Pondcrete is a mixture of cement and sludge. Its role is to immobilize hazardous waste and, in some cases, low-level and mixed-level radioactive waste, in the form of solid material. The material was used by the United States Department of Energy and its contractor, Rockwell International, in an attempt to handle the radioactive waste from contaminated ponds in the Rocky Flats Plant for burial in Nevada desert. Portland cement is mixed with sludge to solidify into “pondcrete” blocks and placed into large, plastic lined boxes. The sludge is taken from solar evaporation ponds which are used to remove moisture from waste materials, therefore reducing their weight. To do this, liquid waste is poured into artificial, shallow ponds. The waste is heated by solar radiation and any moisture is evaporated, leaving behind the waste. These ponds contained low level radioactive process waste as well as sanitary sewage sludge and wastes, which categorize them and the Pondcrete as a mixed waste.

The Rocky Flats Plant, a former U.S. nuclear weapons production facility located about 15 miles (24 km) northwest of Denver, caused radioactive contamination within and outside its boundaries. The contamination primarily resulted from two major plutonium fires in 1957 and 1969 and from wind-blown plutonium that leaked from barrels of radioactive waste. Much lower concentrations of radioactive isotopes were released throughout the operational life of the plant from 1952 to 1992, from smaller accidents and from normal operational releases of plutonium particles too small to be filtered. Prevailing winds from the plant carried airborne contamination south and east, into populated areas northwest of Denver.