# Microstructure, Durability and Mechanical Properties of
## 4 Discussion  
### Microstructure Characterization  
Regarding mercury intrusion porosimetry results (see Figures 1 and 2), the lower total porosities noted at 28 days for the REF and S mortars are indicative of the development of slag and clinker hydration [19; 46], which started since setting, once both slag and clinker were in contact with water. This effect was more evident in these mortars because no other components apart from clinker or slag were present in them. The higher total porosity noted in the short term for L specimens could be explained by the fact the limestone acts as an inert filler material, without pozzolanic activity [47]. With respect to the F and FL binders, the larger porosities values at 28 days could be due to the delay of the development of fly ash pozzolanic reactions [15], compared to clinker and slag hydrations. The effect of this delay of fly ash pozzolanic reactions at short times can be also observed in the SF specimens, which showed higher porosity at that age, compared to binary binder with only slag (S series). The mortars prepared with the ternary binder with slag and limestone (SL series) also showed higher total porosity at 28 days compared to the S specimens; this would be related to its lower content of an active addition with hydraulic activity such as slag and the presence of limestone as a filler inert addition [21; 47]. With respect to the pore size distributions at 28 days, the higher percentage of finer pores noted for specimens with fly ash and ground granulated blast furnace slag in the binder would indicate a higher refinement of their microstructure, probably due to formation of solid phases as products of slag hydration [19; 46] and fly ash pozzolanic reactions [15]. This was especially noticeable for the SF binder, in which both the abovemen  
Figure 10: Evolution of ultrasonic pulse velocity for the types of mortars studied. tioned active additions were incorporated, being more evident the synergetic effects of both additions in the microstructure development. The less refined pore network noted for the L mortars would be in keeping with total porosity results, showing the effect of this not active addition. The higher pore refinement noted for the majority of mortars studied at 250 days compared to 28 days would suggest the progressive closing of the microstructure due to the development of clinker and slag hydration [48; 49; 50], as well as fly ash pozzolanic reactions [14; 15]. These reactions would produce new solid phases as products, entailing an increase of the relative volume of finer pores, as was observed in the pore size distributions. The fact that the increase of the proportion of finer pores from 28 to 250 days was more noticeable for mortars with only fly ash as active addition (F and FL series), could be related to the abovementioned delay of fly ash pozzolanic reactions [15; 51] in comparison with clinker and slag hydration, because these pozzolanic reactions need the presence of enough portlandite for starting them [15]. For the FL series, in its relatively high pore refinement it could have influenced the nucleation site effect of limestone addition, as has been reported by other authors [52]. The results of the area of portlandite peak (see Figures 4 and 5) would reveal the development of fly ash pozzolanic reactions from 28 to 250 days, as suggested by the portlandite consumption along this period of time, noted for binders with fly ash. This delay in the development of these reactions would entail that the effects of fly ash in the microstructure would be observed at later ages [53], being in keeping with the results obtained. The long-term effects of the hydration of slag in binary and ternary binders were also noticeable, as suggest the more refined microstructure noted for them compared to reference mortars.