Carbonated recycled concrete paste as an alternative to limestone and its impact on the hydration of ternary cement with metakaolin

Demolished concrete produces large volumes of waste that challenge the environment. However, its abundant hydration products offer significant CO2 capture potential, forming CaCO3 in the cement system. This study investigated the effect of the carbonated recycled concrete paste (cRCP) to metakaolin ratio on the hydration of ternary cement. The incorporation of cRCP into the ternary cements delayed sulfate depletion and reduced the heat of hydration. A 7-day synergistic effect was observed at a metakaolin-to-cRCP ratio of 0.875:1. The pozzolanic reaction and carboaluminates formation were inhibited after 7 days in the presence of cRCP, and porosity remained largely unchanged. Consequently, the compressive strength of the ternary cements with cRCP was found to be inferior to that of the samples containing limestone. These findings underscore the importance of internal alkali deficiency for the ternary cement with two kinds of pozzolanic materials.