The application of Natural Hydraulic Limes modified by the addition of calcined clay for the conservation of historic fabric
The use of lime mortars in masonry construction has a long tradition over millenia and was the dominant binder until the development of Portland cements in the last century. Its use declined to the extent that, by the middle of this century, the knowledge, materials and skill base for its successful use had all but disappeared in the UK. Indeed, it is only recently that a UK company has started manufacturing hydraulic lime again for construction purposes. A specialist branch of the construction industry is that concerned with the conservation of historic fabric. In the past, damage has been perpetrated by the use of Portland cements in the conservation of lime or natural cement based materials. This has been attributed to dissimilarities in properties such as shrinkage or moisture movement. Recent work by English Heritage also suggests that even small additions of Portland cement to natural limes can reduce the strength and durability of mortars (Ashall et al, 1996). This is similar to earlier work at Bradford on the addition of Portland cement to Rapide, a natural cement.
Conservators are left with two options:-
1.the use of original material to effect the repairs
2.the use of modern material from a range of materials not necessarily produced for conservation and, thereby, attempt to create equivalent materials
English Heritage has interests in both fields. The problems posed by the first option are obvious and may prove insurmountable. The level of understanding underpinning the latter option is little better; the comparability of each of the modern materials is at a verty empirical level. The realisation that lime mortar technology should be re-introduced has preceeded the research effort necessary to underpin its practical application. However, there is evidence of a new research community becoming established in this important field and this project is an opportunity to become involved and make a significant contribution.
The aim of the project is to address the lack of understanding of the microstructure and related engineering properties of conservation materials; in particular the use of natural hydraulic limes with the addition of calcined clays, such as brick dust, as a pozzolana. Although the initial focus is on conservation it is anticipated that the data will also be applicable to the construction of modern structures.
Pozzolanic limes develop their properties through a combination of hydration, pozzolanic reaction and carbonation. The two former reactions are “internal” whereas the latter will be controlled by the diffusion of CO2 from the atmosphere. Ashall et al (1996) have suggested that both the firing temperature of the brick and the range of size fractions of brick dust may influence one or other of the latter two processes. However, that appears to be the limit of understanding within the Lime community.
A start has been made to review the materials science literature on the use of calcined clays as pozzolanas, as well as conducting preliminary tests on a range of hydraulic limes. It is apparent that
· there is much literature of which the Lime community is unaware.
· most binding media have been either pure Ca(OH)2 or Portland Cement and most clays have been relatively pure.
· clay mineralogy is important; kaolin is more reactive than montmorillonite for example.
· firing temperature is crucial. Modern bricks are fired at far too high a temperature to maintain pozzolanic properties. This temperature should be in the range 700 - 850oC.
· there appears to be two types of hydraulic lime with either a high or low Ca(OH)2 content. This suggests that pozzolanicity is also going to be lime dependant as well as clay dependant.
· there is a need to investigate hydraulic limes as the binder phase and to use commercial calcined clays wherever possible.
Based on this initial work a study is underway to show the influence of firing temperature on pozzolanic activity. Two brick companies have donated clay which will be fired at a range of temperatures and mixed with a single hydraulic lime (Chaux St Astier NHL 2). These data will be compared with commercial brick dust and it is hoped will influence practice. This study will justify the final selection of materials for use in the main project.
In anticipation of these results, three further brick dusts and calcined clays have already been acquired for further study. One is a metakaolin; one a dust currently sold as a pozzolana and the third is a diatomaceous earth from Norway. Of these only the middle dust is capable of segregation to test the hypothesis of dual pozzolanic and carbonation reactions.
The selection of hydraulic limes will also require pre-investigation and has already commenced. The European Prestandard ENV 459-1 identifies 3 classes of natural hydraulic lime, i.e. NHL 2, NHL 3.5 and NHL 5. However, since the Standard is not yet in operation there is concern in the conservation field that some limes may be dosed with Portland cement. Thus XRD and TG analysis of potential limes should reveal suitable undosed material for the project. The selections will be made from Chaux St. Astier, Chaux de Paviers, Jurakalk, Unilit Bio.E and Unilit FEN-X. The former encompasses a range of NHL 2, NHL 3.5 and NHL 5. This range of materials covers the previously stated two types.
A single sand will be used (complying with English Heritage guidelines) to produce mortars.
The experimental work will comprise a number of elements:-
1.establish the properties of the “neat” mortars
2.select NHL 2, NHL 3.5 and NHL 5 limes of the high lime type and produce pozzolanic blends using a range of pozzolanas. The first task will be to determine the optimum pozzolana addition for each combination
3.select low lime type materials from a variety of sources and similarly produce pozzolanic blends.
4.sieve the coarse brick dust to isolate two fractions, i.e. <75 microns and >300 microns - i.e. those provisionally identified with pozzolanicity and carbonation respectively. Appropriate limes will be used to test the two-mechanism hypothesis.
Properties will be established at ages of 7 and 14 days, 1, 3, 6, 9 and 12 months. The following properties will be measured:-
· compressive strength
· density
· water permeability
· shrinkage
· frost and suplhate resistance
· Ca(OH)2, CaCO3 and combined water contents
· pore structure
The key to understanding the performance of a material is to relate it to the developing microstructure and forms the heart of the approach to the work. The microstructure will be determined with respect to depth from the surface of the samples and it is hoped to separate the effects of pozzolanic reaction and carbonation. The mineralogy will be monitored using thermogravimetry whilst the pore structure will be assessed using solvent techniques and/or mercury intrusion porosimetry. Previous work in the laboratory has shown that natural cements have an excessive reaction with methanol and propan-2-ol. Early indications are that the lime may be stable in propan-2-ol such that further trials are warranted.
The work will develop an understanding of natural hydraulic limes so that their selection can be placed on a rational basis and yield sympathetic conservation processes. Simon Swann (Secretary of the Building Lime Forum) will contribute to the management of the project and provide background on the conservation aspects of the work.
The current need is for people to do the work
This is a new field of research and it is obvious that there is a lot of work to be undertaken; more than a single researcher can cope with. There is currently a Departmental Research Studentship available to support the investigation. However, should other self-financed students or those supported by other means be interested they are most welcome to join this programme and participate in work that will have worldwide application. Precise programmes of study will be finalised following further discussions.
The Department is a 5 graded research Department (ie the top grade) and is offering a 3 year studentship to UK or EU nationals who possess or hope to gain a good Honours degree. Its annual value is £5,400 which may be supplemented by paid tutorial and laboratory assistance in undergraduate classes. Tuition fees will aslo be paid by the Department.
Should you wish to discuss the project further before submitting an application (CV indicating any expertise/previous interest in this area of research plus the names and addresses of two academic referees) please contact Dr Dave Hughes by phone (01274 233855) or email (d.c.hughes@bradford.ac.uk). You might like to visit my Home Page.
ASHALL, G., BUTLIN, R., TEUTONICO, J.M. and MARTIN, W. (1996). Development of lime mortar formulations for use in historic buildings. Durability of Building Materials and Components 7. Stockholm, 19 - 23 May 1996. Ed: C. Sjostrom. London: E&FN Spon. pp 352 - 360.
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