# Microstructure of Mortar with Ballast Waste as a Cement Replacement
## 3 Results and Discussion
### Preparation of Mortars with Ballast Waste
#### 3.2.2 Porosity  
Over time, cement-based materials set, harden, and acquire the properties of a solid material. Various tests must be conducted to assess their stability. One of the key properties is porosity. The porous structure of mortars refers to the system formed by all the pores contained within them and their distribution throughout the material. The response to different aggressive actions (physical, chemical, and biological) from the environment is influenced by the transport of substances within this porous structure.  
Figure 6: Frattini diagram with values obtained from the studied samples.  
Among the different types of porosity, the type related to interconnected pores is particularly important, as it represents the maximum reversible water content. This porosity is directly associated with the transport of liquids and gases, the exchange of dissolved substances, and the tortuosity of the pathways [58].  
Depending on pore size, they can be classified as macropores, mesopores or capillary pores, and micropores. The first category corresponds to air bubbles naturally trapped (compaction pores) and/or those intentionally included (entrained air pores). The second category includes pores located outside the cement gel compounds; these pores have variable shapes and may or may not be interconnected and open to the exterior. The last category consists of pores within the hydrated and hardened paste, which generally do not exchange fluids with the surrounding environment.  
The pore types most directly related to durability and substance transport are macropores and capillary pores. A quantification of macropores in the samples yielded the results shown in Figure 7, highlighting the high number of small pores in the 20% substitution at the beginning of the material's hydration reaction. After one year, these pores had reduced to less than half. This is associated with the particle size of the ballast residue, which is smaller than that of OPC. Larger pores are most abundant in the 10% substitution after one year of reaction.  
Large-volume pores are scarce at all reaction times and residue substitution proportions. Medium-volume pores in OPC remain constant throughout the entire hydration reaction. Small-volume pores are the most numerous, increasing in number throughout the test for both substitutions (Figure 8).  
Figure 7: Quantification of macroporosity (as a percentage of pore number) grouped into three pore volume intervals (>0.09 mm\({}^{3}\), <0.09 mm\({}^{3}\) >0.001 mm\({}^{3}\), and <0.001 mm\({}^{3}\)).  
The total porosity values and the average pore size in mortars with residue substitution changed after one year compared to OPC, with a decrease in average pore size [13]. Regarding pore volume, the largest pores were the most abundant throughout the entire period considered. Intermediate pores were almost non-existent (Figure 9). Porosity is linked to the formation of C-S-H gels, whose morphology varies depending on the degree of hydration [59; 60; 61; 62].  
Figure 8: Quantification of macroporosity (as a percentage of pore volume) grouped into three pore volume intervals (>0.09 mm\({}^{3}\), <0.09 mm\({}^{3}\)>0.001 mm\({}^{3}\), and <0.001 mm\({}^{3}\)).  
Figure 9: Representation of volume defects in the studied samples.  
Small pores, which are predominant in mortars with a substitution, decrease in quantity over time, in contrast to larger-volume pores. SEM/EDX observations indicate that when the cement has no substitution, the contact between grains is formed by C-S-H gels and ettringite fibers, giving it a slightly porous appearance (Figure 10A,C). In contrast, when observing cement with a substitution, less-porous materials are generated (Figure 10B).