Salt bursts can cause extensive spalling in stone structures. Water-soluble salts and their hydrates Na2CO3 H2O (Na2CO3-7H2O, Na2CO3 10H2O), Na2SO4 (Na2SO4 10H2O), and MgSO4-H2O (MgSO4 7H2O) are trapped in pores of stone and concrete by both infiltration upward from the ground and by the reaction of carbonate and silicate rocks with sulfuric and carbonic acids from polluted urban atmospheres. The crystallization of the salts and their recrystalhzation from a lower to a higher hydrate within the range of mineral stability may develop stresses of high magnitude. Theoretical bursting pressures were calculated thcrmo-dynamically from vapor pressures of the hydrates and from humidities of the air for several air temperatures.

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