Development of normal and very high strength geopolymer binders based on concrete waste at ambient environment

The objective of this study was to optimize the composition of concrete waste (CW)-based geopolymer binders at ambient environment. The goal was to evaluate the reliability of a new algorithmic mix design method that relies on pre-targeting ratios of SiO₂/Al₂O₃, Na₂O/SiO₂ and liquid/solid of CW precursor powder and sodium silicate and sodium hydroxide alkaline reagents. The effects of elevated temperature curing and the addition of supplementary cementitious materials (SCMs) were also considered. The optimized CW-mixes at ambient curing were subjected to an initial 24 h curing at 50, 75, and 100 °C, and Class C fly ash (FA-C), Class F fly ash (FA-F), metakaolin (MK) and ground granulated blast furnace slag (GGBS) replaced CW at wt.% of 15%, 30% and 45%. The relationship between SiO₂/Al₂O₃ and Na₂O/SiO₂ molar ratios and the fresh properties, including flowability, initial and final setting times, and mechanical strengths were assessed. Furthermore, an extensive microstructural study using X-ray diffraction (XRD), scanning electron microscopy (SEM) coupled with energy-dispersive X-ray (EDS) spectroscopy and Fourier transformed infrared (FTIR) spectroscopy was performed for various mix compositions and curing conditions. The results proved that fresh and mechanical properties are highly dependent on the targeted chemical ratios. Enhanced mechanical strengths of CW-compositions were achieved at adjusted ratios of SiO₂/Al₂O₃ = 12.9 and Na₂O/SiO₂ = 0.23, which were significantly increased by incorporating SCMs with high calcium content. Microstructural assessments indicate that the degree of geopolymerization of CW-based geopolymers and related C(N)-A-S-H formations can be optimized at ambient environment.