Marischal College is a Category A Listed Building located in the centre of Aberdeen city. The structure comprises a large and predominantly Tudor-Gothic granite collegiate complex based around central quadrangle and courtyard. The college itself was founded in 1593 by George Keith, Earl Marischal as a protestant alternative to King's College, founded in 1494. Construction of Marischal College began in 1837 to replace the earlier buildings on the site, and was finally completed in 1906 to become Aberdeen's largest granite building and one of its most defining landmarks.
Scope of Work
Pictured: Marischal College
LTM were contracted by Sir Robert McAlpine to undertake the masonry cleaning and restoration work. The project is essentially a façade retention scheme housing Aberdeen City Council’s new corporate headquarters.
The scope of work comprised careful cleaning of the granite stone, to remove the years of pollution and dirt which had darkened the once ‘sparkling’ granite facades, and re-pointing of the granite in a more appropriate lime mortar, after the later cement pointing was carefully removed. The project also included substantial building of granite faced walls in lime mortar, and some complex structural alterations to form new openings behind the facades. Selective stone replacement was required, utilising a local Kemnay granite (to match the existing stone) extracted specifically for this project.
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Challenges and Considerations
A key consideration was the sheer size and scale of the works – Marischal Collage is the second largest granite building in the world! This was particularly challenging, especially regarding the careful cutting out of approximately 12,000 linear metres of hard, dense cementitious mortar between 2-8mm thick on high quality Kemnay granite with sharp arises…and then repointing it!
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Solutions to Problems
Pictured: Scraping back surface mortar for carbonation
The pointing mortars were carefully selected to meet the current and future performance requirements of the masonry walls. These were based on an NHL3.5 lime and local sand with natural additive to help air entrain the mortar and minimise the risk of any drying shrinkage. The proportion of additive is critical in ensuring the required mortar performance in such exposed conditions.
LTM Materials were therefore tasked with preparing all the pointing mortars off-site which were very carefully and accurately measured and (dry) premixed using appropriate kiln-dried sands. This not only ensured accuracy and consistency in the mortars throughout the project, but also increased the efficiency of all site operatives, ensuring that the project was completed on-time and within budget.
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