Extensive rheological evaluation of geopolymer mortars incorporating maximum amounts of recycled concrete as precursors and aggregates

Abstract

Due to accelerated geopolymerization kinetics, the control and prediction of rheological behavior of geopolymers (GPs) is quite complicated compared to their Portland cement-based counterparts. A comprehensive understanding of the crucial design factors influencing the rheological characteristics of GPs can provide important insights into their adoption in large scale applications. This study aims to comprehensively investigate the rheological behavior of geopolymer mortars (GPM) incorporating maximum amounts of construction and demolitions wastes (CDW) as precursors and aggregates. The binder composition was designed based on recycled concrete (RC) as the main precursor, while investigating its combined use with recycled clay brick (RB) and recycled ceramic tile (RT) at mono, binary and ternary mixtures. However, metakaolin (MK) was also incorporated in all compositions at a 30% replacement rates with CDWs to provide the required targeted ratio of SiO2/Al2O3. The effect of including 100% recycled concrete aggregate (RA) as compared to silica sand (SS) and natural sand (NS) was assessed for all CDW-based mortars prepared with the same water to binder (W/B) and aggregates to binder (A/B) ratios. All synthesized GPMs exhibited pseudoplastic or shear-thinning behavior, where the maximum shear and yield stresses, apparent and plastic viscosities and thixotropy were obtained with the increased amount of RT compared to RB into RC-based binary and ternary binder compositions. Although the addition of 100% RA in GPMs caused enhanced yield stresses, plastic viscosity and rigidity compared to SS and NS-based geopolymer mortars, it also attained higher thixotropy, which could play an important role in enhanced buildability at the early stages of geopolymerization.