Role of syngenite as an internal sulfur source in treated cement kiln bypass dust (T-CBPD) on hydration and Cl− immobilization in CSA cement systems

This study examined the influence of syngenite (K 2 Ca(SO 4 ) 2 ·H 2 O), which exists in recycled T-CBPD, on the hydration behavior and Cl - immobilization capacity in the CSA–T-CBPD–gypsum system, focusing on its role as an internal sulfur source. The results showed that syngenite created an alkali-rich, K-rich environment, which altered hydration kinetics depending on the total sulfur availability and gypsum substitution level. When the total sulfur content was low (i.e., low (gypsum + syngenite)/ye’elimite ratios), hydration was accelerated within the first hour. However, this rapid early hydration was alleviated when the total sulfur level increased, and heat evolution during 1–72 h increased with higher gypsum substitution due to enhanced ettringite formation, leading to improved compressive strength. In addition, dissolved K + ions stabilized AFm phases, thereby promoting Cl - immobilization through Cl-AFm formation. In contrast, higher gypsum substitution favored ettringite formation over Cl-AFm, reducing Cl - immobilization capacity but enhancing Pb 2+ immobilization.