Coupling effects of recycled aggregate and GGBS/metakaolin on physicochemical properties of geopolymer concrete

Reuse of construction and demolition (C&D) wastes is essential for the protection of environment and a sustainable way for recycled materials. However, the recycled aggregate concrete with Ordinary Portland cement (OPC) paste usually had worse physical properties than the normal concrete. In this study, GGBS/metakaolin based geopolymer matrix was used to fully replace the OPC paste in the recycled aggregate concretes. The effects of GGBS/metakaolin ratio and content of recycled aggregates on the slump, setting time, compressive strength, elastic modulus, toughness, Poisson's ratio and failure mode under compression were studied. The geopolymerization products were characterized through XRD test and the crack development at the micro-level was revealed through SEM. Moreover, the carbon emissions of geopolymeric recycled aggregate concretes were calculated and compared with those of OPC-based concretes. The results show that using GGBS/metakaolin based geopolymer to replace the OPC matrix in recycled concretes could provide a better workability, a quicker reaction rate, a higher compressive strength and a better crack resistance. The full replacement of geopolymer matrix with GGBS/MK ratio of 1:1 to the OPC paste in the concrete with 100% recycled aggregate could increase the slump value by 75% and the 28-day compressive strength by 35%, respectively. The proposed green mixtures exhibiting acceptable fresh and hardened mechanical performance are able to be used in appropriate construction projects and hence reduce the need for cement manufacture as well as disposal of C&D wastes. (C) 2019 Elsevier Ltd. All rights reserved.