Portland Cement  Vs Historic Gravestones    

The precise historical origins of Portland Cement are very complex and beyond the scope of this abstract, however its beginnings can be traced back into the late 1700s.  

It was not until 1878, that a standard on Portland cement was issued by the Association of German Cement Manufacturers. In the following two decades, to the turn of the 19th century, increasingly large quantities of Portland Cement were imported into America at an exponential rate. It became the next best thing since “sliced bread” in the construction industry. 
Portland Cement hardened much more quickly then lime based materials, from hydration rather then carbonation, becoming extremely dense in a short time span. It quickly became the material of choice for not only new construction, but also in many forms of “restoration” work. Little was know about its longevity and even less was about its future reversibility, a term not yet even invented in the emerging fields of historic preservation and object conservation.
It was not until quite recently, over the last few decades that gravestone conservation has become an accepted scientific field of study, fostering events like this Cemetery Preservation Summit. Even today, the majority of historic memorials, which are repaired, are not conserved by individuals with adequate training throughout most of America.
Of all the possible gravestone conservation challenges that can be encountered, the one that can be feared the most is gravestone and cemetery monuments that have been previously repaired with a hard Portland Cement based mortar and or concrete.  
The most common type of gravestone to fracture, are the thin marble slab style stones, commonly only about 2 inches in thickness. Even when new, this narrow thickness created a very weak material design, but once weathered and or leaning they are easily snapped off at or above grade. By far the quickest and most common repair performed in this situation is to simply “puddle” or stick the broken stone into a crude liquid concrete, prop it up and let it harden. 
Unfortunately, gravestones that were repaired in this matter often end up being  re‐broken again from storm damage, lawn mowers, etc. The soft marble meeting the very hard Portland Cement concrete also creates a weak point, where these incompatible materials meet. Additionally the transmission of salts and other chemical interactions can contribute to the weakness. 
Large gravestones and multiple piece monuments are also not immune to the misused Portland Cement disease. Leaning monuments are often straightened and then finished off, with a band of concrete around the entire lower base. Sometimes, even new monument installations were simply placed into a wet concrete mass. If these monuments end up leaning and or toppling the concrete can make there resetting extremely difficult.
Finally, Portland Cement also make an extremely strong mortar, grout or glue. One that is nearly impossible to ever remove from a softer stone such as marble, sandstone or slate. Therefore, if past repairs were performed with a very hard mortar they can make re‐working and repairing the stones a second time an almost impossible nightmare. 
This presentation is based on stones that I have encountered with this Portland Cement Disease from all over America. I will offer many varying solutions that I have formulated, to solve this very difficult conservation challenge.