*4.1. Physical Properties*

As per the report from Etiegni and Campbell et al. [62], 80% of WWA consists of particles size less than 1.0 mm and the balance is unburned wood particles with different sizes. Specifically, WWA consists of 25.4% of fine particles, which are less than 75 μm. Wood ash shows a relatively high specific surface area that provides good absorption. The specific gravity of WWA is 2.41, pH 12.57, average particle diameter d50 (mm) is 0.223, bulk density (kg/m3) is 663–997, and specific surface area (m2/kg) is 4200–100,600 (Grau et al., 2015) [61]. An increase in the particle size increases the concentrations of aluminum, arsenic, barium, and copper, and decreases the concentrations of boron, cadmium, manganese, lead, zinc, potassium, magnesium, and calcium [63].
