SALT TRANSPORT IN PLASTER/SUBSTRATE LAYERS

We have investigated how transport and accumulation of salt in a plaster depends on the underlying masonry material. Therefor the same plaster is applied on two substrates of which the pores are either an order of magnitude larger or smaller than those of the plaster, i.e., a plaster layer (lime : cement : sand = 4 : 1 : 10) on a Bentheimer sandstone or on calcium silicate brick. The poresizes as measured for the various layers using mercury porosimetry is given in table 1.
 

Material
nano-pores
micro-pores
volume ratio
Bentheimer sandstone
¾
(40 ± 20) m m
¾
Calcium-silicate brick
(20 ± 10) nm
(20 ± 10) m m
1:1
Plaster
¾
(0.7 ± 0.4) m m
¾

Table 1. Pore sizes measured by mercury-intrusion porosimetry.

Initially the samples were saturated with a salt-solution of different concentrations. Due to the differences in pore structure between the sandstone and the calcium silicate brick totally different drying and crystallization behavior is observed.  In figure 1 the measured moisture distribution for plaster/Bentheimer sandstone as a function of time is given.

Figure 1: Moisture content in the plaster/Bentheimer sandstone system during drying. The
sample was initially saturated with a NaCl solution (c = 4 mol/l ). Dry air is blown
over the top of the sample (x = 4 mm) with a flow of 0.7 l/ min .

In a Bentheimer sandstone the pores are much smaller than the substrate and hence it is drying first. In figure 2 the the Na content as a function of time is given. 


Figure 2. The Na content a function of time for  plaster/Bentheimer
sandstone system during drying.

As can be seen in this case almost all salt is transported from the sandstone into the plaster, i.e, all salt is removed from the sandstone.
In figure 3 the measured moisture distribution for plaster/calcium silicate brick as a function of time is given. Due to the fact that in this case  the brick contains a large number of pores smaller than the plaster,  first the plaster layer dries out.
  

Figure 3: Moisture content in the plaster/calcium-silicate brick system during drying. The
sample was initially saturated with a NaCl solution (c = 4 mol/l ). Dry air is blown
over the top of the sample (x = 4 mm) with a flow of 0.7 l/ min .

In this case a significant amount of salt will stay in the brick and crystallize there. Hence the efficiency for salt removal is very low


Figure 4. The Na content a function of time for  plaster/calcium-silicate brick
 system during drying.


From these we can conclude that the performance of the plaster is not solely determined by the properties of the plaster itself. A proper matching of the pore-size distribution of the plaster with that of the substrate  is needed. Applying a plaster without knowledge of the substrate might even lead to more damage instead of less.