# Susceptibility to Expansive Reactions of a _Greener_ Uhpc: Micro to Macro-Scale Study
## 3 Results and Discussion  
### Porosity and Microstructure  
The pore size distribution of UHPC obtained from the MIP test is depicted in Figure 3a. From 3.5 nm (lower limit of MIP), the UHPC pore size distribution is bimodal, with a higher pore peak around 8 nm, followed by a sharp decrease in pore diameter size. Then, a smaller peak occurred at 70 nm, and no pores larger than 100 nm were found. The average pore diameter corresponds to 8.8 nm, and the total porosity is 4.8%, which seems to agree with previous studies [29]. In Figure 3b, the measured pore distribution is divided into three size ranges: gel or micropores (<10 nm), capillary pores (10-5000 nm), and macro pores (>5000 nm), according to [59]. However, it must be stressed that an official or standard pore size classification for cement-based materials still does not exist, so different pore size classifications may be found in the literature [60]. Still, the consensus is that gel pores correspond to interconnected spaces between the gel particles, usually presented by needle-plate or foil-shaped [61; 62]. On the other hand, capillary pores may be defined as the empty spaces not filled by cementitious materials or hydration products [61; 62]. Since the hydration products occupy more than two times the volume of the original solid phase, the volume of the capillary system is reduced as the hydration progresses. Thus, capillary pores strongly depend on the w/c ratio and the degree of hydration [60].  
Previous research suggested that pores lower than 100 nm slightly impact the strength and durability of cement-based materials [33; 63; 64]. Figure 3b indicates that the main porosity of UHPC came from gel (<10 nm), and only 1% of total pores are capillary pores (10-5000 nm). The very low volume of pores and the absence of capillary pores are the main reasons for the expected high durability against harmful gas and water agents. In addition, it is generally accepted that the pozzolanic additions, in this case, silica fume and ECat, improve the pore structure by its filler effect, filling the void between cement grains and between cement grains and aggregates particles; and the chemical effect, i.e., the pozzolanic reaction, in which SCM reacts with calcium hydroxide from cement hydration producing additional CSH, thus contributing to porosity reduction.  
Microstructure photos of UHPC samples are presented in Figures 4 and 5. The lighter zones in the matrix (Figure 4a) correspond to the ECat particles completely surrounded by hydration products and well bonded to the matrix. As can be perceived, the microstructure seems very dense and compact. Moreover, hydration products were found, namely, calcium hydroxide, but with aluminates, as shown in Figure 5.  
Figure 3: Porosity distribution of an UHPC in the reference dry state after 28 curing age days: (**a**) Differencial pore size distribution; (**b**) Pores size distribution in terms of pore size classification according [60].  
Figure 4: SEM images and XRD diffraction analysis of UHPC microstructure: (**a**) site of ECat particles; (**b**) location of zone Z1; (**c**) ECat particle connection to cementitious matrix; and (**d**) XRD diffraction analysis of material in zone Z1.  
Figure 5: \(Cont\).