Caracterisation mineralogique, geochimique et geotechnique des produits d’alteration sur quartzites micaces de Sa’a (Centre Cameroun) et leur valorisation potentielle

Abstract

The main objective of the present work is to establish from petrological data that the macromorphological characteristics of crystallophyllous rocks, especially those of micaceous quartzites in the equatorial zone, control the weathering processes and that the soils, lateritic gravels and lateritic clays, derived from pedogenetic processes that affect these rocks are usable in road construction and building industry, respectively. The study was carried out in the Sa'a area (Central Cameroon) on a weathering profile overlying micaceous quartzites and on 13 lateritic gravel samples, 10 lateritic clay samples, and 01 alluvial clay sample from Sanaga River. These materials were subjected to petrographic analysis by optical microscopy, mineralogical analysis by X-ray diffraction, geochemical analysis by X-ray fluorescence and atomic mass spectrometry, as well as to geotechnical and mechanical tests, in particular for lateritic gravels and lateritic clays. Statistical tests were performed only on the lateritic gravels. The results obtained show that the studied micaceous quartzites consist of 1 – 2 cm thick quartz platelets separated by millimeter to multi-millimeter ferromagnesian mineral beds. The weathering profile shows the general macromorphological organization saprolite/nodular set/superficial loose clayey horizon. The saprolitic set is atypical because it presents a succession of coarse saprolite/soil material. The main pedogenetic processes along the studied weathering profile are transformation of primary minerals into neoformed-clays and Fe-oxide-hydroxides formation, translocation of rare earth elements (REE), associated mainly with light REE losses, and addition of organic matter in surface horizons, associated with an accumulation of transition metals, high field strength elements, and heavy REE. Lateritic gravels derived from Sa'a micaceous quartzites have high SiO2 contents and low sesquioxide contents, which reduces their geotechnical performance. These materials are of medium to high plasticity. A 6-parameter model of the California bearing ratio (CBR) determination equation was established using mineralogical, geochemical, and geotechnical parameters. The equation for this mathematical model is: CBR = 822,64 – 295,95 x γs + 1,23 x (<2 μm) – 23,21 x VBS + 4,72 x MDD + 2,25 x Fe2O3t – 3,52 x Goethite. The average CBR value (33%) in the studied lateritic gravels, associated with the values of the other geotechnical parameters indicates that these materials can be used as a sub-base for low to medium traffic T2/T3 and as a base course for low traffic T1. The raw lateritic clays from Sa'a are not suitable for the production of bricks and/or tiles due to their mechanical characteristics. The comparison of the parameters after thermal stabilization, with alluvial clays and with lime, shows that the stabilization with alluvial clays is best suited for the studied lateritic clays. However, two modes, thermal stabilization and alluvial clay stabilization meet the requirements for bricks manufacture. Lime stabilization does not allow the production of tiles with acceptable characteristics.