Some cases of restoration and conservation works on historic buildings require the application of mortars that must harden in conditions of no air contact and usually high humidity; however, some setting speed is needed, mainly in order to assure the adequate performance of the whole preservation work. In building conservation practice, there is some difficulty in formulating compatible mortars for use in renders and joints, due to requisites of low elastic modulus, sufficient flexural and compressive strength and adequate behaviour in terms of water intake and drying. For these reasons air lime is the most adequate binder; however it encloses some problems such as slow setting, inability to harden under water, microcracking and lack of durability. The use of pozzolanic materials in lime mortars such as metakaolin is a matter of recent studies and the results suggest that, in adequate proportions, they produce an increase in mechanical strength and durability of mortars, meeting water intake and drying requirements.
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