Development of steel slag composite grouts for underground engineering

The grouting materials are new fields where cement clinker (CC) can be replaced by high amounts of steel slag (SS). To develop steel slag composite grouts (SSCG) for constructions and repairs in underground engineering, it is necessary to determine and optimize the performance of SSCG. In this study, the SS and blast furnace slag (BFS) contents in ternary grouts were all 55–75%. The fly ash (FA) contents were 10% and 15% in quaternary grouts, and the SS and BFS contents were all 20–40%. The FA, SS and BFS contents in quinary grouts were 5–10%, 25–40% and 25–30%. The flue gas desulfurization gypsum (FGDG) content was 5%. The water-solid (W/S) range was selected as 0.65–1.2. The properties investigated were: fresh-state properties i.e., mini-slump, effective W/S, stone rate, flowability losing time, setting time; mechanical performance i.e., flexural strength (FS), unconfined compressive strength (UCS), FS/UCS and fracture characteristic; hydration products; microstructure and pore size distribution. The results show that the SS was suggested to combine with BFS to enhance the strengths, the 3-day and 28-day UCSs of the quinary SSCG (19#) increased by 105.3% and 167.7% with 2% B + 6% AA + 0.4% SP. With the approximate activation, the bigger pores (>100 nm) have been transformed into small pores under 30 or 100 nm effectively, and the compactness of gel structure has been improved. The workability, mechanical performance, and microstructures of optimized SSCG with high amount of SS (40%) and industrial residue (80%) are acceptable, it can meet the requirements of grouting practices for underground engineering.