Green cementitious materials: Utilization of soybean hull ash as a supplementary cementitious material

Cement, the main raw material for concrete production, is the second most widely used material by humanity. With an estimated production of between 3.7 and 4.4 billion tons by 2050, its manufacture represents one of the largest sources of CO₂ emissions. To reduce environmental impacts, the partial replacement of clinker with supplementary cementitious materials (SCM) has been the most efficient alternative adopted by industry. This study evaluated the potential of soybean hull ash, an agro-industrial waste, as a pozzolanic SCM. Characteristics such as chemical composition, particle size distribution, and mortar performance in both fresh and hardened states were analyzed. Mineralogical analysis indicated that the ash contains high levels of silica (SiO₂), alkaline oxides, and magnesium, components that favor pozzolanic reactions. This potential was further demonstrated in the X-ray diffraction test, which indicated a significant proportion of amorphous silica. In the fresh state, increasing the ash content reduced the mortar's workability due to its higher specific surface area (BET: 1.66 m²/g for ash compared to 1.28 m²/g for cement) and fines content. Isothermal calorimetry analysis was also performed, which demonstrated greater heat release with a 5% replacement, suggesting accelerated initial hydration due to the K₂O content, while higher levels exhibited a dilution effect. In the hardened state, replacing 5% of the cement with ash resulted in improved density, reduced water absorption, and increased compressive strength, including after preliminary chloride exposure. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) results confirm this finding. The results demonstrate that soybean hull ash is a technically viable alternative for partially replacing Portland cement.