A Quality-Assurance Procedure for Use at Treatment Plants to Predict the Long-Term Suitability of Cement-Based Solidified Hazardous Wastes Deposited in Landfill Sites
Authors: Rushbrook PE , Baldwin G , Dent CB 
Published: December 2008

This paper is the first detailed presentation of the work conducted during a three-year research program at Harwell Laboratory and sponsored by the U.K. Department of the Environment. The purpose of the work was to investigate the physical and leaching characteristics of the solidified products derived from mixing hazardous wastes with cement and pulverized fuel ash (PFA). The proportions of cement, PFA, and hazardous waste were varied over a wide range. Where possible, relationships between the physical characteristics of the solidified waste and its chemical composition were elucidated. The work has culminated in the development of a new concept to predict, at the time of mixing, whether the future physical and leaching behavior of a batch of solidified hazardous waste will satisfactorily meet regulatory requirements.

It has long been appreciated by regulatory authorities and operators in the United Kingdom that using engineering “standards,” such as setting rate and compressive strength determinations, to define the acceptability of a newly prepared waste mix presents many disadvantages. Some of the components commonly found in hazardous wastes are known to retard the setting of cements such that the long-term physical characteristics of a solidified waste mix are not apparent until many days after preparation. The “quality-assurance” method described in this paper is considered to be a significant advance in enabling both operators to assess the acceptability of their waste mixes, and regulators to verify that any waste mix is likely to conform to the relevant regulatory requirements.

The technique presented is based upon establishing those proportions of waste, cement, and PFA which will produce a solidified product within the locally relevant “acceptable” limits (as laid down in site licenses or local regulations). Findings are based on the results of many trial mixes which were prepared using actual hazardous wastes commonly treated by solidification in the United Kingdom. For each mix, five parameters were measured: rate of setting, hydraulic conductivity, compressive strength, leachate composition, and supernatant retention. The results were used to prepare a “quality-control chart” based upon reasonable standards which could be expected in the United Kingdom for a solidified product. Naturally, different standards may be set by other regulatory agencies outside the United Kingdom. The method of derivation of a quality-control chart and the situations where an existing chart should be revised are discussed. On each control chart there is a “region” defining acceptable proportions of waste, cement, and PFA. This region of acceptability is likely to be dependent upon the chemical nature of the bulk waste streams at a treatment plant site. It will be necessary to prepare a separate quality-control chart for each individual plant; this chart will have to be redrawn if the plant is modified or if the composition of a bulk waste constituent, the cement, or PFA varies by more than a predetermined percentage.