A cement mix suitable for blocking or plugging an abandoned pipeline or back filling a mine shaft, tunnel or excavations contains (i) Portland cement or a mixture of at least two components selected from Portland cement, fly ash, slag, silica fume, gypsum, bentonite and limestone; (ii) diatomaceous earth; (iii) zeolite, (iv) an aluminum silicate and (v) 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.