Very Brief Overview of Mortars and Mortar History
It is helpful for perspective to see an overview of the general history of mortar. All mortars are not the same. Thousands of years ago, it was discovered that burning certain clays and stones produced substances that worked very well for use as binders in mortar. Burning limestone with clay in it produced cement. Burning purer limestone produced lime.
The success and performance of the cements and limes that were produced was highly varied. Some of the variances were caused by different compositions of the minerals that were burned, and some were caused by different processes being used. There are many, many variables in the process of making cement or lime, and just as many variables in the rock or mineral deposits that are used. It is a very technical study of manufacturing technology and chemistry.
It has only recently been known why some mortars survive thousands of years and some only survive half a century. The Romans approached the subject of masonry with perhaps a more scientific approach than had been used previously, and they built masonry structures that have lasted 2,000 years.
In the U.S., people began making binders 300+ years ago, and many of them brought multi-generational expertise with them. But making lime out of a particular limestone deposit in one region or country will not involve the same techniques as making lime out of a different deposit somewhere else. The same techniques may be used, but an entirely different product with different behaviors might be produced.
Lime has been the main binder in mortars in the western world for thousands of years. Cements have been used in certain areas and for certain types of masonry, but less widely. The reason for that is that when a good lime is made (by burning limestone in a certain way), it performs very, very well in masonry walls. It is plenty strong (with the right sand), it is very flexible (in fact, it heals its own cracks), and it allows moisture to pass through it easily (while repelling water on the exterior).
The best limes for mortars are made out of very high-calcium limestone. Perhaps because high-calcium limestone is not universally available, and because it was not widely known before 1900 how to control all the variables in the process of making good lime, it was easily outsold by a scientifically formulated product that hardens quickly and could be made virtually anywhere: Portland cement.
It is no wonder that, with the industrial revolution of the 19th century, Portland cement began to be mass-manufactured by a few, and soon was widely depended on. This very standardized product was patented in the 1870’s in Britain, and by the 1920’s and 1930’s was beginning to replace lime as the main binder for mortar in the U.S.
By the 1960’s, masons had entirely forgotten the techniques of making or using lime as the binder in their mortars. Never scientists themselves, masons forgot all the materials science that had been passed on to them through their traditional trade. Today, masons know very little about the composition of their main product (mortar), except to open a bag of something delivered to them, mix it with some amount of sand (whatever was done in their apprentice years), and lay with it. (We affectionately call it “Wet and Forget”.) The nuances of using different binders: of adding other ingredients to change the behavior of the lime; aging the lime; freezing it; changing the sand; or adding a little cement to make it set under water: these are not part of the masonry trade anymore.
In recent years, mostly because of the efforts of a few tradesman in Europe, lime mortar science and use is making a comeback. People are again learning the basics of lime mortar, finding that it is, in fact, the best mortar for human-scale building and restoration.
For more information on The Basics of Lime, check out our Learning Center. These easy-to-understand articles explain the keys to long-lasting masonry that most masons never knew!