L'influenza del gelo-disgelo e della cristallizzazione salina sul comportamento elettrico di campioni di marmi

The causes of deterìoration of the physical and mechaniical proprìeties of stane mateiials in the Italian climate are chiefly due to two factors: -changes in the status of the water inside the pores following varìations in air temperature; - capillary absorption of water with high load of dissolved salts. These processes promote the formation of saline solutions inside the materials due to condensation. The state of aggregation of stane materìah is represented by porosity, a parameter that affects their physical and mechaniral proprìeties. Porosity impìies thè presence of empty spaces (pores and cracks) through which water and saline solutions can penetrate. Important modifications dìie to wecithmng phenomoia mai oca.tr. Of hese salts are distinctively importuni thè diasulphates (abundant in earth), thè chlorìdes (sea water) and thè nitrates (waste waters). Their presence under certain conditions provoke thè formation of crystals, wich detennines an increase in thè pore water pressure (pressure of crystallisation) that if contini/ed far long perioda of (ime would resuìt in devasta-iing effects (sudi as their qualità deterìomtion). Addi-tionally, many of these salts may exist in different states of hydration (Le. different volumes), so that "hydration pressure" is alno exerted on thè wnlls of thè pore. The precipitation of thè salts ma.y take piace on thè exter-nal surface ofthe storie mate-rìal or within thè porous. If thè veloci ty ofdiffiision ofthe water rapour through thè surface is loiver tìian thè velociti of migmtion of thè solution thmugh thè inside pores, thè salts will cr\slallise on thè surface under thè forni of effloresccnces. Alter-natively. crystfillisntion ma} occur beneath thè external surface giving ?isc lo thè formation of suì>florescences. Saline efflorescences altack and destro} thè surface parta, ti'hile thè formation o