Effect of low ordinary Portland cement content on stiffness, microstructure and mineralogical composition of wood ash pastes

Wood ash (WA) is an abundant by-product of biomass combustion and represents a viable alternative supplementary cementitious material. However, its high physicochemical variability remains a major limitation to its effective utilization. This study investigates the influence of low ordinary Portland cement (PC) contents (0-20 wt%) on the stiffness, microstructural evolution and mineralogical composition of wood ash-based pastes. Four wood ashes of different origins were used to produce sixteen paste formulations. The modulus of elasticity was estimated from compressive strength using the tangent modulus approach, while microstructure and phase evolution were characterized using scanning electron microscopy (SEM), thermogravimetric analysis (TGA) and x-ray diffraction (XRD) at 7 and 28 days of curing. The results show that increasing PC content considerably improves stiffness, compressive strength and bulk density, with optimal performance achieved at 20 wt% PC. Microstructural observations revealed matrix densification associated with the formation of hydration products, including calcium silicate hydrates (C-S-H), ettringite and portlandite. Mineralogical analysis indicated that WFA 8 - and WFA 9 -based pastes exhibited a broader range of crystalline phases compared with WFA 3 - and WBA-based systems, including the formation of additional phases such as alunite. Overall, the findings highlight the critical role of ash variability in governing material behavior and demonstrate that limited PC additions (5-20 wt%) can effectively enhance the stiffness, microstructure and mineralogical characteristics of wood ash-based pastes. These results support the development of sustainable low-strength cementitious materials incorporating high volumes of wood ash.