The capacity of the new builder, delta-Na2Si2O5, used as a substitute for zeolites in detergent formulations to soften wash waters, has been investigated in the present paper. The retention capacity of Ca2+ and Mg2+ on this new builder has been assumed to be caused by an ion exchange process. Then, the ternary equilibrium Ca2+/Mg2+/delta-Na2Si2O5 has been simulated using a model based on the mass action law. The equilibrium constants and the Wilson parameters, as well as the thermodynamic properties for the three binary systems involved in the ion exchange, have been calculated. The equations obtained allow one to deduce the suitable detergent dosage at each washing temperature. Equilibrium data of the ternary system showed that the new builder is more selective for the Mg2+ cation than zeolite 4A, the builder actually used in non-phosphate detergents. It can be concluded that delta-Na2Si2O5 can be used as a builder or cobuilder in nonphosphate detergent formulations, specially in zones with hard water.