Recycling of treated cement kiln bypass dust (T-CBPD) as a new type of supplementary cementitious material (SCM) in calcium sulfoaluminate (CSA) cement

Abstract

This study investigated the potential of recycling cement kiln bypass dust treated via potassium chloride (KCl) extraction (T-CBPD) as a supplementary cementitious material (SCM) in calcium sulfoaluminate (CSA) cement. Isothermal calorimetry revealed intensified initial heat flow and an earlier appearance of the second peak at <= 10 wt% T-CBPD, indicating rapid initial reactions associated with the flash setting. At >= 15 wt%, however, the second peak was delayed, suggesting hydration retardation. This trend was consistent with the setting time measurements, which showed flash setting at 5-15 wt% and significant retardation at 20 wt%. XRD and TG analyses confirmed ettringite as the primary hydration product. Notably, increased ettringite formation during long-term hydration was observed only in the control and in the sample with 20 wt% T-CBPD substitution, whereas lower substitution levels may have hindered further development due to flash setting, which was likely caused by instantaneous ettringite precipitation during the early stage of hydration. These hydration characteristics were reflected in the compressive strength results. At substitution levels up to 15 wt%, strength development was limited because of flash setting. In contrast, 20 wt% T-CBPD substitution exhibited a hydration delay attributed to the high concentrations of KCl and Pb2+, which likely formed surface layers and suppressed early hydration. This led to enhanced long-term hydration and a 28-day compressive strength of 49.1 MPa, which is comparable to that of the control sample (50.6 MPa). Bound chloride content increased with T-CBPD addition, while free chloride content exceeded the established safety limit of 1500 ppm at substitution levels above 10 wt %, highlighting the need for further investigation into its safe use in reinforced concrete applications. ICP-MS analysis confirmed complete Pb2+ immobilization in all samples. Further studies are required to develop effective strategies to suppress flash settings and to optimize the use of T-CBPD for both chloride resistance and longterm strength development.