*3.7. Thermal Properties*

Table 7 shows the WCAP heat capacity and thermal conductivity test results. It is interesting to note that the heat capacity increases with the wood ash content. It can contribute to reducing the heat loss rate of building walls given its relatively low thermal conductivity when used as interior partition. Wood ash level P3 yields a heat capacity 7% higher than that of the control panel. Conversely, the thermal conductivity does not change importantly between 0% and 30% wood ash replacement levels.

**Table 7.** Average thermal properties and density of WCAP as a function of the WA content.


#### *3.8. Microstructure of Mortars*

According to the results shown in Figure 7, there are no clear differences in microstructure between the two samples. They both exhibit a low porosity and pore sizes smaller than 10 μm. The occurrence of spherical particles that have the shape of WA can be observed in Figure 7b as shown by the white arrows.

**Figure 7.** Scanning electron microscopy images of cement control (**a**) and cement + 30% WA (**b**).

#### **4. Discussion**

Although the investigated wood ash does not qualify as a pozzolan, it can be used in replacement of cement up to significant amounts without affecting the physical and mechanical properties of the wood-cement particleboards significantly. In previous studies, maximum wood ash proportions in the order of 15%–20% were reported [10,11]. Compared to the control sample (P0), WCAP prepared with 30% of wood ash in replacement of cement (P3) showed moderate mechanical properties reductions of 10% for bending MOR and 21% for screw-withdrawal resistance. The pH value increases with the hydration of the cement. A high alkaline solution promotes the reactivity of the silica present in the WA, which enhances the pozzolanic activity at the initial stage. Increased pH levels favor the formation of hydrous silica. This compound reacts with Ca2<sup>+</sup> ions and produces insoluble compounds, which are secondary cementitious products [10]. Moreover, WA can act as a filler in the mixtures.

The density of the samples is found to decrease as the WA replacement rate increases, due to the slightly lower density of the ash and, most importantly, the increased amount of water (Tables 4 and 7). As a result of the larger volume of capillary pores, the mechanical and physical properties including density decline. Indeed, water absorption increased significantly from 30% of WA in replacement. It can be explained by the lower amount of cement particles with increasing wood ash contents. Therefore, the hydration reaction was reduced, and the water evaporated quickly in a porous medium with high porosity due to the presence of the wood fibers.

A fraction of the ash of about 7% dissolves in water and contributes to the hydration process. The large surface area associated to the ash particles could also be a factor, as it acts to some degree as nucleation sites for cement hydration. Indeed, based upon SEM examination, no significant difference in the microstructure of a mixture of neat cement and a mixture containing 30% of WA in replacement was found, both exhibiting a dense and uniform microstructure.

The increase in the heat capacity of WCAP after replacement of cement by wood ash has shown that it has the potential to reduce the heat losses of building walls, given the improved insulation it provides. Indeed, WCAP has a low thermal conductivity, about three times lower than that of gypsum boards (0.32 W/m·K) [4]. This low thermal conductivity is mainly due to the higher WCAP porosity compared to that of gypsum because the thermal conductivity of empty voids is very low (about 0.025 W/m·K).

#### **5. Conclusions**

This project studied the physical, thermal, and mechanical properties of wood-cement particleboards incorporating wood ash. Wood ash was found to have an excellent potential for use as partial replacement to Portland cement. Based on the results generated in this study, the optimum replacement rate is about 30% by weight. At this replacement level, the engineering properties of WPCA were moderately reduced (bending MOR by 12%; bending MOE by 20%; screw-withdrawal resistance by 21%) compared to a neat wood-cement control sample. Beyond 30% in replacement, the mechanical and physical properties start to decrease at a significantly higher rate (bending MOR by 43%, bending MOE by 41%, and screw-withdrawal resistance by 60% at a 40% replacement rate). The use of wood ash improves the heat capacity of the WCAP by 11% compared to a neat wood-cement control sample.

The work reported herein is quite promising in view of producing eco-friendly wood cement panels with improved characteristics compared to those of standard gypsum boards. Future work should include the fire-resistance and acoustic properties measurement of this material. The formulation and the processing phases could also be further improved. Notably, the use of a paper surface layer should be studied to enhance the mechanical properties of the panel.

**Author Contributions:** Conceptualization, V.-A.V., A.C., B.B. and P.B.; Data curation, V.-A.V.; Formal analysis, V.-A.V.; Funding acquisition, P.B.; Investigation, V.-A.V., A.C., B.B., P.B. and J.D.; Methodology, V.-A.V., A.C., B.B., P.B. and J.D.; Project administration, A.C. and P.B.; Supervision, A.C. and B.B.; Validation, A.C., B.B., P.B. and J.D.; Writing—original draft, V.-A.V.; Writing—review & editing, A.C., B.B., P.B. and J.D.

**Funding:** This work is part of the research program of the Natural Sciences and Engineering Research Council of Canada (NSERC) Industrial Research Chair on Eco-Construction in Wood (CIRCERB) through programs IRC (IRCPJ 461745-12) and CRD (RDCPJ 445200-12).

**Acknowledgments:** The authors are also grateful to the industrial partners of the NSERC Industrial Chair on Eco-Responsible Wood Construction (CIRCERB) and the SSQ insurance company for providing wood ash from "La Cité Verte".

**Conflicts of Interest:** The authors declare no conflict of interest.
