The recycling of wastes is nowadays a priority in terms of the EU policies. This study aims at the replacement of a certain percentage of clay (natural raw material) by residues produced in a municipal solid waste incineration (MSWI) process referred as Air Pollution Control (APC) residues. The main objective of this work concerns the prediction of the quantity of the waste that can be valorized as lightweight aggregates (LWA). For this analysis the Riley diagram was considered, which consists in plotting the percentage concentration of the main oxides (SiO2, Al2O3, fluxing) in a ternary diagram. The fluxing is defined as the sum of Fe2O3+CaO+MgO+Na2O+K2O. Our results point out that no more than 5% of APC residues should be considered, since the main oxide composition of APC residues is far away from bloating area. A pre-washing treatment improved the waste characteristics being then possible to increase the amount of residue that can be incorporated in percentages below 10%. This theoretical analysis can save experimental work, since it allows the prediction of the maximum amount of wastes to be tested.