Clay Minerals (1996) 31,225-232 STABILITY OF SEPIOLITE IN NEUTRAL AND ALKALINE MEDIA AT ROOM TEMPERATURE S. MARTINEZ-RAMIREZ, F. PUERTAS AND M. T. BLANCO-VARELA Instituto de Ciencias de la Construcci6n "Eduardo Torroja " ( C.S.L C. ), C/ Serrano Galvache s/n, Apartado de Correos 19002, 28033 Madrid, Spain (Received 27 June 1994; revised 11 September 1995) A B S T R A C T: The chemical and structural stability of a Spanish sepiolite from Vallecas, Spain, was studied in 0.04 N NaOH (pH = 12.6), a saturated solution of Ca(OH)2 (pH = 12.6) and deionized/ distilled water (pH = 7). The chemical stability was evaluated by determining the concentrations of Mg, Si and Ca in the solid residues during the kinetic test. The structural stability was studied by determining the changes in specific surface area and 'crystallite' size. The specific surface was determined by the BET method and the 'crystallite' size by the profile of the X-ray diffraction line. The main conclusion was that sepiolite is a material with high chemical stability in neutral and alkaline media, with no significant structural changes. However, in alkaline media, greater dissolution of silica was observed than in neutral media, the silica being derived from the most external tetrahedra attached to the OH- groups. Sepiolite is a hydrated Mg silicate, with the formula, according to Brauner & Preisinger (1956): Si12MgsO3o(OH)a(H20)4.SH20. Its structure is composed of sheets of silica tetrahedra joined by O ions to a central octahedral plane of Mg ions. The silica tetrahedra are inverted every six units, forming longitudinal channels of dimensions 0.36 x 1.06 nm. On the surface of the mineral, cracks in the most external Si-O-Si groups and formation of silanol groups are produced which confer adsorbent properties on the sepiolite. Adequate thermal and acid treatments can modify the channels (Serna et al., 1975; Fern~indez- Alvarez, 1978), the surface silanol groups, the Lewis and Br6nsted centres (Jim6nez-L6pez et al., 1978) etc., modifying in turn the structure, porosity and superficial activity of the sepiolite (Rodriguez- Reinoso et al., 1981; L6pez-Gonz~ilez et al., 198l). Abdul-Latif & Weaver (1969) studied the kinetics of acid dissolution of sepiolite and Corma et al. (1986) proved that at pH<3, the octahedral sheet was attacked, and on increasing the temperature and reaction time, there was also an attack on the tetrahedral silicate sheet. The silanol groups are capable of reacting with certain organic substances by forming covalent links and by modifying the structure and surface of the sepiolite (Serratosa, 1978; Hermosfn & Cornejo, 1986). As can be deduced from the above, there is a very broad and exhaustive knowledge of the behaviour and stability of sepiolite in acid media and under different thermal treatments. However, no data have been published on the behaviour of the sepiolite in an alkaline medium, such as in a system based on Portland cement or lime. The objective of this study was to understand the behaviour and stability of the Vallecas sepiolite in aqueous solutions of neutral and alkaline pH at room temperature. EXPERIMENTAL A sepiolite from Vallecas, Spain, supplied by Tolsa, was used. The material was mineralogically characterized by X-ray diffraction (XRD), X-ray fluorescence (XRF), infrared (IR) spectroscopy, and differential thermal/thermogravimetric analyses (DTA/TGA). Contents of Fe203, A1203, SIO2, CaO and MgO were determined by conventional chemical analyses, weight loss at 1000~ and insoluble residue in hot HC1 (1:5). The fibre length of the initial sepiolite and that of the sepiolite treated with water and basic media for 9 1996 The Mineralogical Society