The role of zinc in metakaolin-based geopolymers. (October 2020)

• Record Type: Journal Article
• Authors: Wang, Lei; Geddes, Daniel A.; Walkley, Brant; Provis, John L.; Mechtcherine, Viktor; Tsang, Daniel C.W.
• Is Part Of: Cement and concrete research. Volume 136(2020)
• Publication Date: 2020-10
• Subjects: Geopolymer -- Alkali-activation -- Reaction kinetics -- Retarding mechanism -- Hazardous waste immobilisation -- Zn leachability; Cement -- Periodicals; Cement -- Research -- Periodicals; Concrete -- Periodicals; Concrete -- Research -- Periodicals; Ciment -- Périodiques; Béton -- Périodiques; Cement; Concrete; Periodicals; 620.135
• DOI: 10.1016/j.cemconres.2020.106194
• Languages: English
• ISSNs: 0008-8846

Abstract

Abstract: Geopolymers are low-calcium, sustainable cementitious materials. The role of Zn, a known retardant used in Portland cement, in geopolymer systems is not well understood. This study scrutinises the effect of Zn on metakaolin-based geopolymer reaction mechanisms and kinetics, and investigates the incorporation mechanism of Zn in geopolymer gels. Isothermal calorimetry and X-ray diffraction analyses show that substitution of ZnO (20 mol% c.f. metakaolin) significantly hinders reaction, likely due to preferential formation of a Na/K-Zn containing phase. Solid-state nuclear magnetic resonance spectroscopy shows that Zn 2+ partially substitutes for Na + /K + in charge-balancing sites within the geopolymer gel. Setting time and leaching tests show that the retarding effect of Zn on reaction kinetics is significantly greater in Na-activated geopolymers compared with K-activated geopolymers, whereas Na-activated geopolymers exhibit superior fixation capacity to Zn. A lab-scale experiment demonstrates that metakaolin-based geopolymers are promising candidates for the stabilisation/solidification of Zn-rich hazardous waste. Graphical abstract: Unlabelled Image Highlights: High-dosage of ZnO significantly hindered the geopolymer reaction process. Crystalline ZnO consumed in alkali-activation reaction to form new amorphous material. Zn 2+ partially replaced Na + /K + in charge-balancing sites within geopolymer gel framework. Retarding effect of Zn on reaction kinetics was significantly greater in Na-activated geopolymers compared with K-activated geopolymers. Na-activated geopolymer performed with superior efficiency in Zn-immobilisation.