The Trouble with Cement

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Introduction

After BAR editor Hershel Shanks criticized recent unsightly repairs to the Temple Mount walls in Jerusalem in his September/October 2010 First Person, archaeological architect Leen Ritmeyer wrote a letter in our January/February 2011 issue to explain how the improper use of cement in the repairs was causing some of the problems on the Temple Mount. This view is now echoed by Edward D. Johnson, the chief conservator of the Archaeological Architectural Field School run by USAID in Luxor, Egypt, who condemns the use of cement in archaeological restorations and explains the dangerous and devastating problems it can cause.
 

 
Re: Leen Ritmeyer’s criticism of the use of cement in repairing the bulges in the Temple Mount wall. The problems with the use of cement for such repairs are even worse than he describes.

Being both an archaeologist and archaeological conservator whose practice emphasizes built heritage and having worked in many places around the world, principally in Egypt, for the last 22 years, I have repeatedly seen cement used on archaeological sites and structures with effects ranging from mildly damaging to completely disastrous. Simply put, cement has no place in the care and conservation of any archaeological site or structure. This is a basic principle of archaeological conservation that I have emphasized continually in my teaching of the subject, most recently in the Archaeological Architectural Field School run by USAID in Luxor, Egypt, where I was assistant director and chief conservator.

Cement damages ancient materials and structures in several ways, all of which work together to accelerate deterioration.

Cement in bulk has a different coefficient of expansion than ancient building materials, which are principally limestone and sandstone. Under warm, sunny conditions it will expand at about twice the rate of the original materials. When used on ancient structures, if put under ancient stone, or used to attach it to an underlying substrate, its expansion will tend to lift that stone off the surface to which it is attached, causing that surface to buckle, detach and slough off.

Cement is also loaded with soluble salts. These will leach out of the cement over time. They will migrate into the pores of the ancient building material, where they will be deposited by evaporation, forming salt crystals. Subsequent exposure to, or absorption of, moisture from the air will cause these crystals to dissolve and reform. This cycle of dissolution and recrystallization exerts immense pressure on the pores of the stone, causing their disruption and destruction. This leads to powdering, spalling off of decorated surfaces, and if not checked, complete disassociation of the stone into sand or powder which can cause larger structural failures in a monument.

Finally, cement is unsightly and always visually clashes with original stones and their patina of age as is dramatically revealed in the pictures in BAR.

Edward D. Johnson
Archaeologist/Archaeological Conservator
La Canada, California