## 3 Results and Discussion
### Autoclaved Aerated Concrete  
Autoclaved aerated concrete mixtures were made with 25, 50, 75 and 100 wt% replacement of quartz for SPL. Foaming of all aerated concrete mixtures occurred gradually during the 4 h preventing stage at 40 \({}^{\circ}\)C. The slow and steady foaming reaction caused by oxidation of the Al powder and reduction of the water in the paste mixtures of Table 2 is beneficial for the robustness of the industrial process. The foaming reaction resulted in a relatively constant density after autoclaving with most samples showing values around 600 kg/m\({}^{3}\), shown in Figure 11. The slight variations in density were mainly caused by the accuracy of weighing the Al powder, as the quantity of Al was the main parameter controlling the porosity next to the water/solids mass ratio, which is more easy to keep accurately constant [19]. The density of all samples falls within the categories defined in EN771-4 for AAC masonry blocks [40]. The slight variations in density, especially the deviation in the samples with 25 and 50 wt% substitution with SPL aluminosilicates and 5 wt% of anhydrite, had to be taken into account when analyzing the strength results. Figure 11: Density of the AAC blocks using several replacement levels of quartz for SPL aluminosilicates after drying at 40 \({}^{\circ}\)C.  
Figure 10: Compressive strength evolution of mortars with 30 wt% spent pot lining aluminosilicates (SPL) as supplementary cementitious material, including 90 days strength (**a**). The relative strength is calculated and compared to the replacement level (**b**). The replacement level is represented by a dotted line as a visual aid (in green for 30 wt% replacement—70% strength and in red for 30 + 15 wt% replacement—55% strength). The compressive strength of the blocks is presented in Figure 12. The strength-density relation is plotted on Figure 13. The strength of the reference mixture (100 wt% quartz, 5 wt% anhydrite) reached approximately 5 MPa, which is a decent strength considering the density [18; 19; 20; 41] and reaches the required strength (4 MPa) for a block of medium density (600-660 kg/m\({}^{3}\)) according to EN771-4 [40]. The strengths and densities are more easily compared with literature and standard values in Figure 13. This strength was not decreased significantly when using only 3 wt% of anhydrite, which would decrease the sulfate leaching when recycling the block at the end-of-life of the building [28]. The effect of the substitution of the quartz for SPL aluminosilicates is most easily seen for the 3 wt% anhydrite samples, as a similar density was obtained for these samples (Figure 11). The strength did not decrease significantly up to 50 wt% of substitution. The compressive strength still reached 4 MPa for a density of 600 kg/m\({}^{3}\). The use of 75 wt% of quartz-SPL substitution resulted in a significant decrease in strength when using 3 wt% of anhydrite. At 5 wt% of anhydrite the compressive strength for the 75 wt% substitution samples was slightly improved, however, also for these samples the strength density relation is not satisfactory and did not reach the values of a commercial AAC strength class according to the EN standard (see Figure 13). Figure 12: Compressive strength of the AAC blocks using several replacement levels of quartz for SPL aluminosilicates. Figure 13: Strength-density relation of the AAC blocks compared to the density classes defined in EN 771-4 and to data from references. The results show that a replacement of quartz for SPL aluminosilicates of 50 wt% was possible without losing significant technical performance in terms of the strength-density relation. Furthermore, the anhydrite content of the mixture could be reduced to 3 wt%, which would enhance the recyclability of the blocks. The 50 wt% of substitution of quartz led to a total SPL content in the raw powder mixture of 25 wt% (see Table 2). The production of AAC is thus a promising route for the valorization of a significant amount of the available SPL.