A cement mix suitable for blocking or plugging an abandoned pipeline or back filling a mine shaft, tunnel or excavations contains Portland cement or a cement blend of two components selected from Portland cement, fly ash, pozzolan, slag, silica fume and gypsum; diatomaceous earth; zeolite and an inorganic salt accelerator. The cement mix may further contain an alkali metasilicate and/or alkali silicate. A cementitious slurry, formulated from the cement mix, may have a density less than or equal to 1500 kg/m 3 , and exhibits good compressive strength.