Method of admixing and placing a substantially non-flowable cementitious material into an underground space, such as a mine shaft or the like. In a preferred embodiment, a cement slurry including about 95% Class F fly ash and about 5% hydraulic cement is admixed with an activator using a high intensity mixer to form a substantially non-flowable cementitious material, wherein the substantially non-flowable material is pumped through a conduit and placed into the underground space. The cementitious material does not flow away from the area of placement.