Natural Cement Makes a Comeback

After disappearing from the construction industry for more than 30 years, natural cement is making a comeback. Designers and builders are rediscovering its benefits in a number of areas, including restoration work.

Michael Edison is president of Edison Coatings, the only producer of natural cement in the United States, and the editor of Natural Cement, which was released last year by ASTM International. He also chaired the task group that updated ASTM C10 – 07, Standard Specification for Natural Cement, a standard that had been discontinued in the 1970s. He discuses the role of natural cement in today’s environment.

Q: What is natural cement?

Michael Edison is president of Edison Coatings, the only producer of natural cement in the United States, and the editor of Natural Cement, which was released last year by ASTM International. He also chaired the task group that updated ASTM C10 – 07, Standard Specification for Natural Cement, a standard that had been discontinued in the 1970s. He discuses the role of natural cement in today’s environment. Natural cement is both technically and historically the material of transition between older lime technology and modern portland cement. The period of its primary use was from 1819-1900, at which point portland cement achieved market dominance. Although natural cement use continued well beyond that, it just never enjoyed the kind of explosive growth in sales that portland cement went through between 1900 and 1920, when it grew almost 40 fold in demand. Portland use went from about 2 million barrels a year to 80 million barrels a year in a very short time while natural cement demand languished at around 2 million barrels a year. The last natural cement operations closed in 1970 but in 2004 we started producing it again.

Q: Why did demand for natural cement go away?

Demand diminished because it doesn’t get as hard as portland cement does in 28 days. It takes more like 90 days. So they could build faster with portland cement and they could also change building designs to much thinner wall construction. If you look at 19th century buildings, whether they’re brick or stone, they generally include multiple courses of brick or massive stones. As we get into the 20th century, we see transitional buildings with structural steel superstructures and single wythe brick veneer. It was portland cement that made that kind of construction possible. And so even though portland cement always cost more than natural cement in its day, the economies in building design that portland cement allowed led to its market dominance.

Q: Then why is it coming back?

It’s coming back because of a couple of things. The primary reason is that we have all these 19th century buildings that are now being restored and the most compatible way to restore something is with the same material that was used to build it. Portland cement and older masonry materials don’t necessarily work well together. So if you have a natural cement building and it’s 150 years old and it’s done really well for 150 years, you wouldn’t repair it with portland cement, you’d repair it with natural cement.

Q: What were people using before natural cement became available again?

They tried to use portland cement, primarily in mixtures with lime for masonry mortars, but it was generally not compatible. There were issues with the portland cement being too hard and too rigid, and with it being too dense and trapping water. An example of the differences in performance properties is a structure like Fort Jefferson, which sits on an island 70 miles off Key West in the Gulf of Mexico. It’s a quarter of a mile around, and there are no expansion joints in that entire structure and no cracks due to thermal expansion because the mortar has remained flexible for the 170-odd years since they began building it. Portland cement-lime mortars carbonate and get brittle with age. In modern construction with portland cement-based masonry mortars, you design in expansion joints to give the building places to relieve stress due to thermal expansion. But expansion joints were typically not needed in buildings of the lime and natural cement eras because the mortars were much more flexible.

Q: Beyond restoration work, is there any interest in using natural cement for new construction?

Yes. In particular, there have been a couple of applications. One is the use of natural cement in the construction of monuments. The reasoning is that we have 150-year-old monuments that were built with natural cement that haven’t required any maintenance, so if you’re building something that’s supposed to last forever, maybe that’s still a really good idea. A secondary niche which has emerged is that natural cement is a greener material than portland cement.

Q: Why is it greener?

Because natural cement requires so much less energy to produce. Instead of firing it like portland cement at temperatures up to 2400 degrees Fahrenheit, natural cement is fired about 500 to 700 degrees lower than that. Also, the grinding is much easier. Natural cement is not a hard material to grind because you’re not actually clinkering the raw materials. So it requires a lot less energy to produce it.

Q: If natural cement is considered to be a green material, do you know if it’s specified or recommended by any green building standards?

I know that’s a consideration and this is almost always an architecturally specified item. Greenness is a priority to a lot of people who specify restoration work—these things just naturally go hand-in-hand. Preserving a building is much greener than knocking it down and building a new one. Long service life is also a consideration. I’m not going to say that green is the number one reason on the list that people are using it, but it certainly helps the cause.

Q: Are there any other reasons people use natural cement?

A really unexpected one is a market that probably not a lot of people are aware of, but there seems to be a number of people who have allergies to portland cement. Apparently, some of the materials that end up in portland cement as grinding aids can cause allergic reactions in certain very sensitive individuals. So I had people starting to call me saying they had hypersensitive clients for whom they were building houses, and they are allergic to portland cement, so they were using lime and the lime is very slow-setting, making it difficult to work with. Natural cement really gives them something more user-friendly, and we aren’t using grinding aids because natural cement is very easy to grind.

There has been a lot of interest in the material. As far as our production capacity goes, demand overtook capacity in late 2007 and every pound we could make was spoken for. We scaled up in early 2008, but I can foresee the need for further expansion in the near future. The National Park Service has come to realize that this material is performing much better than everything else that they’ve tried in the maintenance and restoration of the 19th century natural cement sites that are under their jurisdiction. There have been a couple of state capitals worked on, there have been armories, lighthouses, dams, and bridges. The American Museum of Natural History has been repointed using the original natural cement materials that were used to build it… As a secondary niche, sometimes we’re finding that older structures built with lime do not readily lend themselves to being restored with lime. In structures that have become contaminated with salt, like foundations or sea walls, or in high-moisture environments, lime is not always performing well as a restoration material. Natural cement is providing an alternative that’s more compatible than going to portland cement.

Q: Is natural cement a lot more expensive than portland cement?

It’s a lot more expensive right now because of the limited scale on which it’s being produced. Just in relative terms, natural cement is around 10 to 15 times the cost of ordinary portland cement.

Q: Are you the only producer in the U.S.?

Right now, we’re the only ones. We’re producing natural cement that is very much the same as was made 180 years ago in this country. Cement materials that were used during the same period in Europe really weren’t quite the same. There is one producer of what is being called natural cement in France right now, but their cement does not really qualify as natural cement in the United States. It wouldn’t meet the ASTM standard, and it doesn’t quite meet the definition either. It’s a significantly different material. They call it natural cement, but they’re talking about a material that sets in 90 seconds. That doesn’t really have any great value in masonry restoration or construction. Historically, it was used primarily as a precasting material and it is geologically and chemically different from North American natural cements. There are several requirements of the current natural cement standard that the European material would not meet.

Q: What’s the status of the ASTM standard on natural cement?

The latest 20th century standard had been withdrawn in the 1970s when the material became commercially unavailable. Once it was clear that there was a demand for it again, however, it was appropriately mentioned to us that this really needs to be back under an ASTM standard again or nobody is going to be able to properly specify it or build anything with it. The standard was reissued in September 2006, and a minor revision was released in December 2007.

Q: Can you tell me about the book on natural cement that you edited?

The book is a collection of 10 papers that were presented at the first and second American Natural Cement Conferences, which were held in 2005 and 2006 respectively. I wrote two of the 10 papers, one each for the two conferences, and the other papers represent a broad spectrum of people. There’s a historian, a geologist, a petrographer, a tradesperson, plus some material scientists from Europe who wrote papers about their traditional pre-portland cement-era materials and compared them with American natural cement. There are also case studies of recent restoration projects using natural cement. The supplemental section is a series of reprints. One of the things that we reprinted was the original 1904 natural cement standard, one of the first ones ASTM ever published. There’s also a series of 19th century publications which are otherwise very hard to find.

I think this is going to be viewed as a very valuable resource for anybody working on these 19th century buildings, and for anyone who has a real interest in historic preservation. It’s also a technical primer as to where this whole technology has come up to now. I think it forms the basis for whatever future discussions we will have because we’re now redefining performance in terms of contemporary ASTM performance testing.

Q: Are there tests that have been performed to determine the performance of natural cement based on modern parameters?

Yes. Part of the discussion is that metrics that would have been in the original 1904 standard may not be considered valid anymore. Those methods themselves have evolved and changed. The real focus of the reinstatement effort was how do we take those parts of the standard and update them to reflect current testing methods and current terminology, while remaining faithful to the nature of what the material was 100 years ago. I think we very effectively walked that line, providing a standard that’s usable today, but making sure that the material that will meet that standard today is essentially the same material that it was 100 or 150 years ago.

Q: Any final thoughts?

There are some real benefits to this material even today, particularly in our changing world, where lower energy, greener products, and highly durable materials can really have some important advantages. Natural cement is a remarkable material that was dominant during a particular period of history—and maybe its day passed too quickly.

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