Engineering properties and sustainability of alkali-activated fly ash-slag mortar containing carbon-negative aggregates

In recent years, an increasing number of attempts have been made on the utilisation of carbon-negative aggregates (CNA) produced from calcium/magnesium-rich solid wastes through carbon mineralisation for lowcarbon concrete production. This paper presents an experimental study on the effect of CNA as an alternative to river sand on the engineering properties of alkali-activated fly ash-slag (AAFS) mortar, including flowability, setting time, density, ultrasonic pulse velocity, drying shrinkage, compressive strength and splitting tensile strength. The studied influencing factors include CNA replacement level for river sand (0 %, 25 %, 50 %, 75 % and 100 %) and moisture condition of CNA (air-dry (AD) and saturated-surface-dry (SSD)). The experimental results indicate that replacing sand with CNA in AAFS mortar enhances the flowability and extends setting time when CNA at SSD, but both properties decline when CNA at AD. As the CNA content increases, the density and drying shrinkage of AAFS mortar drop, regardless of moisture conditions. For AAFS mortar with AD CNA, the 28d compressive strength can reach 42.51 MPa at 50 % CNA replacement but reduces with the higher replacement. AAFS mortar with SSD CNA exhibits a continuous rise in 28-d compressive strength, peaking at 39.79 MPa. The maximum splitting tensile strength for AAFS mortar with AD CNA occurs at 50 % CNA replacement, while the splitting tensile strength of AAFS mortar with SSD CNA drops continuously with the increasing content of CNA. The sustainability assessment indicates that the incorporation of CNA into AAFS mortar can reduce the carbon footprint and embodied energy by 63.89 % and 18.25 %, respectively.