# Monitoring and Calibrating Building Materials Drying Kinetics with Capacitance Sensors
## 3 Results and Discussion  
### Materials' Microstructural Characteristics  
The materials' microstructural characteristics, as calculated with the aid of mercury intrusion porosimetry, are presented in Table 2. In addition, Figure 5 shows the distribution of mercury relative volume (Rel. Vol. (%)) for different selected pore ranges.  
The bricks, although both fired and handmade, present totally different microstructural characteristics. _BRI_ presents an open total porosity of about 17.5%, while for _BRM_ this percentage is about 32.6%. Concerning their pore size distribution, both materials present a unimodal distribution around the pore average radius. For _BRI_, the highest percentage of pores is found in the range of 0.1-1.0 \(\upmu\)m, while for _BRM_ it is found in the 2.0-5.0 \(\upmu\)m pore range.  
As far as stones are concerned, the two sedimentary stones also exhibit completely different microstructures. _SRY_ presents a high open porosity value (35.2%), while _SST_ is a very low-porosity stone, with a percentage of 3.91%. Although both stones present their highest percentage of pores in the 0.1-1.0 \(\upmu\)m range, _SRY_ presents in general smaller  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Material**} & **Total Cumulative** & **Bulk Density** & **Total Open** & **Average Pore** & **Total Specific** \\  & **Volume** & _dbulk_ & **Porosity** \(P\) **(\%)** & **Radius** \(r\) **(\(\upmu\)m)** & **Surface Area** \(s\) **(m\({}^{\prime}\)g)** \\ \hline _BRI_ & 85.0 & 2.06 & 17.5 & 0.97 & 0.43 \\ _BRM_ & 174.0 & 1.87 & 32.6 & 2.01 & 1.46 \\ _SRY_ & 197.0 & 1.79 & 35.2 & 2.83 & 2.28 \\ _SST_ & 15.7 & 2.49 & 3.91 & 10.7 & 0.28 \\ _ML_ & 169.2 & 1.77 & 29.9 & 19.3 & 1.20 \\ _MHL_ & 210.4 & 1.60 & 33.6 & 23.9 & 2.23 \\ _MLP_ & 85.0 & 1.85 & 28.4 & 0.03 & 6.28 \\ _MC_ & 174.0 & 2.20 & 14.8 & 0.18 & 3.85 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Materials microstructural properties.  
Figure 4: General conceptual shapes of calibration curves for the capacitance sensors. (**a**) Ideal calibration curve; (**b**) Non-linear relationship between moisture content to capacitance; (**c**) Threshold levels, without any relationship between the two sizes; and (**d**) Unstable calibration [28].  
pores, in a unimodal distribution around its average pore radius of 2.83 \(\upmu\)m, while _SST_ has larger-sized pores, distributed throughout the whole pore range, with an average pore radius of about 10.7 \(\upmu\)m.  
The mortars also show different microstructural characteristics. The natural hydraulic lime mortar (_MHL_) presents the highest open porosity value (about 33.6%), with a bimodal distribution, with a main peak around its average pore radius of 23.9 \(\upmu\)m and a second peak  
Figure 5: Distribution of the percentage of relative volume (Rel. Vol. (%)) at the different pore ranges (from <0.01 to >5 \(\upmu\)m) for the bricks, the stones, and the different mortars investigated in this study.  
at 0.3 \(\upmu\)m. _ML_ also presents high porosity value (29.9%), with a bimodal distribution around its average pore radius of 19.3\(\upmu\)m and around 1\(\upmu\)m. The _MLP_ mortar presents a porosity of about 28.4%, with a bimodal distribution around its average pore radius 0.03 \(\upmu\)m and around 0.8 \(\upmu\)m. The cement-based mortar (_MC_) presents the lowest open porosity, about 14.8%, with a unimodal pore size distribution around its average pore radius of 0.2 \(\upmu\)m.  
In general, it is known that the drying rate of building materials is controlled by a material's microstructural parameters, in conjunction with the effect of environmental conditions [29].