Summary
The Magnesian Limestone Project, a collaboration with English Heritage and Princeton University, lead to better understanding of the rapid deterioration of magnesian limestone structures in the north of England through research on three fronts: stone characterization, decay mechanism or damage process research, and treatment testing. It provided a scientific foundation for the development of conservation interventions for magnesian limestone including a detailed understanding of the cause of decay, the need for pre-treatments to reduce near surface soluble salt concentrations, and the fundamental need in porous building materials to avoid combining a source of moisture, sulfate and soluble magnesium. This work was undertaken through collaboration and exchange between conservators (English Heritage), conservation scientists (GCI), and engineers (Princeton University).

conservation image
 

Results from the GCI's Desalination of Porous Building Materials project provided useful solutions for magnesian limestone treatment. The desalination project was an independent undertaking to improve the body of knowledge concerning poulticing of salts from carved stone and to establish guidelines for the use of desalinization systems for building materials.

Background
The rapid deterioration of magnesian limestone structures in the north of England has been a serious problem for more than one hundred years. While air quality in England has improved during this period, the rate of stone loss in these structures has not slowed. Thus far, conventional stone conservation treatments have not been successful in mitigating this decay, and large-scale stone replacement has been proposed to deal with the problem for important structures such as York Minster and the World Heritage site of Fountains Abbey.

Project Description
Beginning in 2004, the GCI collaborated with English Heritage to study the rapid stone decay found at structures built from magnesian limestone. To this end, the team used Howden Minster—a ruined monastery and chapter house dating to 1388 and managed by English Heritage—as a field laboratory. Located a few miles from Drax, Europe's second largest coal burning powerplant, Howden Minster suffers from rapid flaking of stone surfaces thought to be related to past air pollution and accumulation of salts in stone pores. The project team found that while current air pollution levels are much improved, the accumulated salts are continuing to cause extensive damage, especially in areas sheltered from rain.

In 2005, the GCI characterized the physical and chemical properties in a range of magnesian limestones and determined which of these is the most durable stone for stone replacement projects. The following year the GCI delineated magnesian limestones decay process based on analyses of samples from Howden Minster and successfully simulated the primary decay mechanism—cyclic crystallization of accumulated magnesium sulfate salts from historic acidic air pollution—in the GCI laboratory.

Team members from English Heritage analyzed the structure of the walls at Howden and completed a condition survey, the preliminary results of which were then presented to the UK Cathedral Architects Association.

In 2007, treatment trials for salt removal were undertaken by English Heritage at Howden and evaluated by the GCI. The rate of decay was documented via a time-lapse field camera installed by the GCI. A weather monitoring station was installed at Howden Minster by English Heritage to correlate climate data with decay parameters and patterns. English Heritage also investigated the structure of the building and performed test treatments. The project team held a workshop in York to share current research results on the problem with architects, scientists, and conservators.

Also in 2007, the research group of materials scientist George Scherer of Princeton University joined the project to help investigate important aspects of the different damage mechanisms and to test potential interventions.

In 2008, the time-lapse photography and environmental monitoring systems data revealed the prominent role of condensation events in the salt decay process in Howden Minster’s chapter house. The installation of a roof over the chapter house walls in 1984 led to the drying out of interior walls and the accumulation of surface salt efflorescences. Removal of salts and management of moisture are the most promising conservation approaches. Project results were presented at several international meetings (see Related Materials), These include recent laboratory research focused on clays in magnesian limestone, treatment trials and decay documentation, simulation and modeling.

In 2009 the project characterized a wider range of magnesian limestone samples, determining the role of clay in magnesian limestone decay and documenting the behavior of the complex salt mixtures found at sites such as Howden Minster. Results showed that the clay content of magnesian limestone selected for use in buildings was generally low (0.1 to 1 weight percent) and that barium sulfate was a common component. These low clay levels do not appear to have a significant effect on magnesian limestone behavior. The wide range of composition of magnesium limestone documented from quarries is matched on buildings, with highly variable rates of loss.

More detailed work on salt behavior included thermomechanical analysis, which showed that expansion of stone contaminated with magnesium sulfate salts occurs during drying, followed by relaxation of the stress during dehydration of the precipitated salts. ESEM/STEM experiments show that hydration of single crystals of the lower hydrates of magnesium sulfate is a through-solution crystallization process that is only visible at a small scale (~µm). It is followed by growth of the crystal prior to deliquescence. This demonstrates that crystallization pressure is the main cause of the stress induced by salt hydration and re-wetting of lower hydrate salts. In addition, we found that drying-induced crystallization is kinetically hindered at high concentration, which we attribute to the low nucleation rate in a highly viscous magnesium sulfate solutions.

The components of the project include:

Component One: Characterization of the Building Materials

  • study of the material properties of magnesian limestone, such as compressive strength, thin section analysis, and chemistry
  • measurements of the behavior of magnesian limestone, such as hygric swelling, water uptake, and freeze-thaw resistance
  • evaluation of the variability of magnesian limestone and its suitability for stone replacement

Component Two: Field and Lab Evaluation of Decay Mechanisms of Magnesian Limestone

  • study of damage patterns in the field
  • documentation of stone loss using repeat photography
  • laboratory simulation of rapid flaking using humidity fluctuations in an environmental chamber
  • modeling of salt decay mechanism for magnesian limestone
  • monitoring of stone loss and correlation with environmental conditions using time-lapse system and weather station

Component Three: Field and Lab Evaluation of Interventions to Reduce Damage to Magnesian Limestone

  • desalination trial at Howden Minster to reduce salt concentrations
  • analysis of poultice and stone samples from Howden Minster
  • laboratory testing of preventive treatments for salt weathering

Component Four: Education and Dissemination
The results of the project are being disseminated to those most interested in magnesian limestone problems—UK conservators and architects.

  • presentation at September 2005 UK Cathedral Architects Association meeting in York, England
  • presentation at October 2007 meeting in York on Magnesian Limestone Conservation sponsored by English Heritage
  • presentation at ART2008 meeting in Jerusalem
  • presentations at Stone 2008 meeting in Torun, Poland
  • articles in applied journals: Construction and Building Materials and Materials and Construction

Last updated: January 2011