Effects of pretreated recycled powder substitution on mechanical properties and microstructures of alkali-activated cement

In this study, recycled powder (RP) was used to replace alkali-activated cement (AAC)’s precursors [i.e., fly ash (FA) and ground granulated blast furnace slag (GGBFS)] to produce recycled AAC (RAAC). Two pretreatment methods [i.e., calcination or carbonation] were used to improve the reactivity of RP. This study investigated the effects of the replacement ratio [i.e., the ratio of RP to FA/GGBFS], pretreatment methods, and alkali modulus on the fresh and hardened properties, phase assembly, and microstructure of RAAC. The mechanical properties of RAAC gradually decreased with increasing replacement ratio, as alkaline solution reacted with the RP less than with FA/GGBFS. Meanwhile, both calcination and carbonation pretreatment improved the properties of RP and thus enhanced the mechanical properties of RAAC. Compared with carbonation, calcination of RP at 600 °C exhibited better mechanical properties and lower critical pore size, because calcination at this temperature improved the reactivity of RP by decomposing calcite and clay minerals, and the filler effect of prepared RP enhanced the pore size distribution of the RAAC paste. Moreover, increasing the RP carbonation time increased the solid [i.e., calcite] content of the RP and moderately improved the strength properties of RAAC although it increased the critical pore size of RAAC.