Enhancing the performance of metakaolin blended cement mortar through in-situ production of nano to sub-micro calcium carbonate particles

Metakaolin has emerged as a popular supplementary cementitious material to partially replace ordinary Portland cement (OPC) due to its high pozzolanic reactivity and surface area. In this study, a new method, pre-carbonation method, is proposed to exploit the synergistic effect between the metakaolin blended cement and calcium carbonate so that the performance of the cement mortar can be further improved. This new method in-situ produces nano to sub-micro calcium carbonate particles in a slaked lime slurry through bubbling carbon dioxide. The produced carbonated slurry is then mixed with other ingredients of the mortar. The in-situ produced nano to sub-micro calcium carbonate particles possess high reactivity due to their high surface area, leading to faster and more complete reaction between the cement and the calcium carbonate. Experimental studies show both the mechanical properties and durability of the metakaolin blended mortar have been improved by the proposed method. The calorimetry results show that the carbonated slaked lime slurry can accelerate the hydration of the metakaolin blended cement due to the extra heterogenous nucleation sites provided by the calcium carbonate nano to sub-micro particles. TGA/XRD results suggest that more ettringite is produced in the mortar/concrete with pre-carbonated slaked lime slurry, which increases the total volume of the hydration products, leading to a denser microstructure of the mortar/concrete. All these findings suggest that in-situ production of nano to submicro calcium carbonate particles provides a viable way to improve the performance of metakaolin blended cement. (C) 2018 Elsevier Ltd. All rights reserved.