Abstract: In this paper, the influence of the "black liquid" on the behaviour of the cement paste is investigated. The"black liquid"is the pulp-waste- liquid of a paper mill in which short cotton-wool is used as the raw material for making paper. The results obtained show that the"black liquid"does not cause any change in the phase of the hydration product, it suppresses the hydra-tion of the cement paste only at the early age and its waterreducing effect and dispersing effect improve the pore structure of the cement paste, homogenize the distribution of the cement hydrate and increase the strength of the hardened cement by 20-30 percent.