Interfacial reactions between humic-like substances and lateritic clay: Application to the preparation of “geomimetic” materials

Schematic representation of the interaction between fulvic acid and clay mineral in acid or alkaline conditions. [Display omitted] •An acid medium is favorable for the adsorption of humic substances on lateritic clay.•Iron oxides, in the form of goethite, enhance the formation of organo-clay complexes.•The Langmuir model describes well the adsorption between mineral and organic matter.•Cementitious phases like katoite and richterite appear in the consolidated material. The aim of this study was to understand the mechanisms responsible for the strengthening of “geomimetic” materials, especially the chemical bonding between clay and humic substances. The mineral matter is lateritic clay which mainly consists in kaolinite, goethite, hematite and quartz. The other starting products are fulvic acid (FA) and lime. The preparation of these geomimetic materials is inspired from the natural stabilization of soils by humic substances occurring over thousands of years. The present process involves acidic and alkaline reactions followed by a curing period of 18days at 60°C under a water saturated atmosphere. The acceleration of the strengthening process usually observed in soils makes this an original process for treatment of soils. The consolidation of the “geomimetic” materials could result from two major phenomena: (i) chemical bonding at the interface between the clay particles and iron compounds and the functional groups of the fulvic acid, (ii) a partial dissolution of the clay grains followed by the precipitation of the cementitious phases, namely calcium silicate hydrates, calcium aluminate hydrates and mixed calcium silicum and aluminum hydrates. Indeed, the decrease of the BET specific area of the lateritic clay after 24h of reaction with FA added to the structural reorganization observed between 900 and 1000°C in the “geomimetic” material, and to the results of adsorption measurements, confirm the formation of organo-ferric complexes [1]. The presence of iron oxides in clay, in the form of goethite, appears to be another parameter in favor of a ligand exchange process and the creation of binding bridges between FA and the mineral matter. Indeed all faces of goethite are likely to be involved in complexation reactions whereas in lateritic clay only lateral faces could be involved [2]. The results of the adsorption experiments realized at a local scale will improve our understandings about the process of adsorption of FA on lateritic clays and its involvement