## 3 Results and Discussion
### Properties of Composite Portland Cements (CPCs) Manufactured by 20% Slag
#### 3.2.1 Compressive Strength  
The early, late and long term compressive strengths development of the CPCs with 20 wt.% slag are presented in Table 6 and Figure 6a,b for the air and water-cooled slags, respectively. In addition, the results are compared to the OPC (95% clinker and 5% gypsum). All investigated mixtures were found to be within the values of EN 197-1 [17]. The early strengths (2 days of curing) were found higher than 2 Pa and 28-day curing strengths were in the range of 48.2-59.4 MPa. The preservation of relatively high early strengths observed in all the composite cements despite their limited and/or no C\({}_{3}\)S content is most possibly attributed to the contained calcium aluminates. Concerning the effect of iron oxide content, in general, for high FeO\({}_{\mathrm{x}}\) containing slags, the reduction of the % FeO\({}_{\mathrm{x}}\) is expected to enhance its cementitious properties, as this increases mainly the mass of residual calcium and silicon oxides, the combination of which leads to the formation of desired hydraulic phases. According to the results of this study, the %FeO\({}_{\mathrm{x}}\) of the slags, within the studied concentration limits, 3.85 wt.%-21.1 wt.%, did not present significant influence on the compressive strengths of the produced cements. The highest values were obtained using slag 4-2, with 3.85% FeO\({}_{\mathrm{x}}\) content both for the air (a) and water-cooled slag-product (b). In fact, the CPC prepared by 4-2b slag acquired slightly higher compressive strengths than those of the blank OPC. This can be attributed not only to the lower FeO\({}_{\mathrm{x}}\) content, but also to most the hydraulic phases formed in these slags as presented above. These slags were abundant in calcium silicates (C\({}_{2}\)S, C\({}_{3}\)S) and calcium aluminates (C\({}_{3}\)A, C\({}_{12}\)A\({}_{7}\)). The strengths of the rest of the composite cements lay at similar levels, despite the difference in their FeO\({}_{\mathrm{x}}\) content. Even the cement prepared with the slag having the highest FeO\({}_{\mathrm{x}}\) content (\(\sim\)21%), presented considerable compressive strength values. This can be correlated not only with the calcium silicate compounds (C\({}_{3}\)S, C\({}_{2}\)S) detected in this slag, but also with the fact that part of the iron was in trivalent state resulting in the formation of brownmillerite, C\({}_{4}\)AF, which exhibits good and intermediate contribution in the early in the final cement compressive strengths, respectively. This observation demonstrates that the state of Fe in the slag influences the morphology and its latent hydraulic properties in agreement with the findings of [18]. It is also noticed that the main peaks of the XRD pattern of this slag were attributed to magnesiowustite. However, its negative effect on strengths development was counterbalanced by the presence of the other active phases in the slag mass. These results manifest that the strength development of the manufactured composite cement is highly affected by the mineralogical composition of the used slag. According to these results, in case of slags with crystalline structure, the most important feature for positive contribution to the compressive strength of the cement is the formation of mineralogical phases similar to those met in clinker.