Ultra-fine slag activated by sodium carbonate at ambient temperature

Abstract Na2CO3 (NC)-activated slag was one of the main directions for low carbon cementitious materials. However, its application in the civil engineering field was restricted because of the slow compressive strength development at ambient temperature. To solve this problem, slag with high fineness was used in the present study. The compressive strength and hydration process of NC-activated slag with various fineness slag was systematically researched, and the related mechanisms was studied by hydration heat, X-ray diffractometer (XRD), Thermogravimetric (TG), Nuclear magnetic resonance (NMR), and Mercury intrusion porosimetry (MIP). Results showed that the compressive strength of slag with D50 = 3.1 μm activated by 4% sodium oxide equivalent (Na2O-E) of NC reached 46.3 MPa and 91.6 MPa at 28 d and 90 d age. Decreasing slag D50 (i.e. increasing fineness) augmented the compressive strength obviously, and more obvious enhancing effect was observed over time. On the one hand, the more reaction sites and the higher reactivity offered by slag with lower D50 could accelerate the activation process significantly, leading to a higher reaction degree of slag; on the other hand, the greater filling effect of slag with lower D50 was more beneficial to refine the microstructure. The optimization of slag fineness was an efficient method to augment the mechanical property of NC-activated slag system at ambient temperature.