Microstructure, hydration process, and compressive strength assessment of ternary mixtures containing Portland cement, recycled concrete powder, and metakaolin

Abstract

The aim of this study is to evaluate the effect of recycled concrete powder (RCP) and metakaolin (MK) on the microstructure, hydration, and compressive strength of ternary mixtures containing Portland. The methodology involved producing cementitious pastes with different combinations of RCP + MK, where Portland cement was substituted by 30 %, and three sources of RCP were considered: construction, demolition, and laboratory. The materials were characterized physically, chemically, and mineralogically. The characterization of the fresh state properties was done using a rotational viscometer. The hydration of the pastes was evaluated by isothermal calorimetry, X-ray diffraction, and thermogravimetry. The compressive strength and elastic modulus were determined for 1, 7, 28, and 120 days. Finally, an analysis of the microstructure of the prepared pastes was conducted by scanning electron microscopy. The results show that RCP and MK have negative influence on the fresh state properties, increasing both the yield stress and plastic viscosity. The additions also promote early hydration of Portland cement (first hours), which decreases due to the dilution effect. The quantity of hydrated products tends to be comparable to the reference, except for calcium hydroxide, which is reduced due to the pozzolanic activity of MK. The compressive strength and elastic modulus of the RCP + MK mixtures can be equal and/or superior the reference, demonstrating a synergistic effect between both materials. The use of RCP + MK mixtures is presented as a technically viable alternative to replace Portland cement without compromising performance.