Zinc oxide in alkali-activated slag (AAS): retardation mechanism, reaction kinetics and immobilization

The relatively fast setting of alkali-activated slag (AAS) greatly restricts its wide application. ZnO can serve as the retardant for AAS. However, the role of ZnO in AAS is not well understood. This paper investigates the retardation mechanism of ZnO in the AAS system and the effects of ZnO on the reaction and performance of AAS. Results showed that a ZnO dosage of 2 wt% prolonged the setting of AAS paste by 72 min and inhibited the growth of C-(A)-S-H gels within the first 6 h of the reaction. The retardation of ZnO in AAS could be attributed to the 'nucleation inhibition', in which Zn2+ inhibits the generation of calcium hydroxide precipitation owing to the preferential incorporation of Ca in the calcium zincate (CZ) phase, thus reducing the nucleation sites and slowing down C-(A)-S-H gels growth, until all Zn ions are transformed into CZ phase or bonded in the AAS framework. ZnO slightly reduced the compressive strength and reaction extent of AAS, and was immobilized in AAS through various physicochemical approaches. This study provides a new look into the interaction between ZnO and AAS systems.