Ancient and modern mortars in the restoration of historical buildings

In the restoration of old stone buildings the materials used should show a strong resistanoe to weathering factors as well as being easily replaceable and compatible with the original materials. Useful information on ancient lime burning and slaking methods and on the mixing and use of old mortars has been found in historical literature. The influence of these widely differing methods on the properties of mortar cannot, however, be clearly ascertained frnm the literature. Modern literature, on the other hand, specifically discusses the workability and hardening of lime mortars in addition to different overall properties and durability. Using this information an attempt has been made to explain the reasons for the durability or deterioration of old mortars and the type of requirements which modern mortars should fulfil in the restoration of historical buildings. In this case the main concern is the resistance of the mortar to frost and sulphur dioxide (durability) and its moisture properties. Fifteen buildings are examined for failures in order to get a general idea of the condition of mortars in Finnish historical stone buildings. The majority of failures observed have been caused by the effects of frost and moisture. The physical properties of approximately sixty old Finnish mortars taken from these buildings are studied and their composition analysed. Approximately one hundred laboratory-made mortars are also examined. This research gives a substantial amount of new information an the properties of mortars, which can be applied to the production of modern mortars. The mortars tested on-site on the masonry walls of Suomenlinna and Merikasarmi have been selected from the most promising laboratory-made mortars. These include two lime fly ash mortars, one lime mortar and one lime cement mortar, all with air-entraining agents. During the course of the project, new methods were developed for the analysis of mortars. A large amount of information pertaining both to old and new mortars has been obtained by microscopic techniques and deals with binders, aggregates, cracks and bond strength. Appendix 1 describes the influence of salt on the deterioration of bricks and mortars, and deals with methods of prevention.