Influence of Wood Ash on the Mechanical Properties and Durability of Cement Mortars

This study investigates the influence of wood fly ash (WFA) and wood bottom ash (WBA) as a partial replacement of Portland cement (PC) on the mechanical performance and durability of cement mortars. Mortar mixtures containing 20% WFA (FA-20) and 20% WBA (BA-20) were compared with a reference mixture (REF) using bending and compressive strength tests, ultrasonic pulse velocity (UPV), total water absorption (TWA), and durability tests such as alkali–silica reaction (ASR) and carbonation resistance. The results showed that BA-20 exhibited higher mechanical performance and a denser microstructure than FA-20, as confirmed by UPV and TWA. At 365 days, compressive strength reached 71.9 MPa for REF, 64.3 MPa for BA-20, and 43.7 MPa for FA-20. Durability results indicated that after 365 days, REF exhibited the highest ASR expansion (~0.50%), whereas the incorporation of wood ash reduced expansion to approximately 0.41% for FA-20 and 0.29% for BA-20. In terms of carbonation resistance, FA-20 showed the greatest accelerated carbonation depth (10–14 mm), while REF exhibited the lowest carbonation depth (~3 mm). The differences were attributed to PC dilution and microstructural variations affecting porosity and transport properties. WBA demonstrated better performance than WFA, highlighting the importance of wood ash particle characteristics in PC replacement applications.