Degradation of wall paints due to sodium sulphate and sodium chloride crystallization
Keywords:paints, salt crystallization, sodium sulphate, sodium chloride, ancient buildings
A test method for evaluating wall paints behaviour to soluble salts crystallization was developed at LNEC. in the present paper, a recent set of tests is described and discussed. The major objectives were: analysing and comparing the behaviour of a common emulsion ("plastic") paint and a silicate-based paint; observing and comparing the effect of sodium sulphate, sodium chloride and distilled water on the paints and on a non-painted stone; evaluating this test method adequacy and effectiveness. The silicate-based paint showed a resistance to soluble salts crystallization greater than the one of the plastic paint. However, the degradation pattern of the silicate-based paint (blistering of a filmic layer) was similar to the one of organic paints and distinct from the one of pure mineral paints. The amount of damage that a saline solution can cause to wall paints cannot be inferred from the amount of damage it can cause to stone. Sodium chloride seems to be able to cause more severe degradation to wall paints than sodium sulphate. To the unpainted stone, sodium sulphate seems to be more damaging than sodium chloride. The test method seems adequate to observe and compare the behaviour of wall paints under soluble salts action. However, lower (around 0.5%) concentrations for both sodium sulphate and sodium chloride should be tested in the future.
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