# Microstructure, Durability and Mechanical Properties of
## 4 Discussion
### Durability and Mechanical Parameters
As has been discussed, this could be due to the more noticeable effects at short ages of slag and clinker hydration [19, 46], compared to fly ash pozzolanic reactions. The compressive strength increased with age for all the studied mortars, this also agreed with the microstructure refinement and the reduction of diffusion coefficient with time observed, which would be produced as a consequence of the abovementioned reactions development. Again, the most considerable increase of compressive strength from 28 to 250 days was noted for mortars with fly ash (F, FL and SF), which would be in keeping with the rise of their relative volume of finer pores and with the noticeable reduction of their diffusion coefficient. Then, the effects of new solids formation as products of fly ash pozzolanic reactions would also have beneficial effects in the compressive strength of the mortars. In relation to the ternary binders, it is worth to highlight the good compressive strength performance of SF mortars at 250 days, showing similar values as the REF mortars. Thiswould show the synergetic effects in this parameter of incorporating two active additions, such as fly ash and slag, in the binder [58], which also happened for the diffusion coefficient. However, when one of both active additions was combined with limestone in the ternary binder (SL and FL series), the compressive strength showed lower values. This influence of the presence of limestone in the binder was more noticeable in compressive strength, compared to other parameters analyzed in this work [47]. As has been explained for the results previously discussed, this could be due to the fact the limestone is not an active addition [47], so its presence would not produce additional solids formation, which would progressively fill the pore network of the materials, improving their properties. This effect was more evident for binary binder with only limestone as addition (L series), which showed the lowest compressive strengths along the studied time period. The differences between the different tested binders regarding the flexural strength were smaller (see Figure 9). However, there were similarities with the results of the previously analyzed parameters. On one hand, at 28 days, this strength was higher for the REF and S mortars, which was previously linked to the earlier development of slag and clinker hydration. On the other hand, for the majority of the binders studied, the flexural strength improved with age, in keeping with most of the previously discussed parameters. The ultrasonic pulse velocity (UPV) is a useful parameter for getting data related to the mechanical performance of the material, as well the presence of defects and voids [45; 59]. The lowest UPV values noted for the L mortars along the studied time period (see Figure 10) would suggest a worse mechanical behavior, which would agree with the lowest compressive strength values noted for this binder, as has been already explained. The increasing tendency of UPV for all the mortars would also be in keeping with the rise with time of compressive and flexural strengths observed. At initial ages, UPV was higher for the REF and S mortars, highlighting the noticeable rise until 28 days for the S mortars. This result again coincides with those discussed for other parameters, such as compressive strength, diffusion coefficient and total porosity, which were related to the early effects of clinker and slag hydration [19; 46]. The slower UPV growth with time noted for binary and ternary binders with fly ash was also in consonance with the results of the abovementioned parameters, showing the delay in the development of the pozzolanic reactions of this addition [15]. At later ages, the relatively high UPV values observed for the F, S and SF series would indicate that their microstructure would be more compact and their mechanical performance would be better in comparison with other analyzed mortars.