PHYSICOCHEMICAL TRANSFORMATIONS OF SULFATED COMPOUNDS DURING THE LEACHING OF HIGHLY SULFATED CEMENTED WASTES

Cementation of sulphated evaporator concentrates leads to highly sulph ated low level wastes, (ca. 25% w/w sodium sulphate solution as mix wa ter), which exhibit the presence of ''U-phase,'' a sodium-bearing calc ium monosulphoaluminate-like phase. During the leaching of simulated h ighly sulphated OPC/ BFS cements, cured at room temperature and contai ning U-phase, sodium sulphate, and ettringite, physico-chemical transf ormations have been pointed out (transformation of U-phase into ettrin gite). Samples having the same chemical composition, but cured at high temperature (maximal temperature during curing: 120 degrees C), do no t contain ettringite initially, but secondary ettringite is formed dur ing leaching. XRD spectra point out the existence of precipitation fro nts (or of phase formation fronts) varying linearly versus the square root of time. The analysis of leaching solutions has provided compleme ntary data used in a code, the aim of which is to assess cement degrad ation, based on coupling between transport by diffusion and chemical r eactions (DIFFUZON code). The U-phase-ettringite transformation is con firmed. (C) 1997 Elsevier Science Ltd.