Properties and microstructure of Ca-rich slag-based mortar prepared with a one-part sodium carbonate–silica fume solid activating blend

Sodium carbonate (Na 2 CO 3 , SC)-activated slag systems typically exhibit slow setting, delayed hardening, and limited early-age strength, whereas the activation of cement-containing Ca-rich precursors can induce rapid setting without a corresponding increase in early-age strength. In this study, a one-part SC–SF solid activating blend was employed, in which silica fume (SF) partially replaced SC. The influence of the solid activating blend-to-precursor mass ratio (A/P) and the sodium carbonate-to-silica fume mass ratio (N/S) on flowability, setting behavior, and mechanical properties of cement-containing Ca-rich slag-based mortars was systematically investigated. Microstructural characterization was conducted to examine the associated phase assemblage and microstructural features. The results indicated that, in the pure SC-activated system, flowability initially increased slightly and then decreased as A/P increased, whereas setting time progressively shortened. In contrast, partial replacement of SC with SF at suitable proportions markedly enhanced flowability, prolonged setting time, and promoted early-age strength development. At an A/P of 0.3, the SC–SF mortar with an N/S of 1.0 exhibited approximately 36.7% higher flowability, an extended setting time from 29 to 133 min, and a 151.9% increase in 3-day compressive strength relative to the pure SC mixture. The 28-day and 56-day compressive strengths were further improved by 24.6% and 29.1%, respectively. Microstructural analyses at A/P = 0.3 and 28 d showed that, compared with the pure SC mixture, specimens with partial replacement of SC by SF exhibited weaker carbonate-related diffraction features and a more continuous and compact gel-like matrix in the observed regions. Overall, appropriate adjustment of the sodium carbonate and silica fume dosages can improve the flowability, setting behavior, and mechanical properties of the mortar, providing a useful reference for the mixture design and further investigation of one-part sodium carbonate-activated systems.