Salt load in Tanzanian historic lime mortars

Abstract eng:
For many centuries salt remains the object of international scientific debate due to its destructive behaviour in porous building materials. Using the same procedures as the ones used by the current studies on the climatology of salt crystallisation, we replicated this effect on more specific and less subjective two buildings, selected based on their location, age, authenticity, historical value, and possible results deployment hence, the study of two Tanzanian historic buildings in salt prone areas and those located away from the ocean. Despite receiving only minor restorations, these buildings have lasted for more than 100 years. The main concern is their future existence, which is threatened by daily receipt of salty oceanic sprays. The objective of this study was to characterise them, determine the salt profile, salt load and salt damage on external walls and to predict environmental conditions necessary to limit salt deterioration. Chemical mineralogical techniques characterised samples collected at different heights and depths from building walls. The study used wet chemical methods to obtain water soluble salt contents (Cl -, SO42-, NO3-) supported by X-Ray Diffraction (XRD), Mercury Intrusion Porosimetry (MIP) and microscopical analysis using thin sections. The ECOS program RUNSALT determined the behaviour of salt mixtures in tropical climates for each building. Analytical results were correlated to provide the salt loads of the walls. Due to the porous nature of lime mortars (25 – 37vol%) used, the walls exposed to climatic conditions such as those facing the ocean revealed more surface crystallisation of salt, than the ones not facing the ocean. While, halite occupies the major part of the walls, there are traces of nitrate at the level of one meter on the walls of desolated buildings and sulphate is predominant at 3 m, especially in buildings located in urban areas. RUNSALT supported these results by showing that the local environmental conditions caused the crystallisation of halite, sylvite, niter and gypsum.

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