Manufacturing of lightweight aggregates from biomass fly ash, beer bagasse, Zn-rich industrial sludge and clay by slow firing

A 70:30 black-white clay mixture (C) has been blended with different proportions of three wastes: bagasse from beer production (BG), a flux sludge used in galvanization (FS) and fly ash enriched to 60% with K 2 O from a biomass plant (FA). A complete characterization of the raw materials was performed. Mixtures of C and FA for 15, 17.5 and 20 wt% of K 2 O were blended with water, shaped into prismatic specimens, oven-dried, muffle-sintered at 1000–1200 °C, and finally crushed into lightweight aggregates (LWAs). The amounts of FA corresponding to K 2 O = 20% for a temperature, T, of 1000 °C (LWA variety C20K-1000) yielded the best technological properties: oven-dry density of 1.22 g/cm 3 , water absorption of 39.8% and open porosity of 48.6%. Therefore, other specimens containing BG and FS were sintered according to the same protocol considering K 2 O = 20% and T = 1000 °C. White aggregates were also obtained at 1000 °C (C-1000 variety) from C-mixture (without FS, FA and BG). The addition of BG (5,7 and 10 wt%) did not mean any improvement with respect to C20K-1000, and in the case of FS (25–40%), the development of red color was fostered, which could be interesting for gardening or ornamental purposes. A leaching test conducted on the aggregate with 40 wt% FS showed that the concentration of heavy metals was below the permitted environmental thresholds. Although the leachable Zn concentration of the aggregate (205 ppm) was reduced by 85% with respect to the original residue, it is still high enough for crops that demand this metal. The outcome of this investigation proves the usefulness of recycling agricultural and industrial wastes to obtain high quality LWAs for agriculture or gardening.