## 3 Results and Discussion
### Spent Pot Lining Reactivity and Influence on Cement Hydration  
The reactivity of SPL as SCM was tested using the R3 heat release test at 40 \({}^{\circ}\)C. The specific heat presented in Figure 4 evolved towards 200 J/g after 7 days. This is similar to class F coal fly ashes, an SCM commercially used and added in Figure 4 as a reference. This implies that sufficient reactivity is present in the SPL aluminosilicates for use as SCM, provided no other non-cementitious exothermal reactions have occurred. Given the phase composition, the reactivity seems to stem from the aluminosilicate amorphous phase in the SPL. The interaction of the SPL aluminosilicates with the hydration of the cement was investigated by studying the reaction kinetics using isothermal calorimetry on cement pastes. The heat flow and heat are both shown in Figure 5, normalized to the amount of CEM I in the mixture. A mixture with 30 wt% of SPL aluminosilicates (70 wt% CEM I, water/binder = 0.4) was compared to two references: one pure CEM I (water/binder = 0.4) and one with 30 wt% quartz filler (70 wt% CEM I, water/binder = 0.4). The sample with quartz filler showed a slight acceleration of the hydration because of the filler effect. This effect also caused a higher cement reaction degree at late age, which can be seen by comparing the total heat at 7-28 days between the CEM I reference and the sample with 30 wt% of quartz filler. The SPL aluminosilicates caused a significant delay in the cement hydration reactions. This is observed most clearly in the heat flow curves, where a shift of the peak is observed. The height and shape of the peak was still quite similar to the references. The shift appeared mainly as a delay of the hydration reaction, when the reactions eventually start, they occur similarly as in the references. Eventually at late age (7-28 days) the cumulative heat was similar to the quartz filler. Whether there was a significant contribution from the reaction of SPL itself is not clear at 28 days, and would require independent measurement of the clinker degree of reaction in the composite cement. Figure 4: R3 heat release of the spent pot lining aluminosilicates determined using isothermal calorimetry compared to class F coal fly ashes as reference. Data for coal fly ashes was obtained from [PERSON] et al. [32]. \begin{table}
\begin{tabular}{c c c c c c} \hline  & **d10 (\(\mu\)m)** & **d50 (\(\mu\)m)** & **d90 (\(\mu\)m)** & **Specific Surface—BET (m\({}^{2}\)/g)** & **Density (kg/m\({}^{3}\))** \\ \hline Milled SPL & 2.3 & 9.8 & 48.0 & 3.05 & 2697 \\ \hline CEM I 52.5R & 1.2 & 7.1 & 26.0 & 1.55 & 3145 \\ \hline \end{tabular}
\end{table} 
Table 5: Physical properties of the milled spent pot lining aluminosilicates. Particle size distribution by laser diffraction, BET specific surface by nitrogen sorption and the true density from He-pycnometry. The hydration products from the reaction of the cement and SPL were characterized using XRD on samples that were hydration stopped after 7 and 28 days. The diffractograms in Figure 6 show the formation of ettringite and calcium monosulfoaluminate as hydration products. The formation of ettringite before 7 days and afterwards the formation of monosulfoaluminate corresponds to a regular behavior of Portland cement if there are no carbonates added to the mixture [33]. The content of sulfate in ettringite is higher and therefore, this phase formed until the systems ran out of sulfates, after which the formation of monosulfoaluminate started. No qualitative differences with the XRD results of the reference including 30 wt% of quartz filler were observed. The diffractograms of the references and the full diffractograms of the sample with 30 wt% SPL are available in the supplementary information in Figures S1 and S2. The amount of Ca(OH)\({}_{2}\) (Portlandite) was investigated in more detail semiquantitatively. A comparison of the height of the peaks in Figure 7 shows that at 7 days a similar amount of Ca(OH)\({}_{2}\) was present.