Revista de Ciências e Engenharia de Materiais

²⁹Si and ²⁷Al MAS NMR Characterization of the Structural Evolution of a Lateritic Clay under Acidic and Alkaline Treatments

Abstract

Herve Goure-Doubi, Valérie Montouillout, Gisèle L Lecomte-Nana, Benoît Naît-Ali, Léon Koffi Konan and Agnès Smith

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 Yaoundé (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 Al_IV and Al_VI populations together with silicon Q² (0 or 1Al) and Q³ (0 or 1Al) environments located at δ_iso=-83 ppm and δ_iso=-91 ppm respectively.
The acidic reaction during 24 h, does not significantly affect the silicon-rich layers. It induces a conversion of part of Al_IV population into Al_VI 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.