Improving the mechanical properties of cement paste with carbonated blast furnace slag by tailoring CaCO₃ polymorphs and increasing carbonation degree

Carbonating calcium-rich supplementary cementitious materials (SCMs) is increasingly used to reduce concrete's carbon footprint. However, significant knowledge gaps persist: (1) Carbonation forms a CaCO₃ layer on SCM particles, which may hinder active silica dissolution and pozzolanic reactions, potentially affecting concrete performance. The extent of this impact is unknown; (2) Carbonation produces calcite, aragonite, and vaterite polymorphs on SCM surfaces, but their effects on concrete's strength and microstructure remain unclear; (3) Increasing carbonation degree of SCM before concrete use may reduce carbon emissions, but its effect on mechanical and long-term properties is uncertain. This study aims to bridge these gaps and optimize carbonated SCM use in low-carbon concrete. Blast furnace slag (slag) underwent wet carbonation at 23 ± 2 °C and 60 °C to form pure calcite and aragonite on its surface, respectively. Carbonation degrees of 3 %, 6 %, and 9 % were achieved by varying carbonation duration. The carbonated slag replaced 30 % of cement in cement paste preparation. This study addresses three key questions: (1) How does increasing carbonation degree affects slag's pozzolanic reactivity (2) How different calcium carbonate polymorphs (e.g., calcite and aragonite)? on slag surfaces affect cement paste properties? and (3) The impact of varying carbonation degree on cement paste performance. Results show that 6 % carbonation enhances pozzolanic reactivity, contributing positively to cement paste performance. Aragonite increases 1d strength by 23 %, while calcite boosts 28d strength by 17 %. At 9 % carbonation, autogenous shrinkage decreases by 15 %, and aragonite raises flexural strength by 70 %. Tailoring carbonation degree and CaCO₃ polymorphs optimize strength and shrinkage in cement paste.