# 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)  
Table 4 compiles the bloating index values obtained from the mixtures. The results indicate high dispersion in the \(BI\) data, with values ranging from \(-43.5\%\) to \(29.4\%\), both corresponding to sample R100 when sintered at \(1150\)\({}^{\circ}\)C and \(1250\)\({}^{\circ}\)C, respectively. There was a tendency towards volumetric expansion in the set evaluated, since all the formulations showed positive bloating indices (\(BI\) > 0) in at least one of the sintering temperatures. Notably, sintering at \(1250\)\({}^{\circ}\)C promoted greater volumetric expansion compared to the other thermal conditions tested. Figure 5: Diagram of commercial potential for manufactured lightweight aggregates. Adapted from [PERSON] [8]. Although a general trend of increased bloating at 1250 \({}^{\circ}\)C was observed, sample R50 showed a lower bloating index than expected. This deviation can be attributed to the high variability typically associated with bloating behaviour, as confirmed by the dispersion of BI values across the tested mixtures. As noted by [PERSON] and [PERSON] [4], the manufacturing parameters of lightweight aggregates play a critical role in expansion performance. Key factors such as the granulation technique, the granulometric, chemical, mineralogical and microstructural properties of the raw materials, the formulation and processing conditions of the raw mixes, the water content used during pellet formation and the sintering protocol can significantly influence the extent of bloating [4; 11; 49]. By grouping the results in Table 4 from the sets of mixtures made, we can see that: (a) the R100 formulation, made exclusively with RCW, obtained an expansion index of almost 30% when calcined at 1250 \({}^{\circ}\)C. This result indicates the potential of this residue for the expansion phenomenon, and (b) with the exception of sample R90 at 1200 \({}^{\circ}\)C, all the samples made using a mixture of RCW and RC showed bloating when sintered at 1200 \({}^{\circ}\)C and 1250 \({}^{\circ}\)C. (c) Invariably, the aforementioned samples did not have a chemical composition within the parameters pre-established by [PERSON] [44]. However, the results found attest to the formation of a liquid phase with adequate viscosity to trap the gases. It is therefore possible to assume that the mineralogical composition of this group of samples acted as a substitute for the aluminosilicate matrix [11] and that the prediction of expansion based on the chemical criteria proposed by [PERSON] [44] is not suitable for RCW [12]. Figure 6 illustrates the morphological aspect of the samples, as well as a representative model of their behaviour after the drying and sintering processes. It should be noted that the interpretation of this figure also allows us to infer the presence of relative bloating. Using the R90 formulation as a reference, dimensional shrinkage of close to 30% was observed when sintering at 1100 \({}^{\circ}\)C. However, as the temperature increased, the granular structure progressively grew. This evidence suggests that the expansive capacity of this sample is more expressive than that previously estimated by the bloating index (_BI_). From this perspective, it can be inferred that all the compositions developed in this study showed a degree of volumetric expansion greater than that indicated by the _BI_ values reported in Table 4.  
To improve interpretation of Figure 6, it is important to clarify the parameters represented. The relative bloating index refers to the variation in the average particle volume measured from the point of maximum densification to the stage at which the highest bloating index is observed. The symbol \(\mathcal{O}\) denotes the simulated particle diameter values for each mixture, estimated from the experimentally determined bloating index at each sintering temperature.