Feasibility study of highly alkaline biomass ash to activate alkali-activated grouts

•Three kinds of highly alkaline biomass ash was used as an activator in alkali-activated grouts.•Grouts were successfully prepared by BFS and biomass ash.•The calcium and potassium salts in biomass ash promote the formation of C-S-H and C(K)-A-S-H.•Better mechanical property was obtained compared to KOH-activated grouts.•The increase of hydration products reduces the porosity of grouts. In this study, the feasibility of using highly alkaline biomass ash to activate alkali-activated grouts was systematically evaluated for the first time to promote the development of alkali-activated binders (AABs) with higher performance and cleanliness. Blast furnace slag (BFS) and black rice husk ash (BRHA) were used as precursors for alkali-activated grouts. Ashes of wheat straw, sunflower hull, and sunflower stalk were used as alkali activators owing to high pH values (11.93–12.88 of their aqueous solutions The effects of different activators on the workability and mechanical properties of BFS–BRHA-based grouts were analyzed, and the hydration products and microstructures were characterized by scanning electron microscopy, nuclear magnetic resonance, and X-ray diffraction. The results indicated that the fluidity and setting time of the biomass-activated slurry satisfied the requirements of grouting engineering. Biomass ash accelerates the dissolution of Si4+ and Al3+ from the raw materials, while the potassium and calcium salts promote the formation of C(K)–A–S–H and C–S–H, thereby improving the thermal power and mechanical strength of the system. The 28-d compressive strength of the biomass ash-activated slurry reached 24.67 MPa, which exceeded the 9.12 MPa of the control group. Biomass ash increased the density of the hardened slurry and decreased the porosity by more than 9.1–13.8%. These results are conducive to promoting the development and engineering applications of environmentally friendly AABs.