A Model Linking Compressive Strength and Porosity in Ternary System: Metakaolin, Limestone, Cement

The replacement of traditional cement with high clinker content should be achieved quickly to lower the carbon footprint of mortar and concrete. Cement is responsible for about 70% of the carbon footprint of cementitious materials. Current research suggests that the use of limestone and metakaolin or calcined clay could replace the current four gigatons of clinker produced. Here, binary systems composed of limestone/cement and metakaolin/cement are first studied to determine the individual impact of fine limestone and diverse fine metakaolins on the flow and compressive strength of the material. The flow properties are correlated with the surface areas of clinker and metakaolin and are almost independent of the limestone content. A model based on a linear relationship between compressive strength and porosity is used to estimate the reactivity of cement, limestone and metakaolin. An excellent correlation is obtained with the two binary systems and confirmed with the ternary systems using the same reactivity factors. The presented model allows the determination of the impact of each of the three components on compressive strength development. Limestone and metakaolin accelerate the hydration of clinker, leading to higher early strength, proportionally to their surface area. The reactivity of metakaolin is also found to be directly related to its mean size or surface area.