Development of Highly Reactive Supplementary Cementitious Materials via Carbonated Ternesite: Toward High Cement Performance and CO₂ Emission Reduction

As a low-calcium mineral, ternesite has the potential to reduce the carbon footprint of cement, but its low hydration activity limits its application. The objective of this study is to improve the cementitious activity of ternesite in cement through carbonation treatment, thereby developing highly reactive supplementary cementitious materials (SCMs). The key factors influencing the carbonation of ternesite, including liquid-to-solid (L/S) ratio, carbonation duration, and CO2 pressure, were systematically investigated, and the underlying carbonation mechanism was elucidated. The results indicate that the carbonation of ternesite produced calcite, aragonite, vaterite, gypsum, bassanite, silica gel, and amorphous calcium carbonate (ACC). Carbonation enhanced the early hydration activity of ternesite, while excessive carbonation at higher CO2 pressures inhibited the development of later strength of cement. SCMs with optimal cementitious reactivity were prepared by 30 min of carbonation at an L/S ratio of 0.2 and a CO2 pressure of 0.3 MPa. Ultimately, utilizing carbonated ternesite as SCMs increased the 28 day strength of blended cement by 35.4% compared with Portland cement and resulted in an approximately 10% reduction in CO2 emission. These findings confirm the potential of carbonated ternesite as a highly reactive SCM for the development of novel cementitious materials with enhanced sustainability.