Carmen Vizcayno

Hydromorphic and clay-related processes in soils from the Llanos de Moxos (northern Bolivia)

Abstract The Llanos de Moxos is one of the largest wetlands in the world (more than 100,000 km2) due to seasonal floods. The soil parent materials are fine Quaternary sediments brought by tributaries of the Amazon River. Forests cover some areas, although the dominant vegetation is a pastured savannah and backswamps. At present, the main land use is a very extensive rangeland, and slash and burn agriculture in minor areas. We have studied 15 profiles from several sample areas between Trinidad and San Ignacio. Field and laboratory studies have been carried out in order to elucidate the soil-forming processes taking place and to gain basic knowledge for a sustainable land management. Coarser soils are located near the present river system or in former riverbank areas, often under forest; textures here are loam or silty loam, silty clay loam being dominant. Finer soils may have up to 85% clay and are usually under savannah cover. Illite is the dominant clay mineral in all soils, followed by smectite and kaolinite; vermiculite is absent. Quartz is present in a very significant amount in the clay fraction of all studied soils. Goethite and lepidocrocite are present in all soils, lepidocrocite being a good indicator of poor drainage conditions. Soils are mainly acid, but there are soils with calcium carbonate accumulation in the subsoil, or even saline soils (salitrales). Soils are flooded for a significant period (3–6 months) and bypass flow is very active. Hydromorphic processes are always present and morphologies include hard and soft iron–manganese concretions with diffuse boundaries within peds, impregnative Fe coatings and Fe depletion hypocoatings along pore walls. The micromorphological study shows different degrees of hydromorphism, some of them associated to the present pore system and some with a palaeo character, often disturbed by active slickensides. Microsparite nodules are also present in some low-lying soils. The soils present several morphological features related to the mobility of the fine fraction that can be related to drainage conditions, as recent clay coatings, clay and silt infillings and coatings and clay-depleted hypocoatings. The most clayey soils show striated b-fabrics and frequent slickensides, and some others point to ferrolysis as iron-depleted hypocoatings and low crystallinity clay coatings along pores, although this is not supported by mineralogical data. A moderate cation supply probably prevents strong acidification.

Influence of the calcination temperature of kaolin on the mechanical properties of mortars and concretes containing metakaolin

Abstract The effect of heating, to temperatures between 400 and 1200ºC, on the dehydroxylation of kaolin and the pozzolanic activity of the resulting amorphous material were determined by a variety of analytical techniques. Mixtures of concrete containing variable amounts of kaolin calcined at 700ºC were analysed and the results compared with those for concrete samples containing two different types of imported metakaolin. As shown in this work, Colombian kaolin can be used effectively as a raw material to obtain a highly active product (metakaolin). The optimum heating temperature for the kaolin is between 700 and 800ºC.

Mineralogical, chemical and morphological features of salt accumulations in the Flumen-Monegros district, NE Spain

Abstract Relationships among the mineralogy, morphology and the formation processes of salt accumulations were studied in three sampling areas in the Flumen-Monegros district, which represent three geochemical environments [Na-SO4-Cl, Na-(Mg)-SO4-Cl and Na-CO3-SO4-Cl], according to chemical and mineralogical results. The samples included both natural ones and others that were obtained in the laboratory by the crystallization of brines with and without substrate. A climatic chamber was used to reproduce the environmental conditions. Laboratory crystallization of brines without soil substrate does not always reproduce the natural mineral associations. Such associations were reproduced, however, when the crystallization was carried out with the epipedon. New morphologies are described for bloedite (radiating prismatic, and foliated rosette-like) and burkeite (book plates and spheroidal association of plates), the latter formed through capillary evaporation. X-ray diffraction, scanning electron microscopy (SE and BSE modes), energy dispersive X-ray spectrometry and polarized light microscopy were some of the techniques used, supplemented by chemical data.

Na2Mg(SO4)2.4H2O, the Mg end-member of the bloedite-type of mineral

The crystal structure of a natural sample of disodium magnesium bis(sulfate) tetrahydrate, Na 2 Mg(SO 4 ) 2 .-4H 2 O, the Mg end-member of the bloedite-type of mineral, is reported. The structure can be described in terms of parallel layers built up of MgO 2 (H 2 O) 4 and NaO 4 (H 2 O) 2 octahedra. Octahedral layers are interconnected through SO 4 tetrahedra. The two crystallographically independent water molecules give rise to a three-dimensional network of hydrogen bonds.

Extractable forms of aluminum as affected by gypsum and lime amendments to an acid soil

The influence of gypsum or lime + gypsum amendments on various extractable forms of aluminum (Al) in a reconstructed acid soil (plinthic Palexerult) was investigated. The addition of gypsum depolymerized non-hydrolysable carbon (C) and increased the extraction of Al bound to organic matter. The application of gypsum or lime + gypsum lowered the levels of exchangeable Al; also, the low proportion of Al in outflow solutions suggests the immobilization of Al as a solid phase. Except for exchangeable Al, the gypsum amendment increases the proportion of all forms of Al extracted (bound to organic matter, sorbed to, oxalate and citrate) with various selected reagents relative to unamended samples. The amount of Al extracted increases with increase of gypsum added. The gypsum or lime + gypsum amendments increased soil productivity.

EFFECT OF KAOLINITE AND SULFATE ON THE FORMATION OF HYDROXY-ALUMINUM COMPOUNDS

OH-A1 solutions were prepared by adding appropriate amounts of NaOH to A1C13 to obtain OH/Al mole ratios of 2.0, 2.5, 2.7, 3.0, and 3.3 in the final suspension. Solid Na2SO4 and Georgia kaolinite (KGa-2) were added individually and jointly to the OH-Al solutions. All samples were aged for 30, 70, and 180 d. X-ray diffraction, infrared spectroscopy, scanning electron microscopy, and energy dispersive X-ray spectrometry were used to characterize precipitates. Bayerite, gibbsite, and nordstrandite crystallized at mole ratios of 3.0 and 3.3, with bayerite being the most abundant. A morphology of clusters of triangular pyramids is described for bayerite. Despite the aging duration, only noncrystalline Al compounds were obtained in mole ratios of 2.0, 2.5, or 2.7. The addition of sulfate to OH-A1 solutions in mole ratios of 2.0 and 2.5 produced crystalline basic aluminum sulfates of variable morphology, but with similar chemical compositions. These phases lost crystallinity with aging. The product from a 2.7 OH-Al solution was X-ray amorphous hydroxy sulfate. In contrast, products obtained at mole ratios of 3.0 and 3.3 contained no sulfate ion, which restricted the formation of gibbsite, bayerite, and nordstrandite. The addition of kaolinite to the solutions in OH/A1 mole ratios of 3.0 and 3.3 favored the formation of nordstrandite. The simultaneous addition of sulfate and kaolinite to the OH-A1 solutions in mole ratios of 2.0 and 2.5 produced prevalent sulfate over kaolinite, whereas the opposite occurred at mole ratios 3.0 and 3.3.

Laboratory alteration of gypsiferous materials below a landfill

Some urban solid waste landfill sites in Spain are located on geological substrates of gypsiferous lithology. Although this type of substrate is assumed to be of low permeability, it can develop secondary pores by dissolution and, under favourable environmental conditions, form a karstic system that may pose serious geotechnical problems in the medium and long term. The purpose of this work was to study alterations caused by selective tests in various sections of a gypsum lithological column obtained from the Colmenar de Oreja landfill site (Spain). The tests were used to assess the influence of individual environmental factors and involved the addition of milli-RO water, solutions containing a 1%, 5% or 10% concentration of landfill leachate, and 2, 5, 10 and 15mgl(-1) solutions of NaCl in successive immersion-drying cycles at -15, 20 or 60 degrees C. Differences in alterability between the six studied segments of the lithological column were found to be due to differences in lutite content, specific crystal habit and type of cement. Segments with specular gypsum were the strongest in the tests, while the segment with the highest amount of detrital materials was the most responsive to temperature and moisture changes. The treatments that produce greater alterations are those that contain only milli-RO water.