## 3 Results and Discussion  
### Mechanical Performance and Open Porosity  
Compressive strength tests were performed for all the mortars made with three aggregates (non-reactive siliceous sand, ASJ_2 (Figure 2); siliceous gravel with moderate reactivity, VB (Figure 3); and a reactive siliceous pebble, ASJ_12 (Figure 4)), and blended cements Mechanical performance was assessed at 2, 7 and 28-days following the procedure prescribed in the European standard EN 196-1 [36]. Figure 3: Compressive strength results of the mortars made with the siliceous gravel with moderate reactivity, VB, and four additions mixed in four percentages of 20%, 30%, 40% and 50%: (**a**) natural pozzolan, P; (**b**) siliceous coal fly ash, V; (**c**) silica fume, D; (**d**) blast-furnace slag, S.  
Figure 2: Compressive strength results of the mortars made with the non-reactive siliceous sand, ASJ_2, and four additions mixed in four percentages of 20%, 30%, 40% and 50%: (**a**) natural pozzolan, P; (**b**) siliceous coal fly ash, V; (**c**) silica fume, D; (**d**) blast-furnace slag, S.  
With regard to the mortar mixes made with the non-reactive aggregate, it was found that the reference mortar had the highest compressive strength at 2, 7 and 28 days in comparison with the mortar mixes made with the pozzolanic additions. Moreover, the higher the addition amount in the mortar mix, the lower the mechanical strength. In the case of the natural pozzolan mortar mixes, the reduction of compressive strength is proportional to the increase of addition in the mix from 10% to 50% (Figure 2). However, for the coal fly ash mixes two groups can be differentiated: mortars with low amount of addition (5-20% V) presenting compressive strength results somewhat similar to that of the reference mortar, and mortars with high coal fly ash content (30-50% V) with much lower compressive strength performance and quite similar to those found in the natural pozzolan mixes with high amount of addition (30-50% P). As expected, mortars with a high content of additions present low early strength and a quite slow compressive strength development. Higher compressive strength results were expected in the silica fume mortar mixes with low percentages of replacement (3-10% D) due to the high fineness and amorphous silicon dioxide content found in the silica fume. Both parameters lead to better reactive characteristics. [PERSON] found that the final strength gain in mortars made with silica fume is proportional to the glass content [40]. However, this unexpected result shown in Figure 2 can be attributed to the agglomeration effect reported in the literature [41]. The reaction mechanism of the silica fume can be outlined saying that small particles of silica fume build up agglomerations which will be completely coated on the outside with a gel-like layer. Then, water is incorporated in this layer, which is a key reason why silica fume uptake water quickly [42]. Accordingly, mortars with silica fume exhibit a high-water demand that may result in a stiffer mortar mix, which reduces the workability and promotes potential molding problems. Figure 4: Compressive strength results of the mortars made with the reactive siliceous pebbles, ASJ_12, and four additions mixed in four percentages of 20%, 30%, 40% and 50%: (**a**) natural pozzolan, P; (**b**) siliceous coal fly ash, V; (**c**) silica fume, D; (**d**) blast-furnace slag, S.  
[PERSON] et al. [43] found unreacted silica fume particles in pastes with 5 % of silica fume after 180 days of curing. This supports the low pozzolanic reaction rate of silica fume due to an agglomeration of nanoparticles [44]. In addition, silica fume acts as a filler in mortars with high contents of this blended cement constituent [45]. Therefore, in these cases it does not have a significant role in the strength gain.