Recycled waste concrete and metakaolin based alkali-activated paste: Characterization, optimization, and life cycle assessment

This paper covers the result of physico-mechanical, microstructural, optimization, and environmental impacts of alkali -activated paste (AAP) produced from recycled waste concrete (RWC) powder and metakaolin (MK). The experimental findings revealed that RWC powder up to 40 % effectively reduced setting time. The specimens with Ms ratios of 1.25 and 2.00 had similar setting periods, while the mixes activated with 0.50 had greater setting time variance. The compressive strength significantly increased with an increase in RWC powder, and the mix formulated with 40 % RWC powder and Ms ratio of 2.00 showed maximum strength of 79.1 MPa at 28 days. However, further inclusion of RWC powder beyond 40 % constantly decreased the strength. A statistically significant model to predict compressive strength was obtained by response surface methodology study, and the obtained optimum mix design (RWC=36.33 % and Ms:1.89) was experimentally validated with an absolute error of 0.17 %. The microstructural analysis indicated that N -A -S -H type gels were the major reaction products, and the inclusion of RWC powder led to the generation of C -(A) -S -H type gels along with N -A -S -H type gels. Furthermore, an increase in the RWC powder ratio increased the crystallinity index values of AAP samples, as shown by X-ray diffraction results. Life cycle assessment (LCA) analysis was performed for all AAP mixes, including the optimum mix. LCA results indicated that the addition of RWC powder consistently reduced the CO2 emission, cumulative energy demand, and cost of the resulting AAP mixes; however, these parameters slightly increased with an increase in Ms ratio of alkaline solution.