Did you ever look closely at the exterior of a masonry building and see hairline cracks running up the wall? Or perhaps the mortar joints have lots of cracks running across them, so that the mortar looks 
like it could fall out in places? Your next thought might be, "What’s going on here? Doesn't brick and stone masonry last forever?” What about the castles in Scotland and the palaces in Prussia that are thousands of years old?
Many masons are beginning to ask these questions. For us, our livelihood depends on the answers. It is not a good feeling to see cracked mortar joints in work completed only 5 years ago. Could the mortar that our masons use be flawed?
Fortunately, materials scientists of today have been asking these same questions. After studying those castles in Scotland and masonry buildings all over the pre-modern world, the answers are becoming clear. Turns out the mortar we use today is not at all historic. Time has tested it, and it is failing.
At this point, some mortar history might be helpful. For at least 6,000 years, man had been using (roughly) the same process to make mortar. Burn high-calcium limestone or shells by layering wood and lime stones inside a really fat chimney (kiln), and then light it on fire. The resulting burnt stones are then crushed into powder and mixed with sand and water to make lime mortar. When mixed with water the burnt lime reacts with the water. This causes it to get sticky and slowly harden, lasting for centuries between the stones of a wall. The lime hardens through a chemical reaction in which it is actually turning back into stone!
Natural lime is very different from modern hydrated (mason’s) lime, which cannot be used as the binder (hardener) in mortar because it will never get hard. Portland cement uses lime as an additive to make it a bit more workable.
Now let’s move to the modern history of mortar. In the late 1800's, various inventors began experimenting with new processes and materials for making cement. One man came up with a product called Portland cement. Named because of its similar color to the limestone that had been quarried on the British Isle of Portland for centuries. By 1878, the British government had issued a standard for Portland cement. By 1907, production began in the United States. Today's uses are the main binder in mortar, concrete and stucco.
Portland cement has proven its superiority to natural lime in many departments. In the speed-of-getting-hard department: Portland's the champ. In the waterproof department: no contest. Portland wins. In the hardness department: Portland wins again. Game over? Not yet. According to research, using Portland cement may be a strategic error if longevity is the goal.
Yes, Portland cement seals out water and natural lime allows a little water to move through it. The problem is that most masonry units (like brick, stone, and block) absorb small amounts of moisture from the air and rain. Natural lime mortar acts like a wick to get that water back out -- FAST! Portland cement, on the other hand, doesn’t allow the water to pass, thereby trapping it in the wall. Trapped moisture causes rapid deterioration of the joints, and breaks off the faces of the bricks or stones. Therefore, repointing with Portland cement is not the best option for buildings that were originally built with natural lime mortar.
Another downside to Portland cement is a seemingly positive quality. It is harder than lime mortar. However, this hardness also makes it more brittle and harder than many kinds of brick and stone. Any movement in the building is going to make Portland cement crack, and can break softer brick and stone. The more flexible lime mortar moves with the building without cracking or adding stress to the masonry units.
It gets even better for lime mortar. At the microscopic level, Portland cement contains salts that actually degrade the mortar. This occurs from the inside out so that it starts to decompose as soon as it gets hard. In contrast, natural lime mortar has small amounts of free lime -- lime that never reacted with the water in the beginning, after it was burned. This free lime actually dissolves in the water that is escaping out of the wall. This process fills any cracks that may have formed. The experts call it "autogenous healing." Like a lobster growing back its claw, I guess.
Many restoration mortar recipes call for 8 parts of sand, two parts hydrated lime and one part Portland. Yet even this diluted mixture is neither soft enough nor breathable enough, and reeks havoc on older structures. It’s not surprising that the historic restoration movement is slowly switching from Portland mortars to lime mortars. No wonder they used it for 6,000 years!
The bad news is that natural lime putty for mortar is still difficult to find in this country and it's difficult to find a mason with lime mortar experience.
So the next time you want your chimney re pointed or a historic stone or brick building restored, find a historic restoration contractor who knows about natural lime mortars. As a restoration masonry contractor, the choice is clear to me. What's the point of building new or restoring the old if our work is not going to stand the test of time?