## 4 Materials and Methods
The measurements on concrete samples can depict the combined effect of the binder system and the fractality of the interfacial transition zone more realistically. As a limited amount of sample is used for MIP, it is important to secure a representative sampling. This is more difficult for concrete than for cement paste due to the presence of aggregate particles. In the present study, concrete samples were carefully crushed to particles of about 5 mm. Large aggregate particles were discarded and the mortar particles were increasingly sub-sampled in a riffle splitter until obtaining the required amount of sample. MIP was performed on a Pascal 140/440 (Thermo Fisher Scientific Inc., Milan, Italy) instrument. Samples were pre-conditioned by solvent exchange with isopropanol, as described in [20]. The samples were pre-intruded in the low-pressure device to 200 kPa to fill the interparticle space and then moved to the high-pressure device where intrusion was progressively increased to a maximum of 200 MPa. Then, the pressure was slowly released, and the retraction curve was recorded. Two additional full intruding-retracting cycles were performed on each sample. The assumed surface tension of mercury was 0.48 N/m. The interpretation of the cumulative mercury intrusion is very dependent on the contact angle assumed for the mercury. This contact angle has a direct influence on the calculated pore entry size as indicated in Equation (1), but its exact determination is not straightforward. Even though the physical-chemical characteristics of the pore walls can affect this contact angle, it is generally assumed at a fixed value for comparative purposes. The surface roughness generally increases the effective contact angle [21]. In this sense, the fractal dimension of the pore structure can contribute to explaining differences in the intrudability of samples by changes other than pore throat size. With a fixed contact angle, a higher pressure required to intrude a certain pore volume is directly translated into a finer pore structure. However, this could also be due to an increase in the contact angle due to increased roughness. SCMs can demonstrate pozzolanic activity that affects the pore structure. Such effects are normally defined as caused by pore refinement, but even less noticeable changes in pore wall texture can affect some transport processes. The effect of GGBFS on the surface fractal dimension would suggest the need for a different value for contact angle. An important derivation of the contact angle concerns the hysteresis occurring between the intrusion and retraction hemicycles. By considering a different value for the advancing and the receding contact angles, a significant part of this hysteresis could be explained. However, only if re-intrusion and retraction curves have a similar shape, the hysteresis can be fully attributed to different contact angles. To a lower extent, other elements can also contribute to this hysteresis, such as ink bottle pores and pore connectivity. The literature explains an effect of the pore wall texture that causes a significant increase in the advancing contact angle while the receding contact angle is not significantly affected [22]. Such difference has been evaluated in controlled pore glass and generalised for other porous systems [23]. In fact, a part of the irreversible intrusion of mercury can be attributed to the different values in the contact angle. The pressure required to move the mercury with convex menisci into the pore system is higher than the receding pressure, as the component of the surface tension is proportional to \(cos(\theta)\). In the present study, intrusion and retraction contact angles were estimated in ranges of 140-142\({}^{\circ}\) and 102-104\({}^{\circ}\)[24], respectively. Both ranges of values are not exact, but they are defined as the most probable.