Effect of ZnO on the pozzolanic activity and physico-mechanical properties of modified metakaolin cement mortar composites.

Metakaolin (MK) is commonly added to enhance the mechanical and durability properties of concrete. The pozzolanic activity of MK can be improved by calcining approximately ≈ 1 wt% ZnO with kaolin. However, the role of ZnO in enhancing the pozzolanic activity of MK remains unclear. The aim of this work is to investigate the surface modification of MK caused by ZnO that could enhance the pozzolanic activity of MK. ZnO-modified metakaolin (MMK) was prepared by calcination of kaolin powder (90 μm) with zinc carbonate basic equivalent to 1% by weight of ZnO, at 850 °C, and was analyzed by FTIR, XRD, and SEM techniques. The strength activity index according to ASTM C618, and the physico-chemical properties of blended cement mortars were measured at 28 days. Cement mortar samples were analyzed by FTIR, XRD, and SEM techniques. The XRD and FTIR results of MMK did not detect products of the interaction of ZnO and MK due to the detection limits. The SEM results illustrate the formation of uniform, non-aggregated (MMK) particles. The physico-chemical properties, strength activity index, FTIR, and XRD results of MK blended cement mortars indicated the higher pozzolanic activity of MMK. Whereas the SEM imaging shows the dispersion of cement particles coated with intense honeycomb-like C-S-H without being agglomerated in the case of MMK blended cement mortar. It was concluded that ZnO improves the pozzolanic activity by modifying the surface properties of MK during the calcination process as well as during the hydration process. The proposed mechanisms of surface modification of MK by ZnO were discussed. The addressed mechanism for visualizing the surface chemistry and microstructure of MMK paves the way for future studies on improving the pozzolanic activity of MK and the sustainability of cementitious-pozzolanic compositions.