## 3 Results and Discussion
### Pore Structure  
Characterizations of the pore structures in various paste specimens were carried out with MIP measurements. Figure 4 shows the results of the pore sizes for the 65% slag binder at the age of 1 day to 28 days. Two different pore categories, corresponding to the first and second peaks, are observed. The first peak represents the capillary pore system, which refers to the originally water-filled space, and the pore sizes were normally larger than 0.1 \(\upmu\)m. The second peak represents the gel pore system, which was formed owing to the insufficient packing of hydration products. At 1 day, the amount of hydration products was limited and the majority of the pores belong to capillary pores. With the ongoing hydration process, more hydrates precipitated, filling the capillary pores and, as a result, the first peak is obviously smaller, while the second peak is larger, as can be reflected by the curve at 3 days. It seems that the first peak disappears after slag hydration of 7 days, and only one big bump relating to the gel pore system is observed. The bump moves towards the finer pore sizes as the hydration proceeds to 28 days.  
It is expected that capillary pores will always be present in hydrated cementitious systems [29]. The absence of the first peak for the specimens at later ages can be ascribed to the well-known ink-bottle effect [30]. The MIP technique assumes that all the pores are equally accessible to mercury. This assumption is surely inappropriate for cementitious systems [31], which have a wide range of pore sizes from the nano- to the micro-scale. During the MIP tests, the mercury intruded the throat (small) pores to reach the internal ink-bottle (large) pores. As a consequence, the first peak relating to the large pores cannot be precisely detected, in particular for the specimens with a dense microstructure.  
Figure 5 compares the pore size distributions of 120-day paste specimens with increasing slag contents. The curve of each binder possesses one main peak. The pore size corresponding to the main peak is often noted as the critical pore diameter. It is normally considered that the critical pores, along with the pores larger than the critical pore diameter, are able to form a connected pore network, allowing for mass transfer. As such, the critical pore diameter plays a crucial role in mass transport properties. As indicated in Figure 5, the pore sizes turned out to be significantly finer with the addition of slag, irrespective of slag contents of 25-85%. The critical pore diameter dropped from 0.045 \(\upmu\)m for OPC to 0.009 \(\upmu\)m for 85% slag. The slag can react not only with calcium hydroxide but also with  
Figure 4: Evolution of pore sizes in the paste specimens adding 65% slag up to 28 days.  
water to produce calcium silicate hydrate C-S-H and calcium aluminates [32], leading to the densification of the microstructure. A higher replacement of OPC by slag from 25 to 65% tended to generate more C-S-H hydrates, leading to a remarkable pore structure refinement. This was, however, not the case when the slag replacement level further increased from 65 to 85%. The main reason can be related to the low content of CH in the 85% slag binder; therefore, it is difficult to fully activate the reactivity of the slag, as already proved in Figure 1.