**6. Microstructural Study**

Garcia et al. [71] studied the microstructure properties with the help of a scan electron microscope (SEM) of ground waste wooden ash, obtained from forest regions surrounding a power plant in Portugal. From the SEM images, it was observed that ground waste wooden ash has two governing properties of particles and fibers in layers. The SEM micrographs of the ground waste wooden ash at higher magnification and electron dispersive X-ray (EDX) spectra are shown in Figure 5.

**Figure 5.** SEM micrograph and EDX analysis of waste wood ash (used with permission from Elsevier [71]).

Awolusi et al. [87] investigated the microstructure characteristics of OPC mortar with waste wood ash from sawdust. The mortars were developed with WWA ranging from 0 to 10% with a water to binder ratio of 0.6. SEM micrograph displayed maximum inter-spatial distance between the particles of WWA in comparison to the binder, which was dense with each other [87]. The SEM outcomes of a composite made from corn cob–polypropylene– WWA displayed that the pores present in matrix become smaller as the proportion of WWA was raised. This can be attributed to the pores between the corn cob and polypropylene being filled by the waste wooden ash. Due to the increased dose of waste wooden ash in the mix, the stress concentration stretched, and the shape of WWA was revealed to be considerably small; thus, the distances became very less. The bridging behavior of waste wooden ash could lead to a maximum wrap among corn cobs [88].

It can be observed from the SEM image shown in Figure 6 that waste wood ash with silica fume enhanced the matrix when the ash was dispersed uniformly. Figure 6 shows the study of pores enclosed in the reference sample and sample with different proportions of waste wood ash and silica fume. The doses of waste wood ash and silica fume added to the mix considerably impacted the pores shaped in concrete mortars. The proof of this effect is displayed in the formation of larger pores in the reference specimen. Concrete mixes utilized different proportions of silica fume and waste wood ash by substituting the binder with 15% reduced estimate of pores. This positive test outcome was due to the rich silica in the silica fumes [76].

Chowdhury et al. [80] studied the X-ray diffraction of waste wood ash concrete. Figure 7 displays the X-ray diffraction spectra of waste wood ash concrete. The hump specifies that the specimen was formless, with the peaks of silica demonstrating a crystalline behavior. Thus, waste wood ash comprises silica in crystalline and formless shapes.

Crystals of silica present high peaks at 29 degrees at 2-theta. Formless silica concentrates in the mixture as a suitable binder substitution material due to its pozzolanic behavior [80]. Similar observations were also noted by Elahi et al. [89] from the X-ray diffraction spectra of waste wood ash, which displayed the existence of formless silica, although only in low proportions. A study of the waste wood ash concrete microstructure showed that the inclusion of waste wood ash impacted the pore estimations, and a significant reduction in porosity occurred [90]. The succeeding samples showed a microstructure that was very dense with a lower permeability [90].

**Figure 6.** Pores in the reference concrete and reference sample with different proportions of WWA and SF (used from an open source journal of MDPI [91]).

**Figure 7.** XRD analysis of waste wood ash (used with the permission from Elsevier [71]).

WWA consists of particles sizes that vary between 10 μm and 200 μm, as is observed in Figure 8. The shape and size of particles vary for WWA with small particles adhering to the surface, as is observed. WWA consists of particles with an irregular shape due to the inorganic particles present in it (Etiegni and Campbell, 1991) [62].

**Figure 8.** SEM image of WWA (Grau et al., 2015) (Used from an open source journal of MDPI [61]).
