Mechanical Performance and Embodied Carbon of M40 Sustainable Concrete Containing Recycled Coarse Aggregate, Carbon Nanotubes, Fly Ash, Silica Fume and Geopolymer Binder

Author

Nitish Kumar, Dr. Rachana Bajaj


Abstract

 Recycled coarse aggregate (RCA) lowers the environmental footprint of concrete but weakens it, and few studies show how this loss can be offset by nanomaterials and supplementary binders within one mix series. This paper reports an experimental study of thirteen M40-grade mixes designed to IS 10262:2019, in which RCA replacement (0–60%), multi-walled carbon nanotube (MWCNT) dosage (0–0.15% by weight of cement), fly ash (FA), silica fume (SF) and a fly ash–GGBS geopolymer binder were varied. Workability, compressive strength (3–28 days), split tensile strength, flexural strength and modulus of elasticity were measured, and embodied CO2 and material cost were estimated for ten mixes. Without CNTs, 28-day strength fell from 44.2 to 31.4 MPa as RCA rose from 0 to 60%. At 40% RCA, 0.10% CNT raised strength from 36.1 to 46.3 MPa (+28.2%), restoring the level of the control, although the difference from the control was not statistically significant (Tukey HSD = 3.7 MPa); 0.15% CNT gave lower strength and workability, consistent with agglomeration. Adding 10% silica fume gave 51.2 MPa, and ambient-cured geopolymer mixes reached 50.4 and 53.8 MPa. Embodied CO2 was 13% lower for 40% RCA and 73–77% lower for the geopolymer mixes, while the CNT-containing mixes cost 9–26% more than the control. The results indicate that RCA–CNT–SCM combinations can meet M40 strength, but a CNT-free geopolymer reference, durability data and larger specimen numbers are needed before design recommendations can be made.


Keywords

recycled coarse aggregate; carbon nanotubes; fly ash; silica fume; geopolymer concrete; compressive strength; embodied carbon



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References


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