A cement mix which is suitable for cementing in subterranean formations to provide zonal isolation or for blocking or plugging an abandoned pipeline or back filling a mine shaft, tunnel or excavation contains Portland cement or a mixture of two components selected from Portland cement, fly ash, slag, silica fume, gypsum, limestone and bentonite; and diatomaceous earth, preferably having a BET nitrogen adsorption specific surface area between from about 30 to about 100 m 2 /g. The cement mix may further contain an alkali metasilicate and/or alkali silicate, zeolite and/or aluminum silicate, an accelerator, such as an inorganic salt, and/or an alkaline metal oxide, as well as a lightweight density modifying agent, including glass, ceramic or plastic spheres. A cementitious slurry, formulated from the cement mix, has a density less than or equal to 1500 kg/m 3 and exhibits good compressive strength.