Conveying Performance, Mechanical Strength, and Microstructural Characteristics of Alkali-Activated Fly Ash-Cemented Paste Backfill

Abstract

Fly ash (FA) is widely used in the preparation of cement paste backfill material, but its cement substitution rate is limited by its degree of hydration. In this study, the conveying performance, mechanical strength, and microstructural characteristics of the prepared alkali-activated FA-cemented paste backfill (AAF-CPB) were investigated through bleeding rate tests, slump tests, uniaxial compressive strength tests, scanning electron microscopy (SEM), and X-ray diffraction (XRD). The results show that the bleeding rate range of the prepared AAF-CPB is 1.05%−6.96%, the slump is 162–218 mm, the early strength range is 0.72–2.63 MPa, and the final strength range is 3.28–10.68 MPa; the slurry concentration, gangue particle size, FA replacement rate, and binder-coal gangue (B-CG) ratio have a significant impact on the bleeding rate; slurry concentration also has a significant impact on slump; slurry concentration has significant effects on early strength and less significant effects on final strength of the AAF-CPB, while FA replacement rate has a higher impact on its final strength than early strength. When FA replacement rate is 30%, final strength of the AAF-CPB can be improved significantly. The optimal ratio of the AAF-CPB obtained from the experiment should be slurry concentration of 84%, gangue particle size of 0–5 mm, B-CG ratio of 0.8, FA replacement rate of 30%, and sodium hydroxide concentration C_NaOH of 0.6 mol·L⁻¹.