Influence of different monomer ratios and recycled concrete aggregate on mechanical properties and durability of geopolymer concretes

•Effect of sodium silicate to sodium hydroxide ratio and RCA were determined.•The higher SS/SH ratio, the lower the chloride ion permeability and absorption.•Deboning width at ITZ between RCA-binder decreased with the increase of SS/SH.•Polymer products in the proximity of adequate monomer ratios became uniform. Properties of geopolymer concrete using metakaolin (MK) as the aluminosilicate source and recycled concrete aggregate (RCA) as partial replacement of natural aggregate are presented in this paper. The effects of sodium silicate (SS) to sodium hydroxide (SH) ratios, and RCA in different percentages on the mechanical and durability properties of geopolymer concrete were determined. Microstructural changes of geopolymers as a result of using RCA were evaluated via scanning electron microscopic (SEM) images. Test results showed that compressive strength of geopolymer concrete improved with increasing the SS/SH ratio. Although the use of RCA reduced compressive strength by up to 28%, the strength was still high enough for structural applications. In addition, increasing the SS/SH ratio reduced the chloride ion permeability and absorption of geopolymer concrete. The morphology results showed that the deboning width at the interfacial transition zone (ITZ) between RCA and binder decreased with the increase of SS/SH ratio. The polymer products in the proximity of adequate monomer ratios became more uniform and homogenous. Results also showed that the binder with a SS/SH ratio of 3 exhibited higher density and less porosity than that with smaller ratios. Therefore, the recycled construction and demolition waste can significantly contribute to the sustainability of construction industry from technical, economic and environmental points of view.