Utilization of the thermo-mechanical and chemical activation of waste concrete powder for sustainable construction materials

Abstract

This thesis studies the environmental and mechanical implications of thermo-mechanically treated waste concrete powder (TMWCP) to develop a sustainable clinker-free binder. The study involves an alkali activation TMWCP with metakaolin, and lime activated TMWCP with incorporation of silica fume. In the first case, a novel approach was explored to utilize waste concrete powder (WCP) in conjunction with metakaolin as a precursor in the production of alkali-activated binders for sustainable consumption of construction and demolition waste. A Chapelle test confirms the presence of reactive silica in TMWCP. Different alkali-activated mixtures with metakaolin replacement ranging from 0% to 80% were prepared. The mixture with 40% activated WCP, with a sodium silicate to sodium hydroxide ratio of 2, achieved better compressive strength than the reference sample without WCP. Mineralogical analysis of the mixture pastes revealed that activated WCP-based mixtures developed geopolymer gel and C- S-H gel, contributing to better strength properties in the case of the mixture with 40% activated WCP. Life cycle analysis demonstrated that incorporating 40% thermo-mechanically activated WCP by replacing metakaolin reduces carbon dioxide emissions by 49.5% and 2.2% compared to Portland cement and metakaolin-based binder, respectively. Moreover, the effect of silica fume on the mechanical and hydration properties of the lime-activated TMWCP was investigated. The novel activation method involves thermo-mechanical treatment of WCP, followed by lime and calcium formate chemical activation. The results demonstrated that incorporating silica fume enhanced the pozzolanic reactivity, which led to the formation of C-S-H, pore refinement, and a dramatic increase in compressive strength. For instance, the incorporation of 20% and 25% silica fume exhibited compressive strength of 46 MPa and 43 MPa after 28 days of curing. The developed clinker-free binder with the addition of 20% silica fume demonstrated a reduction in mineral resource consumption of 99.3% and CO2 emissions of 13% and 69.3% compared to the sample without silica fume and ordinary Portland cement, respectively. This study offers a promising avenue for the widespread utilization of WCP as an environmentally friendly clinker-free binder.