Effects of Coconut Shell Ash and Coir Fiber on the Mechanical Properties and Microstructure of Concrete

The utilization of cement is one of the primary sources of carbon emissions in concrete, driving the search for sustainable alternative materials. Although extensive research has been conducted on the use of agricultural waste as supplementary cementitious materials (SCMs), the effects of coconut shell ash (CSA) and coir fiber (CF) on concrete properties have not been extensively investigated. This study systematically investigates the influence of CSA as a SCM (0–20%) and CF as a reinforcement material (0–0.32%) on the workability, density, compressive strength, flexural strength, splitting tensile strength, and failure modes of concrete, complemented by microstructural mechanism analysis. The cement and CSA were characterized using XRF, XRD, and SEM. The results indicate that the incorporation of both CSA and CF reduces the workability and density of concrete. For concrete with CSA only, the compressive strength decreases by up to 24.7% when the replacement level reaches 20%. However, concrete with 10% CSA still maintains 87.2% of the strength of ordinary concrete, which satisfies the C40 requirement. In contrast, CF incorporation alone improves the mechanical properties, with compressive strength, flexural strength, and splitting tensile strength reaching peak increases of 6.4%, 13.9%, and 7.5%, respectively, when the CF content is 0.24%. Incorporating 0.16% CF into 10% CSA concrete mitigates the strength reduction caused by CSA, achieving compressive, flexural, and splitting tensile strengths of 47.99 MPa, 5.63 MPa, and 3.99 MPa, respectively (95.7%, 98.3%, and 96.4% of the strengths of ordinary concrete). Microstructural analysis reveals that CSA deteriorates the interfacial transition zone (ITZ), while CF compensates for partial strength loss through the bridging effect, although its reinforcement efficiency is influenced by fiber dispersion and ITZ quality. This study provides a theoretical foundation and technical reference for the utilization of coconut shell waste in sustainable concrete.