Developing fully recycled alkali-activated mortar made with waste concrete fines as a substitute for both binder and sand: Multi-properties evaluation

Using waste concrete fines (WCF) for preparing sustainable alkali-activated mortar (AAM) is an effective pathway for recycling concrete waste. This study aims to develop fully recycled AAM by substituting GGBS-FA binders and natural sand with WCF in GGBS-FA-based AAM. This approach can decrease the demand for traditional binders and natural sand, while maximizing the reutilization of WCF in AAM. WCF exhibited certain alkali-activation activity and a favorable filling effect in AAM. Replacing binders with WCF was detrimental to the geopolymerization and cementitious products, resulting in the microstructure deterioration of AAM paste. Conversely, substituting natural sand with WCF reduced the actual water-to-binder ratio, which refined the microstructure of AAM paste. The drying shrinkage of AAM increased with the increasing WCF substitution rate for binder, while it initially decreased and then increased with the replacement of natural sand by WCF. The substitution of binders with WCF exerted an adverse impact on the mechanical strength and water transport resistance of AAM, whereas the mechanical strength and water transport resistance were improved as the proportion of natural sand replaced by WCF increased. By optimizing the WCF substitution ratio for both binders and natural sand, fully recycled WCF-AAM with varying strengths and water transport resistances could be produced. Notably, fully recycled AAM prepared by substituting 100 % of both binders and natural sand with WCF exhibited satisfactory mechanical strength. The 28-day compressive strength of AAM-Control, WCF-100FG, WCF-100NS, 50FG+ 50NS, and 100FG+ 100NS were 69.3, 28.7, 85.2, 63.0, and 37.3 MPa, respectively.