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Materials and Structures

, Volume 33, Issue 9, pp 594–600 | Cite as

The petrography and microstructure of medieval lime mortars from the west of Scotland: Implications for the formulation of repair and replacement mortars

  • John J. Hughes
  • Simon J. Cuthbert
Technical Reports

Abstract

Twelfth and thirteenth Century lime mortars from the west of Scotland were examined using optical polarising microscopy and SEM on impregnated, polished thin sections, and of fracture surfaces by secondary electron SEM. The non-hydraulic calcite binder in these mortars is loclly inhomogenouse, exhibiting variations in texture and density. Spongiform binder with porosity of size 10–20μm has sharp transitions with neighbouring zones of dense binder. Sub-isopachus concentrically accreted calcite pore linings delineate extensive areas of secondary porosity created by dissolution of binder. Common lime inclusions (lime lumps) suggest that hot mixing processes were used. Textural characteristics of lime lumps are similar to reaction rims on partially burnt primary source limestone suggesting that provenance for the lime can be deduced from the petrographic features of incompletely burnt relics. The evidence for the dissolution and reprecipitation of carbonate binder materials suggests the need for a careful reconsideration of practical historic mortar analysis. These historic mortars are texturally and compositionally more complex than their modern equivalents. These textures provide evidence for historic lime burning and mortar mixing practices in Scotland during the middle ages, as well as ageing processes, and allows the possibility of a more sophisticated approach to formulating compatible replacement mortars.

Keywords

Calcite Lime Polished Thin Section Lime Mortar Carbonate Source Rock 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Résumé

Les échantillons de mortiers de chaux, utilisés aux douzième et treizième siècles dans l’ouest de l’Écosse, ont été examinés par un microscope optique polarisé et un microscope électronique à balayage (MEB) sur des fines sections polies et des surfaces fracturées par des électrons secondaires d’un MEB. La calcite non-hydraulique dans ces mortiers était hétérogène étant donné la variation de sa texture et de sa densité. La porosité spongieuse, ayant des diamètres de 10–20 μm, présente des transitions d’une zone dense du mélange à une autre zone. Dans la porosité secondaire créée par la dissolution du mélange, la calcite a été déposée sur la surface intérieure. Les inclusions dans la chaux suggèrent qu’un mélange à chaud a été utilisé. Les caractéristiques de la texture de ces inclusions sont similaires à celles d’une calcite issue d’une cuisson primaire partielle. Il semblerait donc que la provenance de la chaux peut être déduite à partir de l’analyse pétrographique des restes de mortier ayant subi une cuisson incomplète. La dissolution et la reprécipitation du carbonate dans le mortier conduisent donc à reconsidérer l’analyse pratique des mortiers historiques, car ils ont une texture et une composition beaucoup plus complexes que leurs équivalents modernes. La texture des mortiers historiques de l’Écosse prouve que la cuisson et les pratiques de malaxage ainsi que le processus de vieillessement permettent une approche plus sophistiquée pour formuler les mortiers de remplacement compatibles.

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Copyright information

© RILEM 2000

Authors and Affiliations

  • John J. Hughes
    • 1
  • Simon J. Cuthbert
    • 1
  1. 1.Advanced Concrete and Masonry Centre, Dept. Civil Structural & Environmental EngineeringUniversity of PaislyPaisleyScotland

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