A new supplementary cementitious material: Walnut shell ash

Partial replacement of supplementary cementitious materials (SCMs) for cement is an effective way to boost the long-lasting sustainability of cement-based construction. However, traditionally used SCMs are facing the problem of low availability. In response, a novel SCM – Walnut shell ash (WSA) – was identified. To validate the feasibility of using WSA as an SCM, frattini tests for pozzolanicity, setting time tests for workability, and compression and flexural strength tests for mechanical properties were conducted. The experimental results have demonstrated that cement paste incorporating no more than 20 % WSA possesses satisfactory pozzolanicity and setting times. Furthermore, this mix achieves an optimal long-term compressive strength of 83 MPa, which it maintains for up to a year. To reveal the hidden mechanisms behind the mechanical performance changes, microscopic and nanoscopic techniques, including scanning electron microscope and 29Si nuclear magnetic resonance, were conducted. Microscopically, the strength enhancement originates from the generation of fibrous C–S–H gel, high hydration level, and low porosity in cement paste with 20 % WSA. Nanoscopically, the changes in mechanical performance result from the shift of polymerization degree and the substitution of Al–Si in cement hydrates. These findings enrich the understanding of the role of WSA in manufacturing sustainable and durable cementitious materials that benefit both biomass waste management and cement industry decarbonization. In addition, the proposed multiscale assessment framework can be a tangible paradigm for screening new SCMs, thereby extending the horizon of sustainable materials research. © 2023 Elsevier Ltd