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Effect of NaCl on the hydric and hygric dilation behaviour of lime-cement mortar

B. Lubelli1, R.P.J. van Hees1, 2 and H.P. Huinink3
1 Delft University of Technology
2 TNO Built Environment and Geosciences
3 Eindhoven University of Technology

The mechanism of damage due to NaCl crystallization has not been clarified yet. Apart from crystallization pressure, other hypotheses have been proposed to explain the decay. Irreversible dilation during NaCl crystallization has been observed in a few cases but has never been studied in a systematic way. In this paper the effect of NaCl on the hydric and hygric behaviour of a lime/cement mortar is extensively studied. NaCl is shown to modify the hydric and hygric dilation behavior of the material. The material contaminated with NaCl shrinks during dissolution and dilates during crystallization of the salt. This dilation is irreversible and its amount is sufficient to cause damage after few dissolution/crystallization cycles. A similar behaviour has been also observed in the presence of NaNO3 and KCl. If a crystallization inhibitor, modifying crystallization habits of the salt, is added to the NaCl, no irreversible dilation occurs. Outcomes of electron microscopy studies suggest that a relation exists between crystallization habit of the salt and dilation behaviour. Salts crystallizing as a layer adhering to the pore walls seem able to cause relevant expansion during drying of the specimen and crystallization of the salt. A similar effect is, in this theory, not possible when the same salt crystallizes without adhering to the material, as in the presence of the crystallization inhibitor.

Key words: Salt crystallization, decay, dilation, hygric behaviour, hydric behaviour