Influence of Retarders on the Properties and Microstructure of an Alkali-Activated Fly Ash–Ground Granulated Blastfurnace Slag–Extracted Titanium Tailing Slag Binder

Alkali-activated materials, serving as alternative cementitious materials, showed great mechanical properties and excellent durability. Nevertheless, their practical application was limited due to their rapid setting and loss of workability. To adjust the workability and setting time, Na2HPO4 and Ba(NO3)2 were used as retarders in the alkali-activated ternary binders incorporating fly ash (FA), ground blastfurnace slag (GGBFS), and extracted titanium tailing slag (TS). The influence of retarder content on the fresh and hardening properties, as well as the microstructure development of the binder, was investigated. The results showed that both Na2HPO4 and Ba(NO3)2 could prolong the setting time of the binder, but the latter was more effective. When these retarders’ content was 1.5 wt.%, the initial setting time was extended by 21% and 45% to 103 min and 123 min, respectively. Ba(NO3)2 was harmful to the strength development of the binder, and the values of specimens containing 1.5 wt.% Ba(NO3)2 decreased by 9.1%, 22.2%, and 22.2% at 1, 3, and 28 days, respectively. Whereas the addition of Na2HPO4 was slightly negative to the 1- and 3-day strength of the binder, it benefitted the 28-day strength. Adding 1.0 wt.% Na2HPO4 would promote the formation of reaction products, resulting in an increase in the 28-day compressive strength by 8.5% to 53.5 MPa. The primary phases of this binder were C-A-S-H and C-(N)-A-H gels.

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