## 4 Discussion
instance, TS2 contained comparable calcium oxide and higher alumina, similar fineness and similar amorphous content with respect to the slags studied by [PERSON] et al. but showed lower reactivity. It is thus not clear how the presence of different oxides contributes to the reactivity and bound water of the slags and their strength performance. As such, a full correlation matrix was developed to see the correlation coefficient between all pairs of oxide content with the R3 7-day heat release and bound water content and the 28- and 91-day strength results for the three studied slags. It should be noted that all slags performed rather similarly in terms of chemical reactivity and strength activity; therefore, the established correlations may not be easily extrapolated to a broader scope of materials covering a wider variation in reactivity. In addition, since the number of studied compositions is low (i.e., 3), the \(p\)-value of the obtained correlation coefficients should be calculated accordingly, taking into account the low number of observations. A \(p\)-value lower than 0.1 is considered a signal for the presence of a potentially meaningful correlation between the two parameters. In this regard, no significant correlation was observed between the 7-day cumulative heat release and the FeO, SiO\({}_{2}\) and CaO contents of the slags. The CaO content of the slags was found to directly correlate with the 28-day and especially the 91-day compressive strength of the mortars (r = 0.998; \(p\)-value = 0.04). MgO also seemed to inversely affect the 7-day heat release (r = 0.991; \(p\)-value = 0.09). However, no significant correlation was observed between the MgO content and strength development. K\({}_{2}\)O and SO\({}_{3}\) were also found to be directly correlated, with both inversely affecting the 28-day compressive strength. Although a one-to-one correlation between the R3 test and the strength development results was not identified, the R3 test results are in line with value ranges for low-reactive materials, as reported for a broader set of SCMs, such as GGBFS, coal fly ash and calcined clay [31; 33; 44; 45], which corresponds with the fairly modest strength activity results. Therefore, the R3 test seems to properly estimate the reactivity of NFM slags and can thus be recommended for further use for the same purpose. [PERSON] et al. [21] also concluded that the R3 test can be successfully used for the assessment of the reactivity of NFM slags. In contrast, TGA measurements for determining the portlandite consumption of the blended pastes were found to be similar for different slags and may not be easily related to slag reactivity, given the variation in the CaO content of the slags and the need for additional measurements or assumptions on hydration product stoichiometry. As such, due to their laboriousness and lack of contribution to the conclusions about the reactivity of the slags, portlandite consumption measurements are not recommended for future studies on such slags. The reactivity of slag can be further improved by an increase in fineness [20] and more extensive modification of the chemical composition. From the presented results, an optimal region cannot be inferred for the chemical composition at this point. However, it appears that limiting the MgO and K\({}_{2}\)O/SO\({}_{3}\) contents could have a positive effect on the strength development of slags. Literature data point to the potential benefits of further increasing CaO and Al\({}_{2}\)O\({}_{3}\) levels [18]. Rapid water-jet cooling to maximise the amorphous content is also advisable for increasing the amorphous content and boosting reactivity [14].