Alteration textures in historic Scottish lime-mortars and the implications for practical mortar analysis

John J Hughes, Simon J Cuthbert, Peter M.J Bartos

Research output: Chapter in Book/Report/Conference proceedingConference contribution

Abstract

Evidence of dissolution and later precipitation of carbonate binder materials can be found in medieval lime-mortars from the west of Scotland. Examination of impregnated, polished thin sections by polarising microscopy and SEM reveal isopachus concentrically accreted calcite pore linings that delineate extensive areas of secondary porosity created by earlier dissolution of binder. Some pores show no precipitation and are potential sites of recent dissolution. Multiple isopachus pore linings exhibit several stages of growth. Innermost layers consist of acicular calcite growth with crystal axes perpendicular to the pore wall. Other areas show evidence for enhanced porosity, in the 10-20um range, that have sharp transitions with neighbouring zones of dense binder structure. However, the samples examined so far are from unprotected fallen masonry, where rain and ground water penetration is likely to be greater than in well-constrained positions within walls. It is uncertain at present whether the observed structures formed within a wall or after the masonry had fallen. The position of such structures within a wall will be important for identifying the source and pathways of calcite during dissolution and precipitation.

Significant secondary binder porosity within mortars could compromise analysis aimed at characterisation for matching correctly for restoration. Using uncorrected measurements of binder/aggregate proportions in such ancient mortars as a guide to specifying repair mortar would result in unrealistically high aggregate contents. Porosity, permeability and strength would be very difficult to match in these old and environmentally aged mortars. Despite the dissolution of material from the mortars they have high apparent durabilities, perhaps resulting from the coarsely crystalline re-precipitated calcite. Hence solution-reprecipitation processes are important factors in the maturation and enhanced durability of mortars.
Original languageEnglish
Title of host publicationProceedings of the 7th Eueroseminar on Microscopy Applied to Building Materials
EditorsJoe A Larbi, Hans.S Pietersen, Hans.J Janssen
Place of PublicationNetherlands
PublisherDelft University of Technology
Pages417-426
Number of pages10
ISBN (Print)9076554021
Publication statusPublished - 1 Jul 1999
Externally publishedYes
Event7th Euroseminar on Microscopy APplied to Building Materials - Delft University of Technology, Delft, Netherlands
Duration: 29 Jun 19992 Jul 1999
Conference number: 7

Conference

Conference7th Euroseminar on Microscopy APplied to Building Materials
Abbreviated titleEMABM
CountryNetherlands
CityDelft
Period29/06/992/07/99

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Keywords

  • Historic Mortar
  • Cultural Heritage
  • microscopy
  • Microscopy, Electron, Scanning
  • Microscopy, optical, transmitted light
  • Inverlochy
  • Castle Sween

Cite this

Hughes, J. J., Cuthbert, S. J., & Bartos, P. M. J. (1999). Alteration textures in historic Scottish lime-mortars and the implications for practical mortar analysis. In J. A. Larbi, H. S. Pietersen, & H. J. Janssen (Eds.), Proceedings of the 7th Eueroseminar on Microscopy Applied to Building Materials (pp. 417-426). Netherlands: Delft University of Technology.