# Long-Term Behavior of Concrete Containing Wood Biomass Fly Ash
## 3 Results and Discussion
### Density, Porosity, and Permeability  
The apparent solid density shows that replacing cement with WFA results in a reduction in the mass of the solid, which is consistent with the lower densities of WFA compared to the cement and aggregates (Table 4). In general, the bulk density decreases with increasing cement replacement, but this parameter also depends on the amount of air entrapped during the compaction process. Porosity increased with the increase in fly ash content, although no general trend between the WFA content and porosity could be observed (Table 4). This is consistent with the results presented in [2], where a larger increase in apparent porosity was observed only for mixes with 45% cement replacement by WFA.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Mix Designation** & **M0** & **M1** & **M2** & **M3** & **M4** \\ \hline Fresh density (kg/m\({}^{3}\)) & 2450 & 2460 & 2440 & 2460 & 2450 \\ Air content (\%) & 1.5 & 1.4 & 1.5 & 1.0 & 1.1 \\ Slump (mm) & 80 & 60 & 50 & 60 & 20 \\ \hline Measured initial temperature (\({}^{\circ}\)C) & 23.5 & 26.8 & 25.0 & 25.1 & 25.0 \\ Calculated initial temperature (\({}^{\circ}\)C) & 19.7 & 21.9 & 20.8 & 19.7 & 18.1 \\ \(\Delta\)T (\({}^{\circ}\)C) & 3.9 & 4.9 & 4.2 & 5.4 & 6.9 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Properties of the fresh and hardened concrete.  
Average values of the gas permeability coefficients with the corresponding coefficient of variation calculated as the ratio of standard deviation and average value are plotted in Figure 3. Since all data in Figure 3 were within a range between 1 \(\times\) 10\({}^{-16}\) and 2 \(\times\) 10\({}^{-16}\) m\({}^{2}\) and taking into account that the coefficient of gas permeability for concrete spans over a several orders of magnitude from <10\({}^{-18}\) m\({}^{2}\) for very low permeable concrete to >10\({}^{-15}\) m\({}^{2}\) for very permeable concrete shows that the replacement of cement with up to 30% with WFA had a very small impact on the gas permeability [54,55]. For mix M2, the gas permeability coefficient slightly increased, which could be due to the loosening of the particle packing by the coarser WFA1 particles. However, this increase was also within the reproducibility limits for the gas permeability method, which expressed through the coefficient of variation was around 30% [56, 57, 58].  
Results of the capillary absorption measurement are given in Figure 4. In mixes M2 and M3, a greater dispersion of results occurred, but regardless, the trend of an increasing capillary absorption rate with an increasing quantity of WFA was clear. Mixes with 30% cement replacement showed the highest capillary absorption rate. This may be attributed to the ability of WFA particles to absorb water, besides unburned carbon particles, since both fly ashes contain much coarser particles than cement (more than 50% of the particles were larger than 0.063 mm), which are probably porous and can absorb a certain amount of water. This could also be the reason for the lower slump of the fresh concrete.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Mix Designation** & **M0** & **M1** & **M2** & **M3** & **M4** \\ \hline Bulk dry density (kg/m\({}^{3}\)) & 2355.9 (\(\pm\)22.6) & 2349.7 (\(\pm\)10.0) & 2297.3 (\(\pm\)23.9) & 2362.1 (\(\pm\)17.4) & 2339.8 (\(\pm\)8.7) \\ \hline Bulk saturated density (kg/m\({}^{3}\)) & 2504.2 (\(\pm\)16.0) & 2493.9 (\(\pm\)6.3) & 2452.6 (\(\pm\)16.9) & 2503.5 (\(\pm\)14.2) & 2487.0 (\(\pm\)6.3) \\ \hline Apparent solid density (kg/m\({}^{3}\)) & 2766.0 (\(\pm\)5.7) & 2745.5 (\(\pm\)2.3) & 2719.9 (\(\pm\)8.7) & 2751.0 (\(\pm\)11.3) & 2743.7 (\(\pm\)11.7) \\ \hline Apparent porosity (\%) & 14.83 (\(\pm\)0.67) & 14.42 (\(\pm\)0.39) & 15.54 (\(\pm\)0.74) & 14.14 (\(\pm\)0.39) & 14.72 (\(\pm\)0.51) \\ \hline \hline \end{tabular} The numbers in brackets are standard deviations.  
\end{table}
Table 4: Densities and porosity of concrete.  
Figure 3: Results of the gas permeability measurements.