The rationale is that polyphosphate-CAC matrices have shown interesting potential to solidify/stabilize heavy metals, owing to the aforementioned acid-base reaction that yields a compact and low porous matrix mainly composed of ACP - amorphous calcium phosphate -, which can be able to retain hazardous compounds. We aimed to take advantage of the reactivity of one of the sludge components: the sludges with relatively large concentrations of phosphate are expected to act themselves as reactants that, interacting with the CAC, could result in a very effective retaining system of the sludge constituents. Sludge samples from two locations have been incorporated in high proportion within the mix. We discussed the effects of the sludge on the CAC mortar and a possible interaction mechanism is provided.