Research paper Effect of iron phase on the strengthening of lateritic-based “geomimetic” materials Gisèle Lecomte-Nana a, ⁎, Hervé Goure-Doubi a , Agnès Smith a , Alain Wattiaux b , Gilles Lecomte a a Groupe d'Etude des Matériaux Hétérogènes, Centre Européen de la Céramique, GEMH, 12 rue Atlantis, 87065 Limoges Cedex, France b Institut de Chimie de la Matière Condensée de Bordeaux, 7 avenue du Docteur A. Schweitzer, 33608 Pessac, France abstract article info Article history: Received 14 May 2011 Received in revised form 21 August 2012 Accepted 16 September 2012 Available online 16 October 2012 Keywords: “Geomimetic” materials Lateritic clay Ferric gels Fulvic acid Iron oxides and oxyhydroxides Phase transformation The present work aims to investigate the action of associated iron species contained in lateritic clays in the strengthening process of “geomimetic” materials (compressive strength of 12 MPa). The starting products are a lateritic clay from Cameroon, fulvic acid and lime. The synthesis involves acidic and alkaline reactions followed by a curing period of 18 days at 60 °C under water-saturated atmosphere. Since goethite (α-FeOOH), hematite (α-Fe 2 O 3 ) and ferrihydrite (5Fe 2 O 3 ·9H 2 O) are the main iron compounds in the raw clay, investigations have been carried out to identify to which extend each of these phases participates in the consolidation process. It appears that hematite and ferrihydrite may contribute to the consolidating mechanism since the as obtained “geomimetic” products exhibit characteristic compressive strength of 8.3 and 4.5 MPa respectively. Besides, the use of goethite leads to consolidated products with characteristic compressive strength of 6.5 MPa. The strengthening is effective when the synthesis of iron species is performed separately (no direct precipitation onto kaolin particles). Moreover, among the various model samples tested, only those elaborated with goethite were resistant towards water seeping and wearing. Hence, goethite is considered as the major active phase in the dissolution precipitation reaction which leads to bridging ferric gels. © 2012 Elsevier B.V. All rights reserved. 1. Introduction 1.1. General background Laterites and lateritic clays are iron-rich kaolinitic clays which arise in tropical and subtropical warm humid regions. They are characterized by typical yellowish or reddish colors and they result from weathering and leaching processes of soils (Banerjee, 1998; Lecomte et al., 2009; Maignien, 1966). Lateritic concretions which originate from chemical reactions, over thousands of years, between lateritic soils and organic acids under varying temperature, pressure and pH value in soils can have strong mechanical characteristics. One interesting scientific chal- lenge is to be able to reproduce this concretion-like consolidation over short periods of time and still obtain materials that are mechanically resistant for building purposes (Banerjee, 1998; Kasthurba et al., 2007). The lateritic soils contain iron and aluminum oxides and oxyhydroxides, with kaolinite as the main clay mineral. Numerous studies have been conducted on the stabilization of lateritic raw materials using hydraulic binders such as Ordinary Portland Cement (OPC) or lime (Akoto, 1986; Alutu, 2006, 2007; Attoh-Okine, 1995; Falade, 1994; Hansen et al., 1999; Lasisi et al., 1984, 1986, 1990; Osula, 1991, 1993, 1996). The use of OPC for the consolidation of lateritic clays presents drawbacks: (1) the final mechanical properties of the obtained products are not very high. Typically, the compressive strength varies from 2 to 10 MPa (Mbumbia et al., 2002) while with concrete and fired clay bricks this parameter lies between 20 and 50 MPa; (2) the OPC production requires firing and grinding cycles, which are energy consuming processes; (3) the high energy and/or high binder requirements to reach such mechanical properties are not readily available in developing countries. Therefore, suitable alternative solutions need to be explored in order to consolidate lateritic clays. Preliminary investigations (Lecomte et al., 2009), performed in our laboratory, were emphasized on the develop- ment of original, sustainable and environmental friendly routes to consolidate lateritic clays. This target was successfully reached by combining our knowledge on the behavior of ferric solutions under various pH media and on the natural occurrence of lateritic concretions that exhibit high hardness values. Our challenge is to perform and control such consolidation at the laboratory scale on a relatively short period of time. The consolidation was achieved here through a three- step chemical route at low temperature (between room temperature and 60 °C) involving acidic and alkaline reactions and using fulvic acid (Pandeya et al., 1998; Wang and Qin, 2007), which is a natural product. Among various properties of the final materials presented in Applied Clay Science 70 (2012) 14–21 ⁎ Corresponding author. Tel.: +33 587 50 25 59; fax: +33 587 50 23 01. E-mail address: gisele.lecomte@unilim.fr (G. Lecomte-Nana). 0169-1317/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.clay.2012.09.014 Contents lists available at SciVerse ScienceDirect Applied Clay Science journal homepage: www.elsevier.com/locate/clay