This paper reports the investigation aimed at verifying the influence of potassium dihydrogen phosphate and ferric chloride hexahydrate on stabilization/solidification (S/S) processes applied to tailing waste from a Pb and Zn mineral processing plant. The use of cement alone (in a percentage equal to 5%) does not permit the obtaining of a final product complying with current Italian legislation. The possibility of using potassium dihydrogen phosphate, singly or combined with ferric chloride hexahydrate, has been investigated under different pH conditions, simulating the aging phase in waste-cement mixtures. The combined effect of the additives mentioned (1% KH2PO4 and 1% FeCl3), together with small quantities of cement, permits production of inert material, through the physical encapsulation and formation of stable compounds. (c) 2013 Elsevier Ltd. All rights reserved.