Burned Wood Ash and Waste Marble Powder as Partial Cement Replacements in Concrete: Engineering Performance, Microstructure, and Environmental Assessment

The increasing environmental burden associated with Portland cement production has encouraged the use of waste-derived materials as partial cement replacements; however, their environmental benefits must be considered alongside changes in concrete performance. This study comparatively investigates burned wood ash (WA) and waste marble powder (WMP) as independent partial replacements for Portland cement (PC) at 20%, 40%, and 60% by mass. The chemical composition of WA indicates the presence of silica-bearing constituents that may contribute to cementitious or pozzolanic reactions; however, its pozzolanic activity was not independently quantified in this study. WMP was considered primarily as a calcium-rich mineral powder whose contribution may include physical filler and particle-packing effects rather than being assumed to possess independently verified cementitious activity. A 100% PC mixture was used as the control. Concrete performance was evaluated through slump, density, compressive strength at 7, 14, and 28 days, splitting tensile strength at 28 days, and water absorption. Microstructural characteristics were examined using scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDX), while embodied energy and carbon emissions were estimated comparatively to assess the environmental implications of cement replacement. The control achieved the highest 28-day compressive strength of 31.6 MPa. At 20% replacement, WMP and WA mixtures achieved 21.1 and 19.5 MPa, respectively, with further strength reductions at higher replacement levels. Workability decreased with increasing replacement, although WMP mixtures retained greater slump than corresponding WA mixtures. Cube water absorption generally increased at higher replacement levels, whereas cylindrical specimens exhibited greater variability. SEM-EDX revealed differences in matrix morphology and local elemental composition associated with replacement material and proportion. Increasing cement replacement reduced the calculated embodied energy and carbon emissions, but these environmental improvements were accompanied by substantial engineering-performance penalties at higher replacement levels. Among the replacement mixtures investigated, 20% WMP provided the most favourable balance between engineering performance and reduced Portland cement consumption. The findings demonstrate that the replacement level should be selected by considering engineering performance together with environmental benefit rather than by maximizing cement replacement alone.