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Researchers are investigating eco-friendly binders like coconut shell ash (CSA), metakaolin (MK), and calcined clay (CC) as supplementary cementitious materials (SCM) in high-strength concrete (HSC). Abundantly available as industrial or agricultural waste, these materials, when combined with Portland cement (PC), offer synergistic benefits. This not only improves concrete performance but also addresses waste disposal issues, presenting a sustainable and environmentally friendly solution for long-term use in HSC production. However, this study performed on fresh and mechanical characteristics of HSC blended with CSA, MK, and CCA alone and together as SCM after 28 days of curing. A total of 504 samples of standard concrete were cast and the cubical samples were tested to achieve the targeted compressive strength about 80 MPa after 28 days. The experimental results indicated that the rise in tensile, flexural and compressive strengths of 9.62%, 8.27%, and 10.71% at 9% of CSA, MK, and CC as SCM after 28 days of curing. As SCM content increases, the density, porosity and water absorption of concrete decrease. Moreover, the workability of fresh concrete is getting reduced when the concentration of SCMs increases in HSC. In addition, the concrete's sustainability assessment revealed that employing 18% MK, CC, and CSA as SCM reduced carbon emissions by approximately 11.78%. It is suggested that using 9% CC, MK and CSA together in HSC yields the best results for practical applications in civil engineering.

This experimental work explored the synergic effect of millet husk ash (MHA) and wheat straw ash (WSA) on the fresh and hardened properties of metakaolin-based self-compacting geopolymer concrete. An equal amount of MHA and WSA contents at 2.5%, 5%, 7.5% and 10% were utilized as a partial replacement of metakaolin in the mixes. The filling ability (slump flow, V-Funnel, and T50 flow) and passing ability (J-Ring and L-box) of fresh SCGC mixture were determined. The study further determined the density, compressive, split tensile and flexural strength and permeability of hardened SCGC mixes. The results showed that both MHA and WSA addition somewhat reduced in workability of fresh SCGC mixes. However, the hardened properties were significantly enhanced, a development that was more pronounced in the mix having 5% each of MHA and WSA with compressive strength, split tensile strength, and flexural strength of 53.80 MPa, 4.82 MPa, and 5.15 MPa, respectively. The water penetration depth and density of SCGC mixes lessen as the MHA and WSA contents were increased beyond 10%. Overall, the study recommends a combined use of 5% each of MHA and WSA for metakaolin-based geopolymer SCC production to ensure optimal performance.