MONUMENTEN EN WATER Diest 2005 - WTA

structure can be supposed. Both explanations are likely to show up in the cumulative
intrusion curve as a steep increase in porosity in the range of smaller pores. It is important to
notice that the presence of very small pores determines the hygroscopicity of this plaster and
increases its sensitivity to RH changes.
Fig. 6: Pore size distribution of the brick substrate and of the plaster
Point counting measurements have been performed on the thin section in order to study
further the porosity. These measurements allow defining both closed and open porosity;
because of the detection limit of the instrument, only pores larger than 1 μm can be
detected. The closed porosity (due to the hollow spheres) constitutes 28 % of the area; voids
and large pores amount to about 19 % of the surface. This last value is comparable to the
porosity obtained by MIP when considering only pores larger than 1 μm.
The pore size distribution of the material strongly influences its absorbing and drying
behaviour (Figure 7). Because of the presence of small pores, the plaster shows a slow
absorption and drying. Besides, the presence of the cellulose, that can retain water even in
secondary moistening, explains the extremely slow drying of the restoration plaster.
a
Fig. 7: Water absorption (a) and drying rate (b) of the plaster
- 6 -
b