Most tuckpoint mortar mix being used today is a mixture of Portland cement and lime along with other aggregates such as sand and masonry cement in compliance with ASTM (American Society for Testing and Materials) standards.
A Little History of the Use of Tuckpoint Mortar Mix
While tuckpoint mortar mix has two components, namely, Portland cement and lime with the former having the higher ratio, older bricks such as those in houses more than 100 years old, had a negligible amount of Portland cement in mix. Back in those days, bricks were commonly made of lime and sand. This is the reason why much older brickwork tends to be softer than new brick construction because it is Portland cement which makes the mortar hard. Plus, the ratio of lime content used back then was very high.
How Tuckpoint Mortar Mix Today Works
Brick walls, with basic physics in mind, are gradually, unnoticeably, moving. The heat of the sun makes them contract and expand and if the mortar adhering the bricks together is too hard, the tendency is for the bricks to eventually start to crack. Hence, lime balances out the effects of Portland cement in the tuckpoint mortar mix.
Another use and value of lime in tuckpoint mortar mix is that lime is water soluble and tends to react with water and carbon dioxide. This means that over time, the lime in between the bricks can resolidify even if small cracking develops because lime reacts with gas and water and heals the cracks itself as water and gas try to permeate into the mortar.
How to Mix Tuckpoint Mortar and Match the Mortar with an Existing Brickwork
The initial steps in mixing new mortar to use existing masonry work, which includes combining lime with Portland cement.
It is important to note that if the mortar mix is too soft and timeworn, you can mix lime and Portland cement at a 6:1 ratio so if the mortar is too light in color, make sure to use white Portland cement. On the other hand, if it is darker, gray Portland cement will do.
The First Steps
First off, mix a very small amount to start with. It is recommended not to blend more than seven to eight ounces in volume. This is only to test and match the mortar with existing brickwork. Then you can mix two parts sand and one part of the blended lime and Portland cement. It may be helpful to write down your proportions so you will not have a difficult time recalling this later.
Now you can blend them together while dry, adding the right amount of water to make the mixture sufficiently wet to enable you to form it into a ball. Note that the color of your new mixture should match, or at least be very close to, the shade of color of the existing mortar while the new mixture is wet. Otherwise, you will have to start all over again. Be mindful of the sand proportions as well, especially if the existing mortar has too much visible sand, then you will have to adjust your blended lime and Portland cement.
Complementing an Old Brickwork with a New Tuckpoint Mortar Mix
In order to create the tuckpoint mortar mix which will perfectly match the existing mortar in old masonry work, you should leave the tuckpoint mortar mix to dry for a couple of weeks. This will serve as the sample you will use to compare with the existing mortar mix.
This is one way to achieve effective and lasting results in tuckpointing old brickwork. You will need to carefully study the old masonry work and be diligent in blending the new tuckpoint mortar mix until the latter is matching with the former. The process will include meticulous evaluation and assessment of old and new materials alike.