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.sup.3, and exhibits good compressive strength.