Influence of calcination temperature on the durability of metakaolin concrete and limitations of conventional testing methods

Abstract

Incorporating metakaolin (MK) as a partial cement replacement offers a viable pathway to reduce carbon emissions while improving concrete performance. However, its durability behavior remains inconsistent, and the applicability of durability test methods to MK‐based concrete is not well established. This study evaluates MK derived from two kaolinites calcined at 750–850°C, with emphasis on reactive amorphous content and temperature‐induced inert phases. The results show that MK calcined at 800°C and used at 15%–20% enhances compressive strength and reduces sorptivity, whereas 10% MK increases capillary absorption due to insufficient pore refinement. A key finding is that the conventional 105°C drying method, developed for OPC, is not suitable for MK concrete, as it alters the microstructure and leads to unintentionally increased water absorption and porosity, as confirmed by SEM observations. Carbonation depth increased with MK content because the reduction in calcium hydroxide lowers buffering capacity, even though the pore structure is refined. This indicates that carbonation results primarily reflect alkalinity loss rather than actual CO 2 transport. Consequently, service life predictions based on conventional models should be interpreted with caution. Overall, 15% MK calcined at 800°C provides the most balanced performance.