Masonry Magazine October 1997 Page. 20
Repointing Historic Masonry Buildings to Last
Return of Lime Putty Mortar
John P. Speweik, CSI
Historic Masonry Specialist
U.S. Heritage Group, Inc.
As a young boy growing up in a family of stone masons, mixing mortar was like brushing my teeth... I did it everyday, at least during summer vacations when school was out. Who would have thought that in the age of technology, speed and convenience that my great great grandfather's 1846 mortar formula would return. The trend seems to be one that is sweeping across Scandinavia, Europe and Canada as architects and heritage masons work together to preserve their country's historic masonry properties built hundreds and often thousands of years ago. They call it the "Lime Revival" Its been 20 years for Sweden, 10 years for England, 5 years for Canada... its America's turn now.
Mortar History
The oldest archaeological sites in the world are, of course, masonry. As early as 2450 B.C., masons began using lime and sand for mortar. Lime is made from limestone which has been heated to temperatures exceeding 1,650F where the heat drives off the carbon dioxide and water turning the limestone into quicklime. Traditionally, this quicklime (sometimes called lump lime) was delivered fresh to construction sites in sealed wooden barrels or in covered wagons. This quicklime had to be slaked, combined with water-a caustic reaction which gives off considerable heat and actually causes the mix to boil. A wet lime putty resulted and was left to mature in a pit or wooden box for at least two to 12 weeks, sometimes a year. Mortar was made from the lime putty combined with local sand generally in a ratio of 1 part lime putty to 3 parts sand by volume. Other ingredients like crushed brick dust, clay, or natural cement were sometimes found in smaller quantities before 1870; however, the basic lime putty and sand mortar formulation has remained unchanged for centuries. Portland cement was first manufactured in America in 1871, but did not become truly widespread until the 20th century. As late as 1883, there were only three portland manufacturing works in the U.S. Up until the turn of the century portland cement was considered an additive, or "minor ingredient" to help accelerate mortar set time. By the 1930s, most masons used a mix of equal parts portland cement and lime putty. Masonry structures built between 1873 and 1930 might be pure lime putty sand mixes or a wide range of lime putty to portland combinations. Masonry cement, a prepackaged combination of portland cement and plasticizers (such as hydrated lime or ground limestone), was introduced as a mortar product around 1930. Hydrated lime, which is available at most building supply yards in 50# bags, was introduced around 1935 replacing the need to slake quicklime into putty at the site.
Properties of Mortar
In general, mortars for repointing should be softer (measured in compressive strength) than the masonry units and no harder than the historic mortar to prevent damage to the masonry units. It is a common error to assume that hardness or high strength is a measure of durability. Stresses within a wall caused by expansion, contraction, moisture migration, or settlement must be accommodated in some manner; in a masonry wall, these stresses should be relieved by the mortar rather than by the masonry units. A mortar that is stronger or harder than the bricks or stones will not "give," thus causing the stresses to be relieved through the masonry units-resulting in cracking and spalling (figure 1). Stresses can also break the bond between the mortar and the units, permitting water to penetrate the resulting hairline cracks. In addition to the compressive strength issues, the importance of breathability is critical.