Multiscale investigation of cement pastes with low and high-grade calcined clays and slag at early and advanced ages

Abstract

Clays are abundant materials worldwide, but the massive demand for pure kaolinitic clays in multiple fields could reduce the availability of high-grade calcined clay. For this reason, the properties of cementitious materials with low-grade calcined clays should be seriously investigated. In this work, binary and ternary binders composed of high-grade or low-grade calcined clays and slag, replacing 30% of Portland cement, are studied using nanoindentation, standard strength measurement, MIP, and SEM in order to demonstrate the impact of calcined clay composition on the microscale and macroscale properties of cementitious materials during both early and mature ages. Results show that in binary binders, low-grade calcined clay leads to reduced porosity and enhanced elastic and viscoelastic properties, as probed by means of nanoindentation, while high-grade calcined clay results in a high amount of low-density calcium silicate hydrate (LD CSH). Moreover, in the ternary binder, a synergistic effect has been highlighted between low-grade calcined clay and slag through the improvement of compressive strength at early age, as well as a refinement of the pore structure. From this study, it can be concluded that low-grade calcined clays give rise to good micromechanical properties and can be particularly effective in ternary blends when associated with slag.