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.sup.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.sup.3 and exhibits good compressive strength.