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The Brooklyn Bridge in 1894.
The Return of American Natural Cement
Learn about the history of American natural cement, the differences between natural cement and portland cement, and why there is an increasing interest in this natural building material. by Susan Brimo-Cox
New York & Rosendale Cement Co.: drillers at the Black Smoke Mine.
New York & Rosendale Cement Co.: the Black Smoke Mine entrance.
New York & Rosendale Cement Co.: at the top of the kilns.
New York & Rosendale Cement Co.: the Black Smoke Mine.
If you don’t know what natural cement is, you’re not alone. Even experts in traditional and historic masonry have been scratching their heads when the topic of “natural cement” is raised. Part of the problem is that natural cement use in the United States had a strong burst of use in the 19th century, but was quickly overtaken by portland cement from 1900 on. Now, as restoration and conservation activities increase on buildings constructed in the 1800s, the industry is relearning about American natural cement.
American natural cement rock was formed a very long time ago. Kurtis Burmeister, assistant professor of geology at University of the Pacific in Stockton, Calif., has studied American natural cement geology in the Rosendale region of New York. “I always get a kick trying to explain that the binder is ground-up remains of sedimentary rock deposited 420 million years ago in a world very different from today,” he says.
The rocks used to make natural cement include dolostone or limestone. Limestone is essentially calcite. Calcite plus magnesium makes dolomite. Dolomite forms dolostone when combined with fossils and some clay content. This ideally balanced, naturally occurring rock makes natural cement that needs no additives.
Because of the natural history of the land mass that became the United States, there are many areas where you find these kinds of rocks, Burmeister explains. Sedimentary rock, like dolostone and limestone, is deposited in shallow seaways. A good portion of the American continent was a shallow sea at one time. Natural cement rock has been found in New York, Pennsylvania, Maryland, Virginia, Georgia, Kentucky, Indiana, Illinois, North Dakota, Wisconsin and other states. In mountainous areas, such as along the Appalachians, originally flat layers of rock were uplifted and folded. Then, centuries of weathering exposed the underlying rock.
A variety of methods, including room-and-pillar mining, were used to extract cement rock from the ground, says Burmeister. It was not a simple process and it was very hazardous. Each quarry area had to be approached individually.
Natural cement takes hold in America
Before natural cement there was lime putty, and there are many examples of cement-free masonry remaining today. But as America entered the industrial age, the industry faced new challenges.
Lime putty, natural cement and portland cement: What’s the difference?
Lime putty is made from pure, nonargillaceous limestone; that is, the limestone does not contain clay. The limestone is fired at about 900 degrees Celsius to burn off the water and carbon dioxide — called the “calcine” process — resulting in straight calcium oxide, called quicklime. When water is added the quicklime reacts with the water in a process defined as “slaking” creating calcium hydroxide. As lime putty cures it absorbs carbon dioxide from the atmosphere, a process called “carbonation.” As a mortar, lime putty when fully cured is durable, like limestone, but able to accommodate movement with limited cracking and remains breathable.
American natural cement is made from semi- or moderately argillaceous limestone; that is, it contains some clay. American natural cement rock also, typically, contains fairly high levels of magnesium (though not all do).
When fired at about 900 degrees Celsius, cement rock remains stable. So, when ground and mixed with water the slaking reaction is reduced. The silica and calcium hydrate, but when hydration is complete there is little or no calcium left for carbonation. This results in a more rapid-setting mortar — and it will harden even if exposed to water — that has good bonding and elastic characteristics.
Portland cement is a synthetic combination of limestone and silicate matter, such as shale. It is fired to more than 1,400 degrees Celsius, which reorganizes the chemical elements of the rock, and you wind up with a clinker that is ground up for use. Portland cement is a very consistent product that is a harder, stronger and less porous mortar than natural cement.
Dietrich Werner, president of the Century House Historical Society in Rosendale, N.Y., points to Canvass White, an engineer working on the building of the Erie Canal, as the man primarily responsible for developing American natural cement. In 1817, when American canal building was in its infancy, White was sent to England to investigate how the British built their canals and to see what kind of mortar they used. After spending several months in England learning about hydraulic cement, White returned to the United States and searched the Erie Canal route for material to use as hydraulic cement. Werner says that White had found the right kind of rock by 1818, and the American natural cement industry had begun.
After the Erie Canal, White went on to work on other canal projects, started a natural cement factory, and in 1820 obtained a federal patent for the manufacture of “Water Lime Cement.” The state of New York purchased White’s patent rights for the public in 1824.
The surge in use of American natural cement also is attributable to the 51 Third System seacoast fortifications built after the War of 1812. These fortifications were devised to prevent another British invasion of the United States. The construction efforts — trial, error and testing — by the U.S. Army Corps of Engineers were, literally, the cutting edge of the technology in the United States.
Werner says that by the 1830 the use of natural cement in the United States had caught on because it was the only cement being made in America.
Masonry restoration expert Ken Uracius reports: “There is no doubt that natural cement was the binder of choice during the 19th century, not only in fortifications and lighthouses, but also in almost all engineered buildings and structures.”
Mike Tate, technical manager for research and development for Graymont Inc., points out that natural cement served a need at the time because it allowed construction to proceed more quickly in wet or dry situations. Lime-based mortars, because they weren’t as strong and strengthened more slowly, allowed you to lay only so many courses before you had to wait for those courses to strengthen before proceeding, he explains.
As it turns out, historically and technically, American natural cement was a transition material between older lime-based mortars and modern portland cements. “Natural cement sped up construction, but it didn’t last long,” Tate observes.
Rare American cement reference is republished
In 1898, Uriah Cummings published a book called “American Cements.” As a reference book, it is highly prized, but it was hard to find — until now.
Dawn Chapman, president of Natural Binders Press in Atlanta, took on the project to republish Cummings' “American Cements,” primarily because of its importance to those involved in historic preservation and building conservation. Cummings was not the only one who wrote about natural cements, but in Chapman's estimation he was the “most knowledgeable writer about natural cements in the late 19th century.” While General Joseph Totten and General Q. A. Gillmore, both with the U.S. Corps of Engineers, produced writings about fortifications and military projects, Cummings wrote for a variety of construction types, giving his book a broader relevance to current audiences. He also had a cement manufacturing facility (the Cummings Cement Co. in New York) and ownership interest in others, so he writes about the subject with a different perspective than previous authors, she adds.
What prompted Chapman to republish the book was her need for the tome in conducting research about natural cement. “Natural cements are not well documented in current historic preservation literature,” Chapman explains. A borrowed copy of the original book was professionally scanned for republication.
Chapman says the book will be useful again today for a variety of reasons. “It begins by providing a brief history of natural cement and defining the differences between lime, natural cement and portland cement. It contains a large section on cement testing, as well as a section about manufacturing facilities operating in the late 19th century. It concludes with an extensive list of structures built with American natural cement, including the source of the materials.” What the new printing has that the original didn’t is a new introduction by John Lambert, president of Abstract Masonry Restoration Inc., and a 20-page index — a useful addition that focuses on specific names, locations and buildings discussed in the book. www.naturalbinders.com
Nonetheless, during its heyday, natural cement was used in masonry mortars, stuccos, concretes, grouts and whitewashes. And besides more than 150 canal systems and 51 Third System forts, it was used to build numerous structures, including the Brooklyn Bridge, a dozen state capitol buildings, the U.S. Capitol, the lower 150 feet of the Washington Monument, the Statue of Liberty pedestal, and various museums and government buildings.
By the early 20th century, portland cement dominated the cement binder market. John Wathne, president of Structures North Consulting Engineers Inc. in Salem, Mass., reports: “This was in part due to the fact that it could be manufactured virtually anywhere that limestone existed, and to the fact that its cured properties surpassed those of natural cement.”
Tate says inconsistency was part of natural cement’s downfall. “It was hard to control the quality of natural cement; some were very hard, some very soft.”
Portland cement could be manufactured very consistently, and it had other advantageous properties as well, such as higher compressive strength. “The cement industry convinced architects and the construction industry that harder was better, and that they could build faster and taller.”
Nonetheless, the demise of American natural cement was a long, slow process. The last natural cement manufacturing facility in the country, the Century Cement Co. in Rosendale, N.Y., closed in 1970. The ASTM C 10 Specification for Natural Cement was discontinued in 1978.
The re-emergence of American natural cement
Flash ahead some 25 years and natural cement is being rediscovered, thanks to the increasing need for restoration and conservation work on structures built in the 1800s. “Masonry repairs of historical buildings using incompatible mortars are decaying and falling apart,” Werner says. “People are asking, ‘Why?’”
Restoration architect Mary Katherine Martin stresses the importance of analyzing historical mortars and restoring historical structures with like materials. “Why would you go to the trouble of using something that could be potentially harmful when you can use appropriate materials?” American natural cement has specific qualities that work with the massive historic structures it was used for, primarily its bonding properties and its flexibility to handle settling and expansion-contraction forces it is subject to.
John Walsh, senior petrographer at Testwell Laboratories in Ossining, N.Y., cautions that conservation and rehabilitation people should be aware that many mortars were used in the United States, and they should be careful in making assumptions about materials used. Fortunately, historic material testing is available from a few select labs in the U.S. — even mortar analysis at the microscopic level to ensure an acceptable level of compatibility.
Recent interest and emerging demand for American natural cement has now restarted the industry.
A few years ago, Ken Uracius obtained some cement rocks from the Rosendale area and, in his garage (which he nearly burned down in the process), experimented until he devised a method to burn and produce small batches of natural cement. Since then, commercial production of Rosendale natural cement has begun again, but it is a boutique product at this point.
Still, the demand for historically accurate natural cement caused ASTM Committee C01 to begin reevaluating the standard for natural cement in 2004. The standard, C 10-06, was reissued in October 2006. These specifications make it easier for individuals involved in restorations using natural cement to ensure they are using appropriate materials.
Fort Jefferson restoration helps revive American natural cement
Some 69 miles off the southwestern tip of Florida is an old fort that was never completed, and never really used for its intended purpose. Who would have guessed that Fort Jefferson, begun in 1846, would revive interest today in American natural cement?
Fort Jefferson was designed to be part of the United States’ coastal defense system after the War of 1812. The fort, built from 16 million bricks, was established as a National Monument in 1935 and is now part of Dry Tortugas National Park. For more than 150 years it has weathered sun, sea and use. In 2004, the National Park Service began an $18 million restoration of the fort.
Mary Catherine Martin, at the time an architect with Lord, Aeck & Sargent, and Ken Uracius, a masonry restoration expert, were among those working on the project in its early stages. Both recall that one of the first challenges was to identify the mortar, which was determined to be natural cement. The material used for the Fort Jefferson mock-up was natural cement from Rosendale (a natural cement region in New York), lime putty and local sand. A formula of one part natural cement, two parts lime putty and six parts sand looked like the original material, Uracius explains, but after the completion of the mock-up, the mortar recipe was further refined to two parts sand to one part natural cement without any lime; this mix has been recommended for the next stage of restoration.
Uracius reports the project is currently on hold awaiting funding, but, he says, the project was the start of recognizing the extensive use of natural cement in the U.S. from about 1825 to almost 1900. “There has been much talk of late about lime mortar and its use in the U.S. It is my opinion that because of the Third System fortifications, lighthouses, and canals built in the first half of the 19th century we stopped using lime mortar on engineered buildings and never looked back.”
