Effects of pretreated recycled fine aggregates on the mechanical properties and microstructure of alkali-activated mortar

Compared with traditional cement-based binders, alkali-activated binders can be improved upon combination with recycled aggregates to produce a promising and new green construction material. However, during this process, a pretreatment method is needed to alleviate the negative impact of recycled aggregates on the properties of alkali-activated materials. In this study, fly ash (FA), ground granulated blast furnace slag (GGBFS) and recycled powder (RP) were used as precursors, and recycled fine aggregates (RFAs) were used to replace natural fine aggregates (NFAs) to prepare recycled alkali-activated mortar (RAAM). Various RFA replacement ratios (0, 25%, 50% and 100%), pretreatment methods (calcination temperature and carbonation time) and alkaline moduli (0.8, 0.95, 1.1 and 1.25) were investigated. The effects of these parameters on the fresh and hardened properties, chemical composition and microstructure of RAAM were investigated. The results showed that the mechanical properties of RAAM gradually decreased with an increasing RFA replacement ratio due to the poor physical properties of RFA and its poor matris binding. Two pretreatment methods (optimal calcination at 400 degrees C and carbonation at 6 h) were effective in improving the properties of RFA and thus the mechanical properties of RAAM. Calcination removed the adhered mortar from RFA and thus improved the mechanical properties of RAAM. Carbonation of RFA enhanced the quality of the attached mortar, which strengthened the RFA and provided better mechanical properties. In addition, microstructural analyses showed that RAAM prepared from pretreated RFA had a dense microstructure and a low critical pore size.