Effect of alkali cation on irreversible gel formation in basic medium Monique Tohoué Tognonvi, Sylvie Rossignol ⁎, Jean-Pierre Bonnet Centre Européen de la Céramique, Groupe d'Etude des Matériaux Hétérogènes (GEMH E.A. 3278), ENSCI, 12 Rue Atlantis, 87068 Limoges Cedex, France abstract article info Article history: Received 16 July 2010 Received in revised form 29 September 2010 Available online 8 November 2010 Keywords: Alkaline silicate solution; Irreversible gel; Cation size; Syneresis; Gelation This study is based on the understanding of the behavior of alkali silicates in a basic medium and aims to identify the mechanisms responsible for the formation of an irreversible gel and its consolidation. Commercial lithium, sodium and potassium silicate solutions were used to reveal the effect of the cation nature on the gelation process. Gels are obtained by acidifying alkaline silicate solutions with hydrochloric acid. A syneresis phenomenon during ripening that leads to the formation of a strongly consolidated solid has been observed. Whatever the cation, the gelation or syneresis mechanism would be similar. However, gelation time decreases and syneresis increases with the cation size. Gelation results from formation of small particles which grow in number and size and then gather to fill the available space. Ripening takes place through a dissolution/ precipitation mechanism. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Soluble silicates know a renewed interest in industry. Considered as one of the oldest class of chemical products, they are nowadays used as raw materials for production of silica precipitate or zeolites, inorganic binders in ceramics industry. They are also used as reagents during formation of geopolymers [1]. Alkali silicates are available in a solid, glass or liquid form and are made in the industry by fusing silica (amorphous or crystallized) in the presence of alkali carbonate or sulphate at a high temperature. These products have been widely studied and are well understood in the solid state, however the chemistry of alkali silicate solutions remained difficult due to the large amount of complex mixtures of silicate species and the complexity of their chemical equilibria [2–4]. Actually, more than twenty silicate species in various forms (linear, cyclic, prismatic…forms) [5] are present in the solution inducing their study very difficult. Infrared and 29 Si nuclear magnetic resonance spectroscopies have been used extensively in structural studies of silicate solutions and correlations between wavenumber and Q n units have been established. Many infrared studies on silicate glasses have shown that bands located at 445–455 and 1125–1200 cm -1 are respectively due to Si–O–Si deformation and asymmetric stretching vibrations at the Q 4 site [6,7]. Bands at 1050–1080 and at 1050–970 cm -1 can be attributed to Si–O–Si asymmetric stretching vibration generated by Q 3 and Q 2 units respectively [6,8–10]. The groups of bands at 570 and 775–800 corresponding respectively to O–Si–O bending and Si–O–Si symmetric stretching vibrations are produced by a ring structure [6]. Available commercial solutions have compositions characterized by two major factors: a Si/M (M = Li, Na or K) molar ratio ranging from 0.9 to 2.5 and a SiO 2 content between 22 and 32 wt.% according to the cation nature. The effects of these two main chemical parameters and the destabilization of these silico-alkaline solutions induced by the addition of calcium ions, for instance, leading to gels or precipitates have been widely studied [11]. Identification of the structure and microstructure of involved polysilicate ions has become therefore an issue of considerable interest. These alkali silicate solutions are also used to form silica gels under an acidic or a weakly basic medium in the diluted system [12,13]. Studies based on the formation of gel in a strongly basic medium (pH N 10) by acidification of a concentrated sodium silicate solution showed, in a range of relatively low pH values (11.2 b pH b 10) and silicon concentration (0.5 b [Si] b 6 mol/l), formation of transparent reversible gels; white soluble gels and irreversible gels [14]. The aim of this study is the formation of irreversible gels in an alkaline medium from various alkali silicate solutions in order to identify the role of the alkali cation on gelation and ripening processes. FTIR spectroscopy was used to reveal variation of different vibration bands during gelation and ripening. According to the literature data, there will be attempts to correlate vibration bands with Q n units. 2. Experimental part 2.1. Raw materials Three commercial alkali silicate solutions were used as starting solutions. The characteristics of the initial solution used are reported in Table 1. In order to work under the same conditions, new lithium (water content = 78%) and sodium (water content = 76%) silicate Journal of Non-Crystalline Solids 357 (2011) 43–49 ⁎ Corresponding author. Tel.: +33 5 87 50 25 64; fax: +33 5 87 50 23 01. E-mail address: Sylvie.rossignol@unilim.fr (S. Rossignol). 0022-3093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2010.10.003 Contents lists available at ScienceDirect Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol