Preparation of supplementary cementitious material by semi-dry carbonated ternesite and its effect on hydration and mechanical properties of Portland cement

Ternesite exhibits significant carbonation reactivity and the resultant carbonation products show favorable effects on the performance of Portland cement. Therefore, this study investigated the effects of semi-dry carbonated ternesite on the hydration and hardening characteristics of Portland cement when utilized as a supplementary cementitious material (SCM). The results indicate that the carbonation reaction of ternesite tended to reach a plateau after 10 min, as the formation of calcium carbonate wrapping layer inhibit further carbonation. The carbonation products include calcite, aragonite, vaterite, poorly crystalline calcium carbonate (PCCC), silica gel, gypsum and bassanite, and all of which can contribute to the formation of calcium silicate hydrate (C-S-H) and ettringite in the cement matrix. Moderately carbonated ternesite appears to accelerate cement hydration and densify the pore structure of matrix, thereby continuously promoting the strength development of hardened cement paste over time while this effect diminished with excessive carbonation. Optimal carbonation of ternesite at a degree of carbonation (DOC) of 40.4% achieved the highest 28-day activity index of 95.8% of SCM. Furthermore, sustainability analysis suggests that utilizing carbonated ternesite as a SCM could reduce CO₂ emission by 107.8 kg per tonne of cement prepared. This research provides new insights for the development of novel low-carbon cement with high strength.