Particle Size and Shape Analysis

Shape analysis by scanning electron microscopy observations revealed that the ash particles were irregular in shape and spherical (Figure 2b). Wood ash is suitable for use as a filler/partial replacement of cement in high-performance concrete due to an enhanced "ball bearing" effect given from the spherical shape of WA. The "ball bearing" effect of wood ash creates a lubricating effect when concrete is in its plastic state. According to the results shown in Figure 3, the D10, D50, and D90 values of the WA were 2.5, 18.5, and 114.1 μm, respectively. Wood ash contains an amount of ultrafine particles of 18% (particle diameter φ < 5 μm).

**Figure 2.** Low magnification (**a**) and high magnification (**b**) scanning electron microscopy of wood ash (WA).

**Figure 3.** Particle size analysis of WA.

#### Chemical Composition

The results of the chemical analysis carried out on the investigated wood ash are shown in Table 2. The combined content in iron oxide (Fe2O3 = 1.22%), aluminum oxide (Al2O3 = 2.25%), and silicium dioxide (SiO2 = 7.80%) is found to be 11.27%, which is considerably less than the minimum amount required to qualify a material as a pozzolan, established at 70% [16].


**Table 2.** Physical and chemical properties of wood ash.

The recorded loss on ignition at 950 ◦C was 14.2%, which exceeds the 12% maximum requirement for pozzolans [16]. This means that the ash contains a significant amount of unburnt carbon, which reduces its pozzolanic activity. The alkali content (%Na2O + 0.658 × %K2O) was found to be 7.18%, a value higher than the maximum alkali content of 1.5% required for pozzolana. The specific gravity of wood ash was found to be 2.97, which is far less than the Portland cement density (3.15). WA contains more than 99% (by weight) of inorganic material and yields a pore solution with a high pH.

### Solubility Test

Table 3 shows the percentage of the wood ash dissolved in water during the solubility test. The solubility of WA is estimated to be 7% including lime and alkali hydroxides that are readily soluble in water in laboratory conditions. This soluble component plays an important role in the hydration reaction.



#### *3.2. Change in Density*

The weight of all panels was recorded at the beginning and at the end of the curing period (3 days in the mold) to determine changes in the panel-specific gravity. It decreased by about 5% during that period, owing to the fact that the mold being used was not perfectly impervious. Some water was probably absorbed by the mold itself, as it was made of plywood.

The panel mass reached a plateau about 6 days after removal from the mold, meaning that most of the free water in the cement paste had evaporated in the conditioning chamber at 23 ◦C and 60% RH by then.

#### *3.3. Workability*

Table 4 shows the results obtained for the consistency test. The results reveal that the water demand increased with the wood ash content. The wood ash introduced into the cement increased the carbon content, thereby increasing the amount of water required to achieve satisfactory workability.


**Table 4.** Consistency test results.

#### *3.4. Bending Properties of the Raw Wood-Cement Particleboard*

As described previously, the panels were tested in bending at 3, 7, and 28 days after manufacturing. Each test was performed on three specimens and the mean value is presented in Table 5.

**Table 5.** Average bending strength test results of wood-cement-ash particleboards (WCAP). Mean values with the same superscript are not significantly different for *p* = 0.05; standard deviation is given in parentheses.


Table 5 and Figure 4 show the bending behavior of the WCAP at different curing times. It shows that the bending strength and stiffness values of the sample panels increase with the curing time. They changed little after 7 days of curing, as generally observed for Portland cement-based materials. The statistical analysis results showed that there is a significant difference among samples in terms of bending strength and stiffness at all stages of curing (3 days of curing: *p* < 0.001, 7 days of curing: *p* < 0.001, 28 days of curing: *p* < 0.05). The bending strength and stiffness of P4 and P5 were significantly lower than for the other panels at all curing stages. Optimum bending strength observed in these tests was obtained at 30% wood ash replacement (P3) after 28 days of moist curing.

**Figure 4.** Evolution of the modulus of rupture in bending of WCAP as a function of the moist curing duration.

#### *3.5. Screw-Withdrawal Resistance*

Figure 5 shows the screw-withdrawal resistance of WCAP as a function of the WA content. It shows that the screw-withdrawal resistance decreases as the WA replacement rate increases. The results of the statistical analysis show that the screw withdrawal-resistance is slightly affected up to a replacement rate of 30% in wood ash. However, beyond that value, it decreases rapidly.

**Figure 5.** Effect of wood ash replacement rate on the screw-withdrawal resistance of the WCAP (mean values with the same superscript are not significantly different for *p* = 0.05; standard deviation is given in parentheses).

#### *3.6. Water Absorption*

The water absorption test results are shown in Figure 6. The value of water absorption increases with the percentage of WA replacement and time of immersion in water. Table 6 shows that the thickness swelling of WCAP in water is small (<2%). According to the results, the water absorption of all boards incorporating wood ash is higher than that of the control sample after 28 days of curing.

Water absorption after 2 h Water absorption after 24 h

**Figure 6.** Water absorption and thickness swelling of WCAP recorded as a function of the WA content.


**Table 6.** Average water absorption and swelling of WCAP as a function of the WA content.
