The concept of using aggregates from contaminated recycled materials is slowly gaining popularity. But contaminated fine grained building waste material (BWM) and industrial residues (e.g., fly ash, residual red mud) are an increasing problem for the environment.
The BWM are enriched in some of the trace elements which potentially affect ground water quality if released into the subsoil in sufficient amount. There is a need to accurately assess the mobilization of elements that results from the weathering of these wastes.
One of the objectives of this study is to prove whether it is technically and chemically feasible to produce a light-weight aggregate (pellet) which can be used in the manufacture of blocks, dam rendering and drainage material in sealing systems of landfills.
After grinding the BWM is mixed with fly ash, red clay or other waste materials. Then it is granulated, dried and bloated at 1250-1350 C in an oven. The light-weight aggregate is characterized by fine porosity, closed outer skin with high compressive strength and low specific gravity. These pellets were subjected to leaching tests using a lysimeter.
The BWM and residue have been exposed to natural weather conditions in lysimeters since Feb. 1993. A geotextile with a fixed synthetic draining layer covers the bottom liner of lysimeter and prevents particles entering the filtrate collecting system. After sampling, conductivity and pH value of the leachates were measured. Then the samples were filtered.
The metal contents were analysed by ICP-AES and ICP-MS and those of sulfate, chloride, bromide and nitrate by ion chromatography.
Leachates of BWM (B) show high pH values and high conductivity in comparison to leachates of pellets (P). The later have considerably reduced concentrations of cations (Ca, Cd, Cr, K, Na and Zn) and anions (SO42-, Cl- and Br-) (Table 1 ). It is presumed that during leaching process hydrolyses of various minerals, e.g., CaCO3, CaSO4, have taken place.
In leachates of BWM conductivity and concentrations of anions like SO42- and Cl- are much higher than in leachates in pellets.
Table 1: Analyses of leachates of BWM before (B) / after (P) pelletisation process.
Element/Filtrate B4 B5 B6 P4 P5 P6
Ca [ppm] 383 345 381 8.5 9.3 8.5
Cd [ppb] 0.047 0.034 0.052 0.038 0.033 0.014
Cr [ppb] 22.5 7.82 15.0 0.70 0.42 0.034
Cu [ppb] 18.5 12.3 21.4 21.6 6.4 15.8
K [ppm] 71.9 80 80 5.3 8.8 2
Na [ppm] 228 142 223 73.9 20.5 2.5
Ni [ppb] 50.1 33.6 51.7 40.3 60.3 31.4
Pb [ppb] <0.005 <0.005 0.023 0.78 0.031 0.017
Zn [ppb] 95.7 59.5 101.0 27.7 17.2 13.4
Fe [ppb] 12.9 10.0 11.8 100.8 22.9 11.8
Sulfate [ppm] 2762 1652 1685 28.01 35.38 25.66
Chloride [ppm] 338 85.47 151.07 1.35 9.97 1.98
Nitrate [ppm] nd nd nd 3.40 0.81 3.39
Bromide [ppm] 1082 247 147 bdl bdl bdl
pH 7.9 8.5 7.8 7.1 7.3 6.7
Conductivity [mS/cm] 4.23 4.13 3.55 0.42 0.21 0.08
bdl = below detection limit, na = not available