Mechanical Properties of Fly Ash Geopolymer with Macadamia Nutshell Aggregates

Macadamia nuts find widespread use in cuisine, cosmetics, and healthcare products, leaving behind a substantial amount of nutshell residue posthulling. Capitalizing on the remarkable compressive strength inherent in macadamia nutshell (MS) compared to other plant shells, this study investigates the utilization of MS as a partial aggregate replacement in fly ash-based geopolymer composites for sustainable construction applications, highlighting the need to optimize the content for balanced mechanical properties. The effects of MS content (0%, 20%, 25%, and 30%) on the mechanical properties (compressive and flexural strength), density, water absorption, and thermal resistance of the composites were evaluated. Results indicated that the compressive and flexural strength of plain geopolymer increased with rising NaOH concentrations. However, an increase in MS content led to a decline in the flexural strength of the composites due to the brittle nature of the aggregate. Conversely, the compressive strength of 20% MS mix displayed a similar trend to plain geopolymer, while 25% and 30% MS mixes exhibited an inferior trend. Scanning electron microscopy revealed that the adhesion between geopolymer and MS resulted from a physical interlocking mechanism. While the compressive strength of plain geopolymer improved after heat treatment at 400°C, a noticeable deterioration was observed in the composites with MS aggregate. Notably, 20% MS and 25% MS mixes (at 4M and 6M) demonstrated suitable compressive strength (16.54-22.50 MPa) and density (1.20-1.26 g/cm3 ) to serve as load-bearing components in accordance with ASTM C90 standard.