Pétrologie et propriétés géotechniques des sables du cours inférieur de la Sanaga, Cameroun

Abstract

This work aims to characterize the sands of the lower Sanaga River from a petrological point of view and to evaluate their geotechnical aptitudes in making hydraulic concrete. The study was conducted on 19 sand samples and standard hydraulic concrete specimens made from the studied sands and coarse basaltic and granitic aggregates. These materials underwent to petrographic analyses by optical microscopy, mineralogical by X-ray diffraction, geochemical by X-ray fluorescence and by atomic mass spectrometry, as well as geotechnical tests. Statistical tests were performed on the geochemical data. Two types of conventional hydraulic concrete formulations were studied: concrete made with alluvial sand and granites (BG) and concrete made with alluvial sand and basalts (BB), according to Dreux- Gorisse method. For each concrete family, the multiparametric cement/water ratio was kept constant and equal to ~0.52. The petrological analyses revealed that these sediments are mainly coarse-grained sands. These sands consist of quartz, microcline, plagioclases, muscovite, ilmenite, anatase, magnetite, opaque oxides and epidote. The Chemical Index of Alteration (53 – 66%), Plagioclase Index of Alteration (57 – 75%), and Mafic Index of Alteration (54 – 67%) values revealed a moderate intensity of weathering for these sediments. The studied sands show enrichment in light rare earth elements relative to heavy rare earth elements and a negative anomaly in Eu (Eu/Eu* = 0.32 – 0.83). These sands were derived from felsic metamorphic rocks dated Neoproterozoic (1000 – 700 Ma) from south Cameroon domain. Th/U ratios (mean = 5.40; n = 19) reveal that these sands originating from felsic source rocks and are low to moderately recycled. The geotechnical tests showed that optimal compressive strength values are reached after 7 days of curing for BG concretes (28.9 MPa) and after 28 days for BB concretes (29.6 MPa). The transition zone between the cement paste and the coarse aggregate allows a faster setting of the mortar in BG concrete in which the coarse aggregate has a coarse-grainy texture and a geochemical composition close to that of the studied sands. The uniaxial compressive strength values of concrete are related to the geological nature of the aggregates and follow a logarithmic evolution for acidic plutonic aggregates and a linear one for basic volcanic aggregates. Based on this experimental study, BG concretes show the best performance at short curing times of about 14 days.