The effect of ZnO has already been studied for Portland cement, but the study of its impact on hybrid pastes is scarce. Thus, in this investigation, the influence of ZnO addition on hydration, compressive strength, microstructure, and structure of hybrid pastes is presented. The analyses were made by setting time tests, compressive strength tests, X-ray diffraction, Fourier-transform infrared spectroscopy, thermogravimetric analysis with differential scanning calorimetry, and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy. The results indicate that the setting time of the cements was delayed up to 39 min with additions of 3 wt% ZnO. Alternatively, the higher values of compressive strength were observed when 0.5 wt% ZnO was added to the cements for all curing days. In addition, no important differences in the microstructure of samples with different additions of ZnO were observed after 28 days of curing. It is expected that the use of ZnO contributes to the delay of the setting time and the increase of the compressive strength without negatively modifying the microstructure of hybrid pastes.

The use of supplementary cementitious materials as partial Portland cement substitutes in concrete is one of the best strategies for reducing carbon dioxide (CO2) emissions in the cement industry, and also improves the properties of concrete. When highly reactive calcined pozzolans such as metakaolin are used, the setting time of concrete is reduced, and the use of retardants is necessary. The effect of zinc oxide (ZnO) as a cement hydration retarder has already been investigated; however, the influence of zinc oxide on ternary mixtures is unknown and needs study. In this research, the effect of adding zinc oxide on the hydration, microstructure and compressive strength of ternary mixtures is studied using setting time and compressive strength tests, X-ray diffraction, Fourier transform infrared spectroscopy, thermogravimetric analysis with differential scanning calorimetry and scanning electron microscopy. The results show that the hydration process of ternary pastes is delayed or even inhibited when the amount of zinc oxide in the paste is increased. With the addition of 0.5 wt.% zinc oxide, the initial setting time and compressive strength of ternary mixtures after 3 days of curing are similar to those of the cement paste without the addition of pozzolans and zinc oxide.