Calcined clays as supplementary cementing materials for sustainable concrete

Abstract

The Portland cement, which is used as a binder material in concrete, has an important share for CO2 emission worldwide. For this purpose, supplementary cementing materials are used to be substituted with Portland cement in specific amounts. The usage of industrial by-products such as fly ash, silica fume, and slag as supplementary cementing materials seems advantageous. However, potential in availability of good quality by-products in a local scale have led to the search for feasible alternatives to these materials. The aim of this study is to evaluate the clay samples obtained from two different deposits in Turkey after calcination in terms of their use as supplementary cementing material. Chemical, mineralogical and thermal characterization of clay samples were made before and after calcination at various temperatures. Pozzolanic activity and reaction kinetics of the clay samples were evaluated with and without limestone addition by thermal analysis and isothermal calorimetry, respectively. Water requirement and strength activity index of calcined clays selected depending on their pozzolanic activity were also determined. The pozzolanic activity of clay containing a relatively higher amount of kaolinite mineral was determined to be higher. Clays calcined at 700 ºC showed the highest pozzolanic and strength activity whereas a calcination temperature of 1100 ºC results in a relatively lower activity. The limestone addition improved the pozzolanic activity, and the heat evolution during hydration. As BET surface area increased, the water requirement for calcined clay also increased.