AC1 Smeaton project on historic mortars durability

This research investigated the effects of additives to non hydraulic (air) lime-based mortars in terms of durability and workability. Phase I of the project was completed in 1995 and results have been published in the newsletter of the British Building Limes Forum, in Europe in EUREKA, Eurocare's Eurolime newsletter, and in the Bulletin of the Association for Preservation Technology.

Conclusions of Phases II and III have been drawn up and papers were presented at the 7th International Conference on the Durability of Building Materials and Components, at Stockholm, Sweden (edited by C. Sjostrom. London: E&FN Spon 1996), and at the APT conference in Chicago (1997) the paper will be published in a future edition of the APT Bulletin. A summary of progress in all phases to date will be published in the Research Transactions Vol II & III,1998.

Indications are that small quantities of cement (ie. less than a proportion of 1:3:12 of cement:lime:sand) are unexpectedly likely to have a detrimental effect on the durability of the mortar. It was also found that High Temperature Insulation (HTI) powder, which English Heritage has advised specifiers and conservators to use for over ten years (Practical Building Conservation Volume 2, brick, terracotta & earth, 1988), is not the constant reactive pozzolan we had assumed. Its qualities appear to vary by batch and its effectiveness may need to be more thoroughly tested before universal application.

More importantly, information has been gained regarding some of the properties affecting the behaviour of lime mortars modified by the addition of crushed brick or tile. If the bricks are fired at under or around 850 degrees centigrade, and are of certain clay types, then the resultant dust can act as a highly reactive pozzolan assisting initial set and increasing durability especially if the dust forms a significant proportion of the mix. In order to find pozzolanic material that will match the appearance of a wide range of historic mortars, the testing programme has also included yellow and white vitrified clay materials with some success.

The larger particles of bricks and tile in mortars seem to act as porous particulates, which assist durability. The pores capture air in the original mix and may therefore entrain carbon dioxide to assist early carbonation of the lime binder. The air gaps act as an insulant to the cold and the voids may permit both salt and ice crystals to grow in wet and winter conditions without rupturing the body of the material.

Buildings Lime Forum:

PO Box 251 Edinburgh EH6 4DW

Association for Preservation Technology:

PO Box 3511, Williamsburg, Virginia, 23187, USA.