“Portland cement was invented in 1830, so everything before that is lime of some sort,” he said, referring to the crucial ingredient in modern concrete, which has come to dominate the building trades.When mixed with water and sand, crushed lime can be used as brick mortar, cement, or plaster.
The roads built by the ancient Romans, the Erie Canal, the mortar holding together the granite blocks of the Brooklyn Bridge, and the pedestal of the Statue of Liberty were built using lime as a cement.
He attributes the demise of lime in construction partly to the way that the Portland Cement Association, a trade group, muscled its competition out of the market in the 1800s.
“In the space of a generation or two, they basically completely got rid of lime,” Ryan said. “And so now, if you talk to a mason or a plasterer, they don’t even know that you can just use lime.”
The other reason Portland Cement has largely replaced lime, he said, is that it cures stronger and faster, allowing for the design of larger structures that can be built more quickly and cheaply.
Where it excels, he said, is as a plaster. “When you think about all the buildings in Europe, or anywhere where it’s old and they have that really beautiful patina charm, it’s all lime,” he said.
At the same time, Ryan says, lime performs the impressive feat of sucking carbon dioxide out of the atmosphere, which works against climate change.
The secret is the lime cycle, a chemical process that starts and ends with limestone.
After limestone is mined from the earth, it is heated in a kiln to around 2,000 degrees Fahrenheit, which drives all of the carbon out if it. After reacting with water, it becomes lime. When that lime is mixed into a plaster and applied to a wall, it begins to cure. In the process, the lime sucks carbon out of the air and traps it as the plaster is transformed back into limestone. Thus the wall becomes as hard as limestone and has recaptured the carbon that was emitted during the manufacturing.
The process is not carbon-neutral, however, as the fuel used to heat the limestone usually coal releases carbon dioxide that is not recaptured. Yet Portland cement is heated to 3,000 degrees, so more carbon dioxide is emitted in that process — and cement doesn’t recapture any of the carbon.1:50 p.m. | Updated Interestingly, the Environmental Protection Agency just released its long-awaited pollution standards for cement plants under the Clean Air Act.