The mortar mix you see in those bags or tubes 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 “usually” 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.
The mortar mix used by today’s brick layers 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. Houses over 100 years old 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 pure white.
Mortar Strength / Self-Healing Properties
Brick walls are constantly on the move because 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 tends to be softer than today’s brick, so 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 re-solidify 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.
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 very little 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.
Installation
First, rake out all loose mortar. You should only need to go down about ½ to ¾ of an inch. If the mortar is in poor shape you should do this in sections to minimize the possibility of it all falling apart.
When installing the new mortar it is a good idea to moisten the area before applying the material. If applied to a dry surface the brick will have a tendency to suck the moisture out of the mortar to quickly weakening the joint. Use a jointer tool to finish the surface and brush away excess with a brush.
After the new mortar sets, clean the surface with a diluted mixture of muriatic acid and rinse thoroughly. Make sure to use proper safety equipment when messing with the acid. It is dangerous stuff.