Being a masonry business in Troy NY, we don’t get bored easily. With brick structures all over the city, most built in the early nineteenth century, we find ourselves getting calls for re-pointing often.
In historic masonry structures, the mortar is weaker and therefore the first element to give. This is intentional so that the bricks don’t break, the mortar does. Between 1873 and the 1930s, brick and mortar actually became stronger. Interestingly for Troy, we had a fire in 1862 that destroyed hundreds of buildings in the city. Much of the cities brick structures that are still around were built between 1880-1920. This leaves us masons in a tough spot to find the right historical mixture of lime, Portland and sand to respect the age and strength of the brick and mortar of that time. We have been using a mix prescribed by the Troy Architectural Program: 1 part Portland cement, 3 parts hydrated lime and 6 parts sand.
Here are some before and after pictures of an 1890 brick structure on 9th Street in Troy. This historical mix is intentionally weaker than regular type n- and type s-mortars you find at the big box stores. It is intentionally weaker because the original mortar is not as strong.
The National Parks Service of the U.S. Government tells the history of mortar and Portland cement as:
“Historical Background Mortar consisting primarily of lime and sand has been used as an integral part of masonry structures for thousands of years. Up until about the mid-19th century, lime or quicklime (sometimes called lump lime) was delivered to construction sites, where it had to be slaked, or combined with water. Mixing with water caused it to boil and resulted in a wet lime putty that was left to mature in a pit or wooden box for several weeks, up to a year. Traditional mortar was made from lime putty, or slaked lime, combined with local sand, generally in a ratio of 1 part lime putty to 3 parts sand by volume. Often other ingredients, such as crushed marine shells (another source of lime), brick dust, clay, natural cements, pigments, and even animal hair were also added to mortar, but the basic formulation for lime putty and sand mortar remained unchanged for centuries until the advent of portland cement or its forerunner, Roman cement, a natural, hydraulic cement.
Portland cement was patented in Great Britain in 1824. It was named after the stone from Portland in Dorset which it resembled when hard. This is a fast-curing, hydraulic cement which hardens under water. Portland cement was first manufactured in the United States in 1872, although it was imported before this date. But it was not in common use throughout the country until the early 20th century. Up until the turn of the century portland cement was considered primarily an additive, or "minor ingredient" to help accelerate mortar set time. By the 1930s, however, most masons used a mix of equal parts portland cement and lime putty. Thus, the mortar found in masonry structures built between 1873 and 1930 can range from pure lime and sand mixes to a wide variety of lime, portland cement, and sand combinations.
In the 1930s more new mortar products intended to hasten and simplify masons' work were introduced in the U.S. These included masonry cement, a premixed, bagged mortar which is a combination of portland cement and ground limestone, and hydrated lime, machine-slaked lime that eliminated the necessity of slaking quicklime into putty at the site.”