Understanding the role of calcium silicate slag in sodium carbonate-activated green materials: Influence on kinetics, strength, and efflorescence

Abstract

This study assessed the impact of calcium silicate slag (CSS), an industrial by-product of aluminum extraction from fly ash, on the reaction kinetics, compressive strength, and efflorescence of one-part sodium carbonate-activated slag (SCS). Various proportions of CSS (0%, 10%, and 20%) were used to substitute for slag. The resulting performances were evaluated through chemical shrinkage, heat liberation, compressive strength, water absorption, phase composition (XRD and TG.), microstructures (BET and BSE-EDS), and simulated efflorescence test (partially and fully immersed in deionized water). The results indicate that the CSS possesses a porous structure. The addition of CSS impairs flowability but accelerates the development of chemical shrinkage and heat liberation. The incorporation of CSS promotes the transformation of gaylussite, leading to an increase in CASH and hydrotalcite formation. Consequently, a higher compressive strength and a lower water adsorption rate are achieved. 10% CSS addition improves the splitting tensile strength of SCS under conditions of partial immersion in the efflorescence test. However, a 20% CSS substitution exacerbates the formation of efflorescence products and reduces the splitting tensile strength. These observations are associated with residual alkalis in CSS, a lower Al/Si ratio of products, and an increase in capillary pores attributed to CSS particles. This paper demonstrates the feasibility of using alkaline solid waste for slag substitution and performance regulation in SCS and reveals the underlying mechanism. The prepared CSS-SCS binder exhibits much lower energy consumption and CO2 emission than traditional cement-based material, as determined by life cycle assessment.