# Recycling Red Ceramic Waste as a Raw Material for Lightweight Aggregates
## 3 Results and Discussion
### Characterisation of Lightweight Aggregates
#### 3.1.1 Bloating Index (BI)
The bloating index (BI) represents the overall expansion behaviour of the material, calculated as the variation in average particle volume from the initial state to the final state after sintering. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Samples**} & \multicolumn{4}{c}{**Bloating Index (_BI_) (\%)**} \\ \cline{2-5}  & **1100 \({}^{\circ}\)C** & **1150 \({}^{\circ}\)C** & **1200 \({}^{\circ}\)C** & **1250 \({}^{\circ}\)C** \\ \hline
**R100** & \(-\)22.5 & \(-\)43.5 & 3.4 & 29.4 \\
**R90** & \(-\)29.5 & \(-\)23.2 & \(-\)6.7 & 19.5 \\
**R80** & \(-\)16.0 & \(-\)24.8 & 7.9 & 10.7 \\
**R70** & \(-\)25.4 & \(-\)27.6 & 7.8 & 15.4 \\
**R60** & \(-\)22.7 & \(-\)22.2 & 17.0 & 23.4 \\
**R50** & \(-\)20.8 & \(-\)13.4 & 15.7 & 16.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Bloating indices of the mixtures. Figure 6 shows that the sample made using only RCW (R100) showed strong contraction when sintered at 1100 and 1150 \({}^{\circ}\)C and progressive bloating when calcined at 1200 and 1250 \({}^{\circ}\)C. Although the specimen in question did not meet the expansion criteria reported in the literature [44], it can be seen that recycling RCW waste into LWA does not require the addition of other materials for the granules to expand. Similarly, previous studies have identified waste with properties suitable for direct use in LWAs [13; 58]. It appears that the mineralogical composition of the RCW strongly influenced the results. Sample R60, made with 60% RCW, also showed significant bloating indices when sintered at 1200 and 1250 \({}^{\circ}\)C (17.0 and 23.4%, respectively). The data found at 1200 \({}^{\circ}\)C show that the use of clay as a supplementary bloating material helped to obtain the highest \(BI\) in the samples fired at this temperature. On the other hand, this temperature range, apparently suitable for sintering R60, is slightly higher than that used by industry during the manufacture of commercial LWAs. However, this may be related to the type of furnace used in the research [13]. Figure 6: Appearance and behaviour of the samples after drying and sintering. According to Figure 6, in general, the samples showed similar behaviour in terms of surface layer formation. In principle, sintering these samples at temperatures above 1150 \({}^{\circ}\)C resulted in granules with shinier surfaces, apparently well vitrified. As previously reported, the formation of glassy phases at high temperatures enhances gas retention. This phenomenon appears to exhibit a robust correlation with the BI identified in these samples. Glass phase formation in these samples can also be attributed to the high iron oxide content identified in the RCW and RC [55; 59]. According to Figure 6, the data collected at 1250 \({}^{\circ}\)C show a predisposition to an increase in \(BI\) with an increase in RCW. It is noteworthy that this temperature range appears to be optimal for sintering the set of samples under consideration. This is evident in the observation that, irrespective of the RCW content applied, the BI values were consistently higher than those identified at the other firing temperatures.