Synergistic Effect of Early-Strength Agents on the Mechanical Strength of Alkali-Free Liquid Accelerator Mortar

As the core admixture of shotcrete, the accelerator promotes the rapid setting of shotcrete. In order to promote the rapid development of strength, the synergistic effect and the synergistic mechanism of early-strength agents [$\mathrm{Al}({\mathrm{NO}}_3{)}_3$ (AN), and $\mathrm{Mg}({\mathrm{NO}}_3{)}_2$ (MN)], and aluminum sulfate liquid accelerator on the strength of mortar were investigated. The aluminum sulfate liquid accelerator used in this paper was named AF. The results show that the 1-day strength of the mortar reached 8.8 and 10.73 MPa when AF was added with 2.4% AN and 1.8% MN, respectively. When AF and 2.4% AN were used, the initial and final setting times of the cement paste were 1 min 32 s and 3 min 50 s, respectively, whereas the AF and a very small amount of MN would prolong the setting time of the paste. The synergistic mechanism of AN/MN and AF is as follows: AF provides a large amount of ${\mathrm{SO}}_4^{2-}$ and ${\mathrm{Al}}^{3+}$, which promotes the rapid formation of ettringite (AFt). In addition, ${\mathrm{NO}}_3^{-}$ in AN and MN promotes the formation of the AFt phase $3\mathrm{CaO}·{\mathrm{Al}}_2{\mathrm{O}}_3·\mathrm{Ca}({\mathrm{NO}}_3{)}_2·{\mathrm{XH}}_2\mathrm{O}$ (${\mathrm{NO}}_3\text{-}\mathrm{AFt}$). Compared with AN, ${\mathrm{Mg}}^{2+}$ dissolved by MN will first precipitate $\mathrm{Mg}(\mathrm{OH}{)}_2$ with ${\mathrm{OH}}^{-}$, providing crystal nuclei for the hydration system and accelerating the hydration of ${\mathrm{C}}_3\mathrm{S}$. It is concluded that AF and MN have a better synergistic effect on improving early strength.