Identification of various salt crystallization and water freezing patterns induced by temperature variation from Na2SO4 - H2O system confined in porous materials

Among inorganic salts, sodium sulfate is considered as one of the most dangerous to porous building materials. The pores of building materials are usually occupied by aqueous salt solution. Hence, the phase transitions of both, water and salt should be considered simultaneously during temperature decrease. In the present paper we describe an extensive experimental study devoted to the analysis of combined salt crystallization and water freezing in the Na2SO4-H2O system confined in the pores of sandstone and red clay brick induced by temperature variation. Salt and water phase transition patterns were investigated using differential scanning calorimetry. Saturated samples were subjected to 20 cooling-heating scans, where the initial temperature was changed in each cycle. Such a procedure enabled us to define three different crystallization paths of Na2SO4 hydrates and ice depending on the initial conditions. Based on the recorded heat flow, the crystallization enthalpies of hepta- and decahydrate of sodium sulfate were estimated.