Characterisation of environmentally exposed cement-based stabilised/solidified industrial waste

A metal-plating waste filter cake treated by stabilisation/solidification (S/S) using ordinary Portland cement (OPC) and pulverised fuel ash (PFA) has been characterised after exposure to the environment in SE England for approximately 10 years. The surface region ( approximately 5cm) was severely degraded, extensively carbonated and had reduced acid neutralisation capacity (ANC) compared to bulk samples. Large 'plate-like' deposits of predominantly calcium hydroxide with a calcium carbonate upper layer were found close to, but below the surface of the exposed S/S waste. Calcium zinc hydroxide (Ca(Zn(OH)(3))(2).2H(2)O) was the major crystalline phase found in the S/S waste in the region below the calcium hydroxide plates (10-15cm). Samples taken from the bulk of the environmentally exposed S/S waste, at a depth of approximately 0.5m, were more amorphous, contained no readily identifiable crystalline phases and had negligible strength but retained high acid neutralisation capacity. Metal analysis of homogenised samples taken from different depths into the S/S waste indicated a reduction in the concentration of heavy metals, such as Zn, Fe and Cr, in the top 5cm of the S/S waste and an increase in concentration of these metals in bulk samples. The majority of crystalline mineral phases detected in the 28-day samples were not identified in the 10-year-old samples.