## 3 Results and Discussion
### Mechanical Performance and Open Porosity
The reason is because silica fume, which is composed of submicron particles consisting of more than 90% amorphous silica, reacts with the Ca(OH)\({}_{2}\) produced by the Portland cement hydration and is consumed faster than in the case of natural pozzolan, P, siliceous coal fly ash, V, and blast-furnace slag, S [41]. Accordingly, only about 10% of silica fume is able to react with the Ca(OH)\({}_{2}\) to form C-S-H gel and all the rest performs as a filler [46]. With regard to the blast-furnace slag cements, it was observed the best mechanical results of the blended cements for all the ages and the three aggregates. Moreover, high 28-days compressive strength results, over 40 MPa, can be achieved with high blast-furnace slag levels of 30-50%. The reference mortars made with CEM142.5N and reactive aggregates (moderate and high-reactive) presented lower compressive strength results (about 50 MPa) than those homologous mortars made with the non-reactive aggregate (60 MPa). In addition, a synergetic effect between both reactive aggregates and the pozzolanic materials has been found at 28-days. Mortars with low amount of addition (3-10%) for P, V and D presented higher 28-days compressive strength results than the reference one. About the blended cements made with blast-furnace slag, mentioned synergetic effect seems to be stronger. Consequently, all the results at 28 days (except the 5% S mortar made with the moderate reactive aggregate) are over the reference value (50 MPa). ASR gel fills the pores and causes an internal tensile stress generating cracks when this tensile stress is higher than the tensile stress of the mortar. Accordingly, mechanical properties of the concrete are affected by ASR gel formation, which can significantly reduce the durability and serviceability of the concrete structures. The level of this reduction depends on the type, size, chemical and physical characteristics, and amount of reactive aggregate [47; 48]. Furthermore, it is different for compressive, flexural, or tensile strength. Given that, tensile strength and modulus of elasticity present the highest susceptibility to ASR attack [49]. Furthermore, mechanical properties could be considered as an indirect method for ASR progress assessment. It has been reported compressive strength reduction in high and moderate reactive aggregates. Nevertheless, they were more significant in highly reactive aggregates compared to the moderately reactive ones [47]. By contrast, some studies reported no significant effect on the compressive strength [50]. These results should be justified by the testing conditions. On the other hand, it has been reported that multiaxial compressive strength of mortars made with reactive aggregates and exposed to 60 \({}^{\circ}\)C and 95% RH is higher than the multiaxial compressive strength of non-reactive mortars (about 25%) [51]. This fact is attributed to the higher the microstructural compactness achieved because the ASR gel fills the capillary pores and the interfacial transition zone (ITZ) and it decreases its weakness. Open porosity of the mortars made with the three aggregates and four additions is shown in Figure 5. The open porosity is found to be about 15% for all the mixes except for mortars made with aggregates with moderate reactivity and natural pozzolan (20%, 30% and 40%) or with non-reactive aggregate and coal fly ash (30% and 40%), which presented higher porosities between 23% and 30%. These results did not follow any clear trend. Mortars with low levels of blast-furnace slag (5% and 10%) or silica fume (3% and 5%) had open porosities slightly below 15%, which means that a denser microstructure is reached with the incorporation of S or D to the mortar. Furthermore, with increasing silica fume content from 10% to 50% the open porosity gradually increases from 15% to slightly above 25%. This fact justifies the low compressive strength of the silica fume mortars recorded at 28 days.