## 3 Results and Discussion
### Spent Pot Lining Reactivity and Influence on Cement Hydration
At 28 days the amount of Ca(OH)\({}_{2}\) in the sample with 30 wt% of SPL was significantly lower than in the sample with 30 wt% quartz filler, indicating the consumption Ca(OH)\({}_{2}\). This suggests the pozzolanic reaction of the SPL. The significant Ca(OH)\({}_{2}\) consumption was in line with the high heat release observed in the R3 test (Figure 4). The addition of SPL did not result in a change in the type of reaction products, nor was the amount of ettringite or monosulfoaluminate significantly impacted by the use of SPL as SCM, but the pozzolanic reaction of SPL was confirmed by the lower Ca(OH)\({}_{2}\) content at 28 days. Figure 5: Heat flow (**a**) and cumulative heat (**b**) during isothermal calorimetry measurements. A measurement using 30 wt% spent pot lining aluminosilicates (70 wt% CEM I) is compared to references composed of 100% CEM I and 70 wt% CEM I with 30 wt% fine quartz filler. Figure 6: X-ray diffractogram of the hydrated pastes including 30 wt% of spent pot lining aluminosilicates. The hydrates are indicated and other peaks result from unreacted SPL. The porosity of the paste samples was studied on 7 and 28 days hydrated cements using MIP. The total porosity decreased by the introduction of 30 wt% SPL as opposed to a quartz filler (Figure 8). The pore volume distribution in Figure 8 shows that the increased porosity is most prominent in the gel porosity below 0.01 \(\upmu\)m (10 nm) [34]. The pore filling effect, which is commonly associated to highly reactive SCMs like metakaolin, should be observed as a shift in the threshold pore size--the pore size below which the porosity increases steeply [35]. This effect is observed when comparing 7 and 28 days results, but this is not seen when comparing the pore size distribution of the samples with 30 wt% of SPL and 30 wt% of quartz filler. The reactivity of SPL appears not sufficient to have a significant pore refinement effect on the cement microstructure. The SPL aluminosilicates showed moderate chemical reactivity in the R3 test. The pozzolanic behavior of the SPL was confirmed by comparing the amount of Ca(OH)\({}_{2}\) using XRD. The pozzolanic reactions did not result in an increase in cumulative heat at 28 days, due to a retarding effect on the heat evolution from Portland cement hydration. Figure 8: Pore volume distribution (**a**) and total porosity (**b**) obtained from mercury intrusion porosimetry measurements. A measurement using 30 wt% SPL 2 nd cut (70 wt% CEM I) is compared to a reference composed of 70 wt% CEM I with 30 wt% fine quartz filler. Figure 7: Detailed image of the Portlandite peak in the X-ray diffractograms of hydrated pastes including 30 wt% of quartz filler and 30 wt% of spent pot lining aluminosilicates at (**a**) 7 and (**b**) 28 days. This retardation is mainly observed as a delay of the heat flow peak associated with the hydration reactions. The main components of the SPL aluminosilicates, presented in the characterization in Section 3.1, cannot explain the extensive delay that is observed using calorimetry. A very effective retarder should be present in trace concentrations in the material. The SPL aluminosilicates contain approximately 1 wt% of cyanides, which can have a retarding effect on the hydration of cement [36]. However, as explained in the Materials section, the cyanides were effectively decomposed in the treatment [9], by oxidation using H2O2. Potential reaction products from this treatment are cyanate CNO\({}^{-}\) or ammonium NH3. Furthermore, the possible presence of AlN traces, which are not extractable with the SPL-CYCLE treatment process, could result in their reaction with water and continuous NH3 formation [37]. Ammonium salts are known to delay the hydration of cement and this might thus be a possible explanation for the observed delay [38].