively cost effective? View all comments |
