Comparison of mechanical properties and microstructure of GGBS-based cementitious materials activated by different combined alkaline wastes

Compound alkaline wastes was used to effectively improve the performance of a single-component activator in alkali-activated slag (AAS) cementitious materials. Macroscopic properties and microstructure evolution were characterized of ground granulated blast-furnace slag (GGBS)-based cementitious materials prepared by using calcium carbide slag (CS), soda residue (SR) and NaOH, SR and CS as activators. The binary alkali activators were found to exhibit superior activation synergies compared to single alkali activators, with CS+SR binary alkali activators demonstrating more pronounced activation effects on slag. The results showed that compared to CS and SR+NaOH, the 28d AAS strength increased by 78.56% and 69.58%, respectively, for CS+SR. When the mass ratios of GGBS, SR, and CS were 75:17.5:7.5, the 28d UCS was determined as 30.20 MPa. The microscopic analysis revealed the formation of C-(A)-S-H gel as the main hydration products, Friedel's salt (FS) is formed when SR is mixed. When SR and NaOH are mixed, the amount of C-(A)-S-H gel decreases with the proportion of increasing SR, whereas the amount of the formed FS and NaCl increases. The large amount of NaCl formation indicates that Ca in SR cannot assist in FS formation, and the increase in FS is insufficient to compensate for the strength loss caused by the absence of C-(A)-S-H gel. When SR and CS are mixed, Ca2+ and Cl- components are added, promoting complete dissolution of GGBS and facilitating the formation of more FS to fill pores and voids. Therefore, the microstructure formed by overlapping C-(A)-S-H gel with FS is more compact.