Heavy metal stabilization by means of innovative alumino-silicate matrix

Powdered tuff mixed with NaOH solution has been hydrothermally cured at temperatures ranging from 90 to 150 degrees C. Hardening takes place due to the formation of an amorphous binding phase. At the lowest temperature tested a non-autoclaved process can be carried out. Values of unconfined compressive strength were found to vary from 15.5 MPa to 28.9 MPa depending on reaction conditions. The matrix was tested as a binder for the stabilization of model systems containing cadmium, chromium and lead and for a real system containing a secondary lead smelter slag. The stabilization process was tested from both the environmental and technological points of view by means of leahcing tests and compressive strength measurement. Basic characterization leaching tests carried out with the model systems showed that metal release from hardened paste is below 1%. Compliance leaching test carried out with the real system showed that lead release is below the limit set by law. From the technological pont of view, it was found that unconfined compressive strength is always higher for the real system. Specifically, this system showed compressive strength increasing with slag content to values exceeding 86.5 MPa.