This paper investigates the possibility of accelerating the slowly progressive early reaction process of sodium carbonate-activated slag mixtures by adding a small amount of sodium hydroxide or sodium silicate to the sodium carbonate activator. The reaction process of sodium carbonate-activated slag mixtures was evaluated by isothermal calorimetry and ultrasonic wave technology. Test results showed that adding a small amount of sodium hydroxide or sodium silicate to the sodium carbonate activator significantly accelerated the reaction process, resulting in shortened setting times and enhanced mechanical properties. Only 1% Na2O contribution from sodium hydroxide addition can lead to a 2-d compressive strength of 20 MPa for sodium carbonate-activated slag mixtures. In addition, the 28 days compressive strength was significantly higher than the sole sodium carbonate activator. The addition of sodium hydroxide or sodium silicate also considerably influenced the rheological behavior of the sodium carbonate-activated slag mixture, enhancing the structural build-up rate and transforming the shear-thinning behavior to Bingham or shear-thickening behavior. A more well-packed microstructure was observed in the mixtures with the addition of sodium hydroxide; however, more micro-cracks were observed in the mixtures with a small incorporation of sodium silicate.