Masonry Magazine January 2006 Page. 23
WORLD OF CONCRETE
C8129
Using History as Your Guide
PORTLAND CEMENT is a synthetic substance, made by heating a limestone and clay mixture in a kiln and pulverizing the resulting material. It was first patented in England in 1824 by John Aspdin, a bricklayer, and further advanced by Isaac C. Johnson in 1845. By 1860, subsequent refinements yielded a product very similar to the Portland cement we use today. In 1872, the United States began manufacturing Portland cement.
Given this information, we have an approximate guide for when Portland cement was first used in the construction of U.S. buildings. There are always exceptions to the rules, but based on my work and observations, if a structure was built between the mid 1880s and sometime in the 1930s, it most likely needs to be repaired with a mortar that consists of lime putty, some Portland cement, sand and color pigment - considered a traditional lime-based mortar.
However, if the building was constructed prior to 1872 or, depending on the region, even up to 1890-it should be repaired with a lime-based mortar consisting only of lime putty, sand and color pigment.
Chemistry 101
IT IS IMPORTANT that masons and mason contractors involved in restoration work understand the properties of lime.
Portland cement, the chief ingredient in today's masonry mortars, sets by adding water, which is called a hydraulic action. Using a quite different process, lime sets over time through a reaction with air, called a carbonation process.
Limestone, chalk and seashells are essentially calcium-calcium carbonate to be precise. Lime is derived from heating limestone (or chalk or sea shells) at high temperatures, which drives off the carbon dioxide that is part of its chemical composition and turns the limestone into quicklime, or calcium oxide.
When water is added to the quicklime, it generates heat, called an exothermic reaction. In the trade, we call this process "slaking" the lime. If the lime is made into a powder at this stage, it will become the lime you may have seen sold at masonry supply stores, called hydrated lime. However, if you add excess water during the slaking process, the lime will turn into putty. (To some confusion, hydrated lime and lime putty are chemically almost the same, and both are referred to as "calcium hydroxide.")
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