## 3 Results and Discussion
### Alkali-Aggregate Mitigation by Pozzolanic Additions in Cement Mortars
Figure 8: Alkali-silica mitigation by pozzolanic additions in mortars made with the reactive siliceous pebble, 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.  
effect on the expansion, while amounts of 15% of silica fume presented expansions about 0.2% at 90 days. According to [PERSON] and [PERSON] [55], the long-term effectiveness of condensed silica fume mitigating the ASR is questioned because the alkali metals are entrapped quite soon in low Ca/Si and high-alkali pozzolanic calcium silicate hydrates (for short C-S-H gel), and then, they are released with the time. Finally, blast-furnace slag had a positive effect but somewhat lower than P, V or D. This can be justified by the lower aluminum content found in the blast-furnace slag (12.16% Al\({}_{2}\)O\({}_{3}\) in Table 2). The reference mortar made with high-reactive aggregate showed expansion results of 0.25% at 14 days and 0.42% at 28 days, which demonstrate the high reactivity of the siliceous pebble (Figure 8). In this case, the most effective additions mitigating the ASR were coal fly ash and silica fume. A high content of coal fly ash (20%, 30% or 40% V) suppresses the ASR effect. In agreement with [PERSON] and [PERSON] [27], coal fly ash with alkali content lower than 4% Na\({}_{2}\)O\({}_{\text{eq}}\) and calcium content lower than 20% CaO was effective in mitigating expansion. They reported that higher amounts of CaO increase the concrete expansion. Coal fly ash consumes calcium hydroxide from pore solution lowering its alkalinity and forming C-S-H gel with low Ca/Si ratio. In addition, this product reduces the capillary porosity. Accordingly, the expansions of mortar bars not only depend on the coal fly ash calcium content, but also on the CaO/SiO\({}_{2}\) ratio [27]. The higher CaO/SiO\({}_{2}\) ratio is, the higher expansion is expected. Regarding the silica fume, amounts as low as 5-15% D were also found quite effective. Accordingly, the content of coal fly ash or silica fume needed to suppress ASR is dependent on the aggregate reactivity. Silica fume is one of the most efficient pozzolanic materials for reducing alkali-silica attack even when utilized at rather low replacement levels (10-15%) by decreasing the Na\({}^{+}\), K\({}^{+}\) and OH\({}^{-}\) ion concentrations in the pore solution of the cement-based material due to the alkali-binding capacity of C-S-H gel in hydrated Portland cement pastes with this pozzolanic addition [56]. The pozzolanic reaction rate of silica fume is higher than that of coal fly ash, natural pozzolan and blast-furnace slag due to its higher fineness and reactive silicon dioxide. However, the long-term effectiveness of condensed silica fume against ASR is questioned in the literature [55]. Silica fume binds alkali ions within the first two days of hydration following a similar rection to that of the alkali-silica reaction [27]. ASR gel located on the silica surface reacts with calcium to form C-S-H gel with a low Ca/Si ratio. Accordingly, most alkali metals are fixed by the silica fume particles and the C-S-H gel formed in the hydration of calcium silicate paste. At a more advanced age, ASR gel will react with calcium leading to release of Na\({}^{+}\) and K\({}^{+}\) into the pore solution. Therefore, they become available for ASR and the alkalinity of the pore solution increases throughout time, i.e., months or years [27]. Accelerated mortar bar expansion measurements have been extended to three months to detect mentioned increased alkali level over time. Nevertheless, one disadvantage in carrying out these trials is the reduction of the alkali level in the mortar pore solution by leaching. High amounts of P (40% and 50%) or S (20%, 30% and 40%) resulted also effective. In order to ensure a positive effect on ASR mitigation, the blended cement normally needs a large volume of clinker replacement by blast-furnace slag.