Introduction Portland cement based materials when subjected to the attack of external magnesium sulphate may be seriously damaged due to the interaction between the hydrated cement paste and the magnesium sulphate solution [1, 2]. The interaction of magnesium sul- phate with the calcium hydroxide (CH) present in the set cement paste generates magnesium hydroxide (brucite – Mg(OH) 2 ) and calcium sulphate (gypsum – CaSO 4 ×2H 2 O). The replacement of Ca by Mg in the hydrated calcium silicates (C–S–H) leads to the formation of M–S–H [2, 3]. These reactions can occur simultaneously causing expansion, cracking or spalling, softening and disintegration of the cementitious material [4]. Sulphate attack can be controlled using low-C3A portland cement and low water/cement ratios. The use the mineral admixtures such as metakaolin improves its sulphate resistance due to the reduction of matrix permeability and consume of the CH formed during calcium silicates hydration [5, 6]. The evaluation of the damage caused by the sulphate attack in cementitious materials are based on visual examination, changes in the mechanical prop- erties or expansion of cement paste or mortars immersed in sulphate solutions [7, 8]. Analytical methods such as X-ray diffraction [9], scanning electron microscopy [10, 11] transmission X-ray microscopy [12] and X-ray microtomography [13] have also been used to evaluate the evidences of the magnesium sulphate attack on cement based materials but they can provide only qualitative information. The characterization of the products of magnesium sulphate attack (brucite, gypsum and M–S–H) by thermal analysis can give quantitative information on the magnitude of the sulphate attack and its combination with mechanical tests consists in a powerful methodology to determine the extent of the chemical and mechanical damage in cement based systems. In the present paper this ap- proach is used to study the resistance to magnesium sulphate attack of mortar mixtures containing 0, 10 and 20% of metakaolin (MK) as Portland cement replace- ment (by mass). Experimental Materials This study made use of a Portland cement, class CPII-F32 under Brazilian standards [14] (defined as containing: 85 mass%<clinker£91 mass%; 3 mass%< gypsum<5 mass%; 6 mass%<filler< 10 mass%) and giving 32 MPa compressive strength at 28 days in the standard test. Its main chemical and physical charac- teristics are presented in Table 1. The TG and DTG curves of the Portland cement are presented in Fig. 1a. It can be seen that the cement presents a slight mass loss between 380–440°C due to the dehydroxylation of the calcium hydroxide, which may be formed from the hydration of the free lime present in the clinker. Between 500–730°C the mass 1388–6150/$20.00 Akadémiai Kiadó, Budapest, Hungary © 2008 Akadémiai Kiadó, Budapest Springer, Dordrecht, The Netherlands Journal of Thermal Analysis and Calorimetry, Vol. 94 (2008) 2, 511–516 EVALUATION OF MAGNESIUM SULPHATE ATTACK IN MORTAR- METAKAOLIN SYSTEM BY THERMAL ANALYSIS J. P. Gon¸alves 1* , R. D. Toledo Filho 2 and E. M. R. Fairbairn 2 1 Universidade Estadual de Feira de Santana, Technology Deparment, Km 03, BR 116, Campus Universitário, Feira de Santana Bahia, Zip Code 44031-460, Brazil 2 COPPE/UFRJ – Universidade Federal do Rio de Janeiro, Civil Engineering Department, Brazil This paper reports an experimental study on the magnesium sulphate resistance of mortar specimens incorporating 0, 10 and 20% of metakaolin (MK). The evidence of the attack was evaluated through the content of calcium hydroxide (portlandite) and formation of magnesium hydroxide (brucite) by thermal analysis (thermogravimetric and derivative thermogravimetric analysis). The mechan- ical degradation of the mortar specimens was evaluated through splitting tensile tests after 200 days of exposition to the magnesium solution. The addition of metakaolin resulted in a reduction in the content of calcium hydroxide and in a smaller formation of brucite in comparison with reference mixture. A tensile strength loss of about 7% was observed for the metakaolin mortars submitted to the magnesium solution attack for 200 days. Keywords: metakaolin, mortar, sulphate attack, TG/DTG * Author for correspondence: jardelpg@gmail.com