Evaluating the Elastic Modulus of Concrete Modified with Coconut Shell Ash

Abstract

This study investigates the potential of coconut shell ash (CSA), an agricultural waste product, as a supplementary cementitious material (SCM) for sustainable concrete production. CSA, obtained from calcined coconut shells and rich in reactive silica, alumina, and iron oxides, was used to replace ordinary Portland cement (OPC) at 0%, 10%, 20%, and 30% by weight. A total of 224 concrete cubes were prepared using a 1:2:4 mix ratio and a constant water-to-cement ratio of 0.5, and were tested over 14 curing periods ranging from 3 to 91 days. Workability was measured using slump tests, while the elastic modulus was estimated based on compressive strength using the ACI 318-14 formula. Findings revealed that increasing CSA content led to reduced slump, indicating lower workability due to the ash’s porous and high-surface-area characteristics. However, the mix containing 10% CSA demonstrated an enhanced elastic modulus, peaking at 19.34 GPa on day 28, surpassing the control. This improvement is attributed to CSA’s pozzolanic activity, which promotes additional calcium silicate hydrate (C-S-H) formation, thereby densifying the concrete matrix. Higher replacement levels (20–30%) resulted in performance decline due to cement dilution. The study concludes that 10% CSA substitution offers an optimal balance between mechanical performance and sustainability in concrete.