Recovered soda residue as alkaline activator of furnace slag: geopolymer prepared from furnace slag incorporated with soda residue and its reuse in composite cement

The accumulation of soda residue (SR) poses a consequential threat to the environment because of the absence of effective methods for treating them on a large scale. SR is composed of NaCl, CaSO₄·0.5H₂O, and Ca(OH)₂ in addition to CaCO₃, which can be potentially used to replace reagent-grade alkaline activators for activating furnace slag (FS). This work investigates the effect of SR addition on the hydration mechanism of FS by preparing geopolymer from FS and SR. The geopolymer’s compressive strength, initial pH value, and hydration products (including yield and morphology) were analyzed; its activity index was also investigated. The analysis results show that the pozzolanic activity of FS was stimulated using SR as alkaline activator. In the presence of SR, FS dissociated to form [SiO₄]⁴⁻ and [Al₂O₄]²⁻ first. Then, it reacted with Ca²⁺, OH⁻, Na⁺, Cl⁻, and SO2 − 4 to form lamellar C(N)–S–H, needle AFt, and platelet Fs. Upon the addition of 70 wt.% SR to 30 wt.% FS, the hydration products reach the highest yield, leading to optimum compressive strengths of 3.43 and 8.84 MPa at 2 and 30 d, respectively. Furthermore, although the activity index of the geopolymer prepared from 70 wt.% SR and 30 wt.% FS is only 50.95%, 0–20 wt.% geopolymer can be used as an additive to prepare a 32.5-grade composite cement. This work realized the large-scale resource management of continuously discharged SR in support of China’s double carbon policy.