# Microstructure, Durability and Mechanical Properties of
## 4 Discussion
### Durability and Mechanical Parameters  
With respect to durability-related parameters, the decrease with time of the steady-state chloride diffusion coefficient (see Figure 7) would be related to the progressive microstructure refinement produced by the development of slag and clinker hydration, and fly ash pozzolanic reactions, according to the previously discussed porosimetry results. At 28 days, the lower values of this parameter for the REF and S series and the higher values for binders with fly ash (F, FL and SF series) would again show the delay of fly ash pozzolanic reactions compared to slag and clinker, also coinciding with microstructural characterization. However, the effects in the long term of fly ash were very noticeable in the diffusion coefficient, as showed the important reduction of this parameter between 28 and 250 days for the F, SF and FL series, even improving at the later studied age the values of this parameter noted for specimens with only slag as active addition. These results would be overall in keeping with the pore refinement in the long term produced by fly ash pozzolanic reactions, as suggested by the pore size distributions, electrical resistivity and differential thermal analysis results, already discussed. This pore refinement would make more difficult the diffusion of chlorides through the microstructure of the mortars, giving as results lower values of this coefficient. The addition of slag in ternary and binary binders also produced a beneficial effect in this coefficient at 250 days, improving the reference mortars. This is due to the progressive development of slag hydration, which also produced a microstructure refinement, as has been previously explained for the mercury intrusion porosimetry results. For ternary binders with slag, it is interesting to highlight the good performance of SF mortars regarding the diffusion coefficient in the long term, probably produced by the combined effects of slag hydration and fly ash pozzolanic reactions. On the other hand, the fact that this parameter was slightly higher for SL mortars at 250 days could be due to the presence of limestone in the binder, an addition without pozzolanic or hydraulic activity. These effects were less noticeable when limestone was combined with fly ash in the binder (FL series), maybe due to the abovementioned influence of the pozzolanic reactions of this addition, as well as the nucleation site effect of limestone addition when it is combined with fly ash [52, 57]. Finally, the reduction with time observed for the L and REF mortars would coincide with the increase of the presence of finer pores, revealed by pore size distributions, and it would be produced by clinker hydration development [46]. The highest diffusion coefficient in the long term for the L mortars would also agree with the previous results, and it would be due to the non-active effect of limestone addition [47]. The water absorption after immersion was similar for the different mortars studied (see Figure 6). With respect to the binders with active additions, at 28 hardening days, SL and FL specimens showed slightly higher values of this parameter compared to the S, F and SF mortars, which would be produced by the addition of limestone in these binders, as has been already explained. However, in the long term the absorption after immersion decreased for mortars made with ternary binders, reaching approximately the values noted for the REF specimens. Therefore, the use of these binders did not produce a worsening in the behavior of the mortars in relation to this parameter. Regarding the mechanical properties, the compressive strength results (see Figure 8) showed similarities with the microstructure characterization and diffusion coefficient results. The highest values of this strength at 28 days observed for the REF and S specimens, would be in keeping with their lower total porosities and lower diffusion coefficients noted at that age.