## 3 Results and Discussion
### Alkali-Aggregate Mitigation by Pozzolanic Additions in Cement Mortars
ASR mitigation was assessed by means of Equation (3), where the expansion recorded in mortars with the pozzolanic material (blended mortar expansion) is compared with the expansion monitored in mortars made with cement without any addition (CEM I mortar expansion):  
\[\text{ASR mitigation }(\%)=\frac{\text{CEM I mortar expansion }-\text{Blended mortar expansion}}{\text{CEM I mortar expansion}}\times 100 \tag{3}\]  
Figure 9: _Cont._  
Reactivity of natural pozzolans is mainly based on the amounts of active constituents, primarily reactive silica, which is part of the total silica of silica fume. Reactive silica reacts with calcium hydroxide (for short CH) produced from Portland cement hydration and forms calcium silicate hydrates (for short C-S-H gel) with lower Ca/Si ratio than the C-S-H gel formed during the Portland cement hydration. Both of them provide binding and strengthening properties to the cement. Then, silica fume has a twofold role in the cementitious system: it will play both a chemical role (pozzolanic reaction), and also, a physical one (filler function) [45]. Nevertheless, small silica fume particles enhance higher water demand that could reduce the cement-based material workability [46]. With regard to the alumina content present in blended cements with coal fly ash, blast-furnace slag or natural pozzolans, it is believed that is ineffective to mitigate ASR expansion probably due to the expansive ettrinigite formation replacing calcium monosulphate in the pore solution [63]. However, coal fly ash with a high amount of alumina has higher alkali binding capacity than low alumina coal fly ash and leads to C-S-H-A gel formation [29]. The coal fly ash used in the present work has 23% Al\({}_{2}\)O\({}_{3}\). Then, high alumina coal fly ash replacements of 20-40% are useful to reduce ASR expansion. Furthermore, coal fly ash cement pastes reduce permeability by pozzolanic reaction and filler effect. Figure 9: Alkali-silica mitigation by four pozzolanic additions in percentages from 3% to 50% in mortars made with three aggregates: (**a**) non-reactive siliceous sand, ASJ_2; (**b**) siliceous gravel with moderate reactivity, VB; (**c**) reactive siliceous pebble, ASJ_12. These two processes reduce the capillary pores formation and avoid the ASR gel accumulation on the reactive aggregates. The pozzolanic reaction consumes calcium hydroxide from the pore solution promoting the alkalinity reduction. In addition, coal fly ash modifies the alkali-silica gel properties, i.e., reduce the ASR gel viscosity and the swelling pressure [64; 65]. Therefore, coal fly ash blended cements mitigate ASR attack in agreement with the literature [19; 22; 27; 57; 65]. The coal fly ash effectiveness depends on the calcium content and on the calcium oxide to silica ratio [27]. Figure 9a represents the ASR mitigation results in mortars with a low expansion. In this case, only high amount of coal fly ash (20%, 30% and 40%) and natural pozzolan (40% and 50%) in the mortar reduce the expansion level from 50% to 80%. As shown in Figure 9b, the mitigation degree is higher in mortars made with moderate-reactive aggregates, i.e., about 90% for silica fume (15% D) and coal fly ash (30% and 40% V). Furthermore, high contents of blast-furnace slag and natural pozzolan (>40%) also achieved mitigation degrees over 50%. Nevertheless, the highest mitigation level was found in mortars with high-reactive aggregates. Finally, mitigation degree in blended cement mortars made with the high-reactive aggregates exhibited the best ASR mitigation performance (Figure 9c). Coal fly ash (40% V) reached more than 95% of efficiency, whereas silica fume reached nearly 95% (10-15% D), natural pozzolan reached over 75% (40-50% P) and blast-furnace slag over 60% (30-40% S).