Upgrading physico-mechanical characteristics and bacterial resistivity for cementless pastes modified with ZnAl2O4 nano-spinel: A comparative study with ZnO

Abstract

The significant negative impact of embedding nano-ZnO in cementitious materials on setting-time and compressive-strength limited its application despite its remarkable self-cleaning properties. Therefore, this study presents a proposal to employ nano-ZnAl₂O₄-spinel instead of nano-ZnO in an attempt to eliminate such defects while maintaining self-cleaning behaviour. Seven mixes were prepared: alkali-activated slag (AAS, control, 0 wt%nano-particles), while other specimens were modified with 0.25, 0.5 and 1 wt% nano-ZnO or nano-ZnAl₂O₄-spinel. The results proved the efficacy of nano-ZnAl₂O₄-spinel in shortening setting-time and improving the compressive-strength at early/later ages compared to nano-ZnO. The AAS-modified with 0.5 wt%nano-ZnAl₂O₄-spinel (optimum dose) have an acceptable setting-time (within standard limit) and the highest compressive-strength in between AAS (control) and other modified with nano-ZnO. The XRD showed that forming Zn(OH)₂ (isolated-barrier) and zinc-alumino-silicate-hydrate is the reason behind the retardation effect and inadequate compressive strength of AAS-modified with nano-ZnO. The TGA/DTG and SEM clarified that the filling/nucleation-site/reactivity effects (high surface-area) of nano-ZnAl₂O₄-spinel cause generating a massive amount from various strength-giving-phases that form compact structures. Regarding the antimicrobial activity, the specimens containing nano-ZnAl₂O₄-spinel have a superior bacterial resistivity similar to others containing nano-ZnO. Finally, it is concluded that nano-ZnAl₂O₄-spinel is preferred to nano-ZnO in developing self-cleaning binding materials with acceptable fresh/hardened properties.