Experimental applications of mortars with pozzolanic additions: Characterization and performance evaluation
Introduction
Whenever possible, conservation strategies are always the best option to consider when degradation affects ancient renders: treatment with biocide, small repairs, fulfilment of lacunae, cracks repair, consolidation to restore loss of cohesion or loss of adhesion [27].
Nevertheless, if defects are severe – generalised desegregation or large detachments – it may be necessary to substitute old renders, partially or completely. In those cases it is essential to choose mortars presenting good compatibility with the background and enough durability concerning the environment.
Old fortresses near the coast of Lisbon have been originally rendered with very durable lime mortars, sometimes showing pozzolanic reactions [19], [21], but their very severe environmental conditions – salty atmosphere and sea water close to foundations and walls, mechanical action of the waves, hot sun and temperature variations, strong wind – and also some lack of knowledge concerning maintenance and conservation techniques, produced the deterioration of these renders and the need for their substitution.
To choose substitution renders for these constructions is a challenging situation, because a very good durability in moist, salty, aggressive environment is necessary and this implies resistance to water, meaning some hydraulicity. On the other hand, cement mortars and highly hydraulic lime mortars are known to be incompatible with old masonry [3], [11], [23], [26], [33]. Lime–cement mortars have been used in a similar case, apparently with acceptable results for the last ten years [24], but there are well based opinions [10], [17], [23], [29] sustaining that cement must be avoided whenever possible, because it favours salt damage due to its content on alkaline ions. Thus the best options in this case were thought to be lime mortars with pozzolanic additions.
Because of the lack of experience with modern pozzolanic mortars on sea environment, it was decided to carry out experimental applications on one of the Fortresses’ walls of renders with different pozzolanic additives and of some comparison mortars with known performance. An experimental program was carried out, both on mortars prepared in laboratory and on the panels applied in situ, to verify the compatibility of the solutions with the existent materials and their performance in the aggressive conditions.
The substitution mortar was chosen considering the tests results, the observations accomplished and the first application trials in work.
Section snippets
Render compositions
The render solutions to test in situ were chosen taking into account the following aspects:
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Compatibility with the old masonry from the physical, mechanical, chemical and aesthetic points of view was a fundamental issue [2], [4], [12], [18], [26].
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Physical compatibility concerning the water behaviour was considered particularly important because of the exposure to sea water and salts.
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Materials available in the market and technology accessible to common construction workers.
The option was by
Selected in situ tests
The tests performed on the panels (Fig. 7) to evaluate the behaviour and characteristics of the applied renders included: (i) sphere impact; (ii) controlled penetration; (iii) adhesion to background; (iv) evaluation of the water content; (v) water permeability under low pressure (with Karsten tubes); (vi) carbonation; and (vii) salt identification with colorimetric identifiers.
These tests have been presented in previous work [13], [15], [25], [29] or in bibliography and they are very briefly
Test results
The results of laboratory tests are summarized in Table 3. The in situ test results are synthesised in Table 4, Table 5 and Graphics 1 and 2.
Mechanical behaviour
In what concerns the mechanical resistance at 6 weeks it is observed that, amongst the renders with lime and pozzolanic materials, the panel with silica fume is the most deformable and the most susceptible to penetration, comparable with the air lime–hydraulic lime panel. The highest behaviour seems to belong to the air lime–Cabo Verde pozzolan panel,
Selection of the final render and application
For the selection of the substitution mortar to use some factors were took in consideration:
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The results of the in situ tests on the panels, especially those on 14 weeks. Actually, the hardening of lime mortars and even of lime–pozzolan mortars, is slow and results at 4 weeks are not representative. Naturally, they have to be high enough to resist to actions during the first period. Lime–Cabo Verde pozzolan panel had the best results, but lime–metakaolin had results almost as good at 14 weeks.
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Conclusions
The work carried out showed the importance of in situ applications and in situ tests to evaluate the performance of lime-based mortars for ancient buildings. Actually, the results obtained in laboratory tests can be misleading, because the curing conditions and the application techniques play a fundamental part in the final characteristics of lime mortars.
The use of comparative materials with known behaviour is also important because the in situ tests results have not a direct correspondence
Acknowledgements
The authors wish to thank LNEC technicians Bento Sabala, Dora Santos and Ana Francisco their collaboration in carrying out of the tests. Also the good collaboration of the application enterprise in fulfilling the application and materials specifications was a precious help. Finally, we wish to thank the Fundação para a Ciência e Tecnology (Portuguese Foundation for Science and Technology) for the financial contribution to this work, within the scope of Research Projects “Conservação de rebocos
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