Abstract
In the present work the modifications induced by the geomimetic processing on the environment of silicon and aluminum atoms are finely characterized by solid state MAS NMR. The raw clay (Lat), a lateritic clay from Yaounde (Cameroon), contains kaolinite, quartz, hematite and goethite as major mineral phases. This material is treated under acidic conditions during 24 h (LatAF) and then under alkaline conditions during 18 days, leading to the final consolidated “geomimetic” product (LatAFCH). The samples have been characterized by 29Si and 27Al solid state NMR at each step of the process.The NMR spectra obtained for the starting clay indicate the presence of AlIV and AlVI populations together with silicon Q2 (0 or 1Al) and Q3 (0 or 1Al) environments located at δiso=-83 ppm and δiso=-91 ppm respectively.The acidic reaction during 24 h, does not significantly affect the silicon-rich layers. It induces a conversion of part of AlIV population into AlVI environment mainly related to the occurrence of six-fold organo-aluminum complexes when using fulvic acid. With inorganic acid, Al dissolution prevails.The neutralization (using lime) of the medium, followed by ageing for 18 days, promotes clay interactions with available calcium ions. The newly formed phases are cementitious CSH, CASH and CAH phases.
Highlights
Clays are natural raw materials characterized by a layered structure and belonging to the phyllosilicate family
There are two groups of phyllosilicates depending on the constitution of an elementary sheet resulting from a pilling up of octahedral (Al and/ or Mg cations) and tetrahedral (Si cations) layers along the c-axis [14]: 1:1 or Te-Oc phyllosilicate group that exhibits an octahedral layer combined to a tetrahedral layer, typically as in kaolinite where the characteristic d-spacing of the elementary sheet is 0.714 nm and the 2:1 phyllosilicate group characterized by an elementary sheet composed of one octahedral layer sandwiched between two tetrahedral layers to form an elementary sheet
The particles are stacked together and appear as aggregates with a mean size of 1 μm [6]. Such high specific surface area may arise from the presence of iron phase nodules onto the surface of clay platelets as well as from the high alteration of this type of lateritic soil
Summary
Clays are natural raw materials characterized by a layered structure and belonging to the phyllosilicate family. The d-spacing is in the range of 0.9 to 1.4 nm in general and typical clay minerals of this group are montmorillonites, vermiculites, talc, illite, mica. These raw materials are readily available and can be recycled, they are involved in sustainable and environmentally friendly applications. Clay is considered as lateritic clay when the amount of iron phases is in the range of 15-25 mass% in association with near to 50% mass clay minerals. Such raw materials are mainly used for road and building purposes. The present work is aiming to contribute to clear up the latter point by the investigation of the evolution of the local environment of the main constitutive atoms namely Si and Al using solid state MAS NMR analyses
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