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The phenomenon of salt crystallization is an important deterioration mechanism in building materials. Damage is caused by crystallisation pressure exerted within pores. Sodium sulfate is considered the most damaging salt, known to have two hydrated phases at ambient conditions. The metastable heptahydrate and the stable decahydrate called mirabilite (Na2SO4.7H2O and Na2SO4.10H2O). Damage is caused by mirabilite crystallisation either directly or via heptahydrate dissolution. The formation of mirabilite is induced at the top of the stone core by seeding with a crystal of mirabilite. We investigate the strain on the porous host from crystallisation, mirabilite crystal growth rate and degree of solution supersaturaiton with respect to mirabilite at the temperature of initiation. Using a thermostatted chamber we measure sample strain with an LVDT transducer and crystal growth rate through the core using thermocouples inserted at regular intervals along the samples. Mirabilite crystallisation is exothermic and we relate the heat produced to the quantitiy of crystals formed. The mirabilite-ice eutectic point occurs at c. –3 oC and results in complete solidification of the remaining liquid phase and further damage.

SALT CRYSTALLISATION IN PORES: THE EFFECT OF CRYSTAL GROWTH RATE ON DAMAGE