Abstract: A novel use of delayed reactivity partly prehydrated lime (“DRQL”), which is comprised of 40 to 98% by weight of CaO and of 60 to 2% by weight of Ca(OH)2, preferably of 80 to 92% by weight of CaO and of 20 to 8% by weight of Ca(OH)2, and more preferably of 85 to 90% by weight of CaO and of 15 to 10% by weight of Ca(OH)2, in the field of the separation of solid matter from the liquid of a suspension. The invention also involves a novel method for treating a sludge, in which the solid matter is concentrated, dried and recovered using the delayed reactivity partly prehydrated lime. It additionally relates to, as a novel industrial product, the purified sludge obtained according to the method.