# Recycling Red Ceramic Waste as a Raw Material for Lightweight Aggregates
## 3 Results and Discussion
### Characterisation of Lightweight Aggregates
#### 3.1.4 Closed Porosity (\(Pc\))  
The data shown in Table 6 indicate that the values of closed porosity (\(Pc\)) varied between 4.8% and 43.0%, observed in samples R50 at 1250 \({}^{\circ}\)C and R100 at 1250 \({}^{\circ}\)C. The results indicate a positive correlation between the increase in sintering temperature and Pc levels, suggesting that thermal parameters such as the heating rate have a direct impact on  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Samples**} & \multicolumn{4}{c}{**Particle Density (\(\rho d\)) (g/cm\({}^{3}\))**} \\ \cline{2-5}  & **1100 \({}^{\circ}\)C** & **1150 \({}^{\circ}\)C** & **1200 \({}^{\circ}\)C** & **1250 \({}^{\circ}\)C** \\ \hline
**R100** & 2.09 & 2.43 & 1.77 & 1.42 \\
**R90** & 2.19 & 2.41 & 1.78 & 1.60 \\
**R80** & 2.22 & 2.38 & 1.64 & 1.63 \\
**R70** & 2.31 & 2.38 & 1.66 & 1.65 \\
**R60** & 2.38 & 2.33 & 1.68 & 1.63 \\
**R50** & 2.46 & 2.24 & 1.67 & 1.64 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Particle density (\(\rho d\)) of the mixtures.  
the development of closed porosity. The data used in the _Pc_ calculations are available in Appendices A and B.  
In general, the samples showed progressive gains in _Pc_ as the temperature rose, regardless of the RCW content applied. In sample R90, for example, the closed porosity increased from 8.4 to 37.3 per cent when the temperature rose from 1100 to 1250 \({}^{\circ}\)C. In this same temperature range, the bloating index and particle density of R90 varied from \(-29.5\) to 19.5 per cent and from 2.19 to 1.60 g/cm\({}^{3}\), respectively. It is known that increased closed porosity has a strong correlation with granule expansion [52]. Consequently, the outcomes observed in this grouping were in alignment with their respective _BI_ and _pd_ data.  
The open porosity data (available in Appendix B) reveal a tendency for _PO_ to decrease with increasing temperature, indicating an intensification of liquid phase formation during sintering. This behaviour suggests that the closure of pores connected to the surface contributes to increased internal porosity. This effect is further intensified by the retention of gases during sintering. In general, the results obtained show high potential for application in the manufacture of lightweight aggregates (LWAs). [PERSON] et al. [52] reported that increased temperature normally reduces porosity due to sintering densification. However, the opposite effect was observed in most of the tested formulations. The increase in closed porosity in this context seems to be strongly related to the trapping of gases during heat treatment.  
The effect of adding RCW on the closed porosity of the mixtures was also analysed. According to Table 6, at 1250 \({}^{\circ}\)C there was a positive correlation between the amount of RCW and the _Pc_ result. For example, in this temperature range, the closed porosity rose from 36.6 to 43.0 per cent when the RCW content increased from 50 to 100 per cent, respectively. According to reports by [PERSON] et al. [62], it can be assumed that the use of RCW can enhance the thermal insulation of the LWAs manufactured. The authors of the study posited that materials characterised by high Pc typically result in a reduction of thermal conductivity, with instances of even convective heat transfer being diminished.