Accelerated carbonation of belitic calcium sulfoaluminate cement: mechanical and microstructural insights

Belitic calcium sulfoaluminate (BCSA) cement is a more environmentally friendly alternative to ordinary Portland cement because of its lower clinkering temperature and reduced limestone demand. However, the carbon dioxide (CO2) sequestration potential of BCSA cement under accelerated carbonation has not been systematically quantified. In this study, the carbonation behaviour, carbon dioxide uptake and performance evolution of BCSA mortars exposed to 20% carbon dioxide, 80% relative humidity and 30°C for up to 28 days after 1 h of initial air curing were investigated. The degree of carbonation (DoC), derived using thermogravimetric analysis (TGA), increased significantly from 5.49% at 3 h to 24.51% at 28 days, while the DoC of the air-cured specimens remained below 3.36%, confirming limited natural carbonation. Accelerated carbonation initially reduced the compressive strength of the mortar due to ettringite destabilisation but promoted subsequent strength recovery, with 28-day strengths comparable to those of the air-cured samples. TGA and X-ray diffraction confirmed progressive ettringite consumption and the formation of monocarboaluminate, hemicarboaluminate and calcium carbonate phases. Microstructural observations revealed early pore refinement followed by the development of a dense carbonate-rich matrix. Overall, the BCSA cement demonstrated high carbon dioxide sequestration capacity under accelerated carbonation while maintaining later-age mechanical performance, highlighting its potential as a ‘low-carbon’ binder.