Figure 1: A schematic drawing of a crystal confined in a pore. Because of the surface tension the crystal it is under pressure.
This mechanical pressure causes the solubility of the salt inside the pore, i.e., a porous material to be different from the bulk solubility:
Hence the crystallization pressure can be assessed experimentally by measuring the solubility increase inside a porous materials during crystallization. In general the solubility will higher and is referred to as supersaturation (Be aware: in this case it is an equilibrium). With NMR we were able to measure the solubility of Na2CO3 and Na2SO4 in a series of model porous materials. In figure 2 the measured solubility is given as measured for Na2CO3 for various pores sizes.
Figure 2. Na2CO3
solubility inside the pores of Nucleosil. The various curves
reflect the bulk literature values or
fits to the data with a supersaturation ratio of 3.1 for the 5 nm pores,
2.3 for the 10 and 12 nm pores and 1.1 for the 30 nm pores.
As can be seen there is a clear increase in the solubility for the 5 and 10,12 nm pores. Using these data the surface energies were determined as gcl=(0.09 ± 0.02) N/m for Na2CO3. The resulting pressure of the crystal can be expressed as P=0.045 (Nm-1)/r . Assuming a tensile strength of 3 MPa for building materials the typical pore size blow which the crystal may cause damage is 30 nm. For Na2SO4 during the experiments a metastable phase was observed (i.e., Na2SO4. 7 H20) which had no excess pressure and consequently will give no damage. However based on the experiments a estimate was made of the surface energy for Na2SO4. 10 H20 :gcl =(0.06 ± 0.02) N/m for Na2SO4. 10 H20. Hence the pressure can be expressed as P=0.03 (Nm-1)/r . Hence it is evident that this damage mechanism can only work for very small pores.
L.A.Rijniers, H.P.Huinink, L.Pel, K.Kopinga, Salt crystallization in porous materials and its implications for stone decay, Euromat 2003, symposium P2 proceedings, http://expert-center.epfl.ch/publications/euromat2003.html, (2003)
L.A.Rijniers, L.Pel, H.P.Huinink, K.Kopinga, Salt crystallization as damage mechanism in masonry, IB2maC 2004, Amsterdam (2004)
L.A.Rijniers, L. Pel, H.P. Huinink, K. Kopinga: Salt crystallization as damage mechanism in porous building materials, an NMR study, MRPM7 Paris (2004) (submitted to be published in special edition of Magnetic Resonance Imaging)
Lourens Rijniers: Salt Crystallization in porous building materials – an NMR study, PhD Thesis (Nov 2004)
L.A.Rijniers, H.P. Huinink, L. Pel, , K. Kopinga, Experimental evidence of crystallization pressure inside porous media, Phys. Rev. Let. 94, 075503, 2005
a Punch, ScienceNow, Feb 2005