## 3 Results and Discussion
### Properties in Hardened State
#### 3.3.2 Porosity  
Given that the mechanical resistance of cement-based materials is closely related to the porosity of the cementing matrix [36], the size, volume, and distribution of the pores in the specimens were determined. Computed tomography was performed (NIKON CT-Scanner X-H-160 with Target of W, filter of 0.375 mm Cu with times of 708 ms per frame with a total of four frames, and the number of projections from 1100 to 155 kV and 57 \(\mu\)A) to obtain images of cuts in the different internal structures and pore size of the systems. [PERSON] et al. [20] consider that substitutions of around 7% waste improve the mechanical and elastic properties when different particle sizes are combined, avoiding homogeneity. In the same line, [PERSON] and [PERSON] [19] express themselves by adding between 20 and 50% of granite powder to cement. The largest number of pores in the mortars with addition corresponds to the small size at the initial moment of the hydration reaction of the PR-1 sample, decreasing markedly later. In Figure 5, the mentioned relationship with the particle size of the granite sawdust  
Figure 4: Variation of the compressive strength values for all studied samples. waste can be seen; which is smaller than in the OPC cement. Pores with greater volume are scarce in all reaction times and additions, except in 20% waste substitution, where there is a greater volume of pores, which then diminish after one year of reaction. The number of intermediate volume pores in the OPC is almost constant throughout the reaction time, perhaps with a slight decrease, per year, and, finally, pores of lower volume are the majority, remaining in the OPC throughout the time. Their number increases when the waste substitution is 10%, a trend that is very markedly repeated for 20% waste substitution. Total porosity and average pore size in the mortars with granite sawdust substitution after one year of curing are modified with regard to OPC, generating a decrease in the average porosity (0.001 to 0.09 mm\({}^{3}\)) of mortars with 10% and 20% waste substitutions. The trend changes after one year when the substitution is 20%. The average pore size changes as the cement hydration process develops towards a smaller size in the case of low-value substitutions, with regard to the initial mortars [51], showing the importance of the pore diameter in relation to the volume total sample. For volume of pores, the most numerous are those greater than 0.09 mm\({}^{3}\). These pores remain constant in number throughout the hydration reaction. Intermediate pores are almost non-existent; they decrease for a 10% waste substitution per year, as shown in Figure 6.  
Small pores are predominant in size in mortars with addition; their quantity decreases over time, unlike those of greater volume. The above observation is clearly detected from the electron microscope images studied in the mortars in Figure 7A, allowing us to appreciate the grain edges of the mortars without substitution. The grains are held together by C-S-H gels and ettringite fibers, which give them a slightly porous appearance, as seen in Figure 7B. Porosity is linked to the existence of C-S-H gels [52]. They are gels that change their morphology according to their degree of hydration, so that the needle growth naturally forms over time, accelerated by rapid drying--their growth being interrupted by slow drying [53]. Figure 8 allows this observation. C-S-H gels are materials highly sensitive to temperature and their apparent density increases with changes in thermal cure from 5 \({}^{\circ}\)C to 50 \({}^{\circ}\)C. Their increase means that gels take up less space, resulting in higher capillary porosity [54] that has a detrimental effect on mechanical properties and durability at high temperature [55]. C-S-H gels at the beginning of their formation are metastable and can evolve, slowly generating water and portlandite [56].