Effects of compounding soda residue with high-calcium activators on the performance of coal gasification coarse slag-based cementitious materials

Coal gasification slag (CGS)-based geopolymers were developed by compounding soda residue (SR) with calcium-based alkali activators to activate ground granulated blast-furnace slag and coal gasification coarse slag precursors. The corresponding effects of combining SR with calcium-based activators and the ratio of SR to desulfurized gypsum on the performance of CGS-based geopolymers were systematically studied. The geopolymer microstructure was characterized by FTIR, TG, XRD, NMR, and SEM. The results showed that when calcium carbide slag, soda lime, or Ca(OH)2 were combined with SR as an alkali activator material, the fluidity was >200 mm and the 28-day ultimate compressive strength (UCS) was >13 MPa. When phosphogypsum (PG) and desulfurized gypsum (DG) were used with SR as the alkali activator material, although the fluidity was reduced, the compressive strength was significantly improved, with the 28-day UCS >27 MPa. The calcium-based activator with high OH− content could promote early depolymerization of ground granulated blast-furnace slag and coal gasification coarse slag. The high Ca2+ concentration facilitated rapid nucleation of C−(A)−S−H and reacted with Cl− in SR and [Al(OH)6]3− in ground granulated blast-furnace slag, forming Friedel's salt (Fs). The calcium−based activator containing SO42− had better activation effects, and the synergistic effects of AFt crystals and cementitious products played a crucial role in strength formation in later ages. With the increasing DG content, the SO42− concentration in the alkaline solution increased, and Al(OH)4− in the slurry reacted with Ca2+ to form AFt, thus inhibiting Fs formation. When the doping of SR and desulfurized gypsum is equal, the C−(A)−S−H gel and the fine rod-like AFt phase form an interlocking effect to fill the low voids and microcracks, which plays a crucial role in strength formation.