Production of carbonates calcined clay cement composites via CO2-assisted vigorous stirring

Abstract

Limestone calcined clay cement (LC3) offers a promising approach to reducing CO2 emissions in the construction industry. However, the early-age compressive strength of LC3 is notably low, primarily due to the high clinker substitution level. This study introduces an innovative carbonate-activated calcined clay cement system, termed C4, produced through a two-step process: pretreating metakaolin (MK) and carbide slag (CS) via CO2-assisted vigorous stirring (CAVS), followed by mixing with cement. C4 demonstrated significant improvements in compressive strength, exhibiting increases of over 300 % at 1 day and 60 % at 28 days compared to LC3. These values also exceeded those of ordinary Portland cement (OPC) by 12.7 % and 8.6 %, respectively. The enhanced performance of C4 can be attributed to particle size refinement, structural modification, and phase transformation induced by the CAVS process. These changes facilitated the formation of C-(A)-S-H, which served as nucleation sites, thereby accelerating hydration reactions in C4. Additionally, the CAVS process improved the reactivity of MK by increasing its surface area and generating more active sites for reaction. Furthermore, the calcium carbonate derived from CS carbonation during CAVS exhibited higher reactivity than the calcite present in limestone, promoting monocarbonate precipitation. Collectively, these factors contributed to a denser microstructure, enhancing compressive strength development.