Mechanical and durability performance of mortars with Portland cement, recycled concrete powder, and metakaolin under accelerated carbonation conditions

This study investigated the behavior of mortars incorporating Portland cement, Recycled Concrete Powder (RCP), and Metakaolin (MK) blends subjected to accelerated carbonation over a period of 180 days. The physical properties, such as water absorption, density, and void index, as well as the mechanical properties, including compressive strength and elastic, were assessed. The extent of the carbonated zone was determined using a 1 % solution of phenolphthalein. Measurements were recorded initially, and subsequently at 7, 28, 90, and 180 days following exposure to CO2. The findings revealed that the initial exposure to CO2 resulted in the densification of the cement matrix due to the formation of calcium carbonate (CaCO3), which led to an increase in compressive strength, and a reduction in water absorption and void index. Even though MK contributed to strength enhancement and microstructural densification, an extended period of CO2 exposure considerably decreased the strength due to decomposition of calcium silicate hydrates (C-S-H) and escalated the porosity. Blends with a higher proportion of RCP exhibited lower resistance to carbonation owing to their heightened porosity and limited availability of calcium hydroxide (CH). The incorporation of MK (up to 10 %) was found to enhance resistance to CO2 penetration during the initial 28 days. However, long-term exposure to CO2 led to a weakening of the matrix, attributable to the consumption of CH through the pozzolanic reaction of MK. While the incorporation of MK enhances properties in the short-term, further detailed investigations are required to devise more durable mortars for long-standing exposure to CO2.