Alteration of secondary minerals along a time series in young alkaline soils derived from carbonatic wastes of soda production

Industrial activities result in increasing amounts of technical substrates being deposited in landfills. These substrates are subject to weathering and pedogenic processes. We studied the chemical and mineralogical transformations on naturally weathered waste deposits of soda industry. Four sites differing in age (15, 19, 57, and 70 years) and derived from carbonatic slurry (mainly CaO.H2O, CaCO3, NaCl) were selected. The formed soils, calcareous spolic Regosols, are weakly to strongly alkaline with pH values ranging from 8 to 12. Within 15 years, the substrate's initial pH of 12 drops rapidly in the topsoil due to the reaction of dissolved Ca either with CO2 from the atmosphere or evolved by microbial respiration and finally stabilizes at around 8.1. All soils show high electrical conductivity, up to 12.3 mS cm− 1 at the youngest site. The electrical conductivity strongly decreases within 70 years of weathering due to leaching processes and the formation of less soluble secondary minerals. The content of organic C in the studied soils ranges from 2.4 to 70.8 g kg− 1 and stocks increase with site age. Soil structure and soil color change distinctly. The binding of CO2 results in large amounts of carbonate, increasing with time. Seventy years after deposition, calcite [CaCO3] dominates the topsoil (0–30 cm depth), comprising about 80% of the soil material. The mineral composition was characterized by X-ray diffraction. Besides calcite, we found different quantities and different distributions of the less common minerals ettringite [Ca6(Al(OH)6)2(SO4)3  26H2O], thaumasite [Ca6(Si(OH)6)2(CO3)2(SO4)2  24H2O], hydrocalumite [Ca2Al(OH)7  2H2O] and hydrotalcite [Mg6Al2(CO3)(OH)16  4H2O]. Formation and alteration of these minerals are basically influenced by changes in the soil pH. With progressing weathering neither thaumasite nor ettringite are stable due to the non-favorable soil reaction (pH ∼ 8.1). In contrast, hydrocalumite and hydrotalcite exist in all investigated soils. They are stable also under weakly alkaline conditions and thus may exist in all carbonatic soils. Results indicate a surprisingly rapid soil development driven by the highly dynamic formation and alteration of minerals in carbonatic substrates under alkaline conditions.