Cement - where would we be without this tough and reliable material? It makes up our sidewalks, roads and bridges; it holds masonry blocks together, and helps us control water use through dams, pipes and aqueducts. Portland cement is the material that we all see and know. However, Portland is a relatively new material that evolved out of ancient traditions and changing demands in the building and engineering fields. As a material, we know very little about the precursors to Portland cement: natural cements and hydraulic limes. Historic mortars, plasters and cements differ dramatically from their modern counterparts. The production technology and skills have changed over time and, while there is an enormous amount of research directed to modern Portland cements, there is little written on the study of cementitious materials produced by more traditional methods.

Historic preservationists, conservators, archaeologists and historians are now struggling with the identification of old cements and mortars. For historians and archaeologists, knowledge of cements can help them understand the technology of the society that produced them. Historic preservationists and conservators face a more practical challenge of repairing old cements. Portland cement has different mechanical and physical properties than historic cements; typically it is harder and denser. This difference in physical properties can lead to the Portland cement damaging the original material.

In order to improve our understanding of this material, a project has been funded by NCPTT (National Center for Preservation Training and Technology), the Samuel Kress and Mellon Foundation to research the Analysis of Historic Cementitious Materials and publish a text book to disseminate the work.  The publication is designed to help conservation scientists, objects and architectural conservators, historic preservationists and archaeologists that work with historic cements.

The project began with laboratory work to develop a protocol for the standard evaluation of historic cementitious materials. It became obvious, very quickly, that the cements and mortars we made in the lab bore very little resemblance to the historic materials we encountered in the field. And so, the search for original natural cements and hydraulic lime samples began.

Rosendale natural cement was the most famous of its day and it set the standard for other natural cements and for early Portland cements, too. Rosendale products were used for the building the base of the Statue of Liberty and the Capitol building to name but a few. But what was it about Rosendale cement that made it so different? It could have been the ingredients used in the raw mix, the kiln temperatures, quality control or other manufacturing procedures.

To solve this and other questions, we turn to the original materials. We have to compare original Rosendale Cements to other natural cements of the day. The samples are analyzed by studying polished sections under a microscope. Microscopy is a powerful tool used within the cement industry to study the primary ingredient of any natural or Portland cement – the hydraulic minerals. When mixtures of clay and lime are heated, a number of reactions begin to take place. At lower temperatures, the lime begins to break down in a process called calcining. As the temperature continues to climb, the mineral constituents of the clay and lime begin to melt and interact to form new minerals. The first hydraulic phases to form are ferrite and aluminate. If the temperature rises higher yet, belite and then alite form from the mix. Each of these phases will impart hydraulic characteristics to the cement product but each has different properties. Belite and alite impart the greatest strengths and superior mechanical properties to cement. Alite is formed at the highest temperatures and it is responsible for high early strength in cements. Belite also gives great strength to cement but it is slower to form so while it does not contribute to early strength, it greatly adds to total strength over time. By viewing the minerals under a microscope, we hope to find un-reacted hydraulic particles. We can then determine a number of parameters like: the composition of the original mix; the kiln temperatures; rate of heating and cooling and grinding techniques. Bulk chemical analyses can give additional information on elemental composition of constituents like total silica, calcium and magnesium.
 
The Portland cement industry has learned, over time, how to manipulate temperature, raw mix, kiln conditions, and particle size to optimize and perfect their product. Makers of natural cements didn’t understand these processes and kilns had yet to reach the level of perfection that they have today. This resulted in an extremely wide variability in cement products. Today, cement products are uniform but in the past, the engineers would have to sample a number of bags from a shipment and test them to determine if the cement could be used in their projects.

By systematically comparing a wide variety of different natural cements, we can begin to understand and answer many of the questions we have about these forgotten products. However, it isn’t easy finding samples. In order to be useful, we have to know the date of the cement (old building or structures being demolished are very helpful for acquiring this sort of sample). It is also extremely useful to know the brand of cement. Luckily, the Rosendale site still exists through the efforts of the Century House Historical Society. Through them we are able to locate original clinker samples and original artifacts and archived materials. Historic preservationists like Ken Uracius have been able to recreate Rosendale cement by taking raw material from the same quarry sites and burning it in a kiln. These are wonderful and rare resources which have helped this project immensely.

Author’s note: Any help, samples or other information in locating old mortar, cement and concrete are most welcome. You can contact Elizabeth S. Goins, Ph.D. at Image Permanence Institute, Rochester Institute of Technology, 70 Lomb Memorial Drive, Rochester, NY 14623-5604