Thermochimica Acta 444 (2006) 16–25 Thermogravimetric investigation on the chloride binding behaviour of MK–lime paste Nabajyoti Saikia ∗ , Shigeru Kato, Toshinori Kojima Department of Applied Chemistry, Faculty of Engineering, Seikei University, 3-3-1, Kichijoji Kitamachi, Musashino-shi, Tokyo 180-8633, Japan Received 1 August 2005; received in revised form 4 February 2006; accepted 8 February 2006 Available online 23 February 2006 Abstract The hydration products of 2.5, 5 and 10% Cl - containing metakaolin (MK)–lime pastes are compared with the same obtained from MK–lime paste to understand the chloride binding behaviour of MK during the hydration of cement. Results indicate that 2.5% Cl - addition into the MK–lime paste initially enhances the formation of Friedel’s salt (Ca 2 Al(OH) 6 Cl·2H 2 O), but Friedel’s salt decomposes at later stages due to the formation of stratlingite (C 2 ASH 8 ). In 5 and 10% chloride containing pastes, Friedel’s salt is observed throughout the reaction periods along with the high amount of CSH. Small amount of stratlingite is also formed on or after 60 day hydration of 5% Cl - containing MK–lime pastes. On the other hand, MK–lime-10% Cl - containing pastes show the complete absence of stratlingite and C 4 AH 13 through out the hydration period, which are the major hydration products of MK–lime paste. Mesuarements of pH of the simulated pore fliuds help to understand the decomposition behaviour of Friedel’s salt. From the experimental results, chloride binding mechanism of MK–lime paste is also discussed. © 2006 Elsevier B.V. All rights reserved. Keywords: Hydration; Metakaolin–lime; Chloride; Friedel’s salt; TG-DTA; XRD Cement chemistry notations C CaO S SiO 2 A Al 2 O 3 H H 2 O C 2 ASH 8 2CaO·Al 2 O 3 ·SiO 2 ·8H 2 O CSH CaO SiO 2 H 2 O C 4 AH 13 4CaO·Al 2 O 3 ·13H 2 O C 3 AH 8 3CaO·Al 2 O 3 ·8H 2 O CH CaO·H 2 O or [Ca(OH) 2 ] 1. Introduction Ingress of chloride ion into the reinforced concrete and cor- responding corrosion behaviour is one of the major problems in cement and building material industry. It is found that the addition of pozzolanic materials like metakaolin (MK), FA can reduce the ingress of chloride by improving the microstruc- ture and chloride binding behaviour [1–3]. In hardened cement, ∗ Corresponding author. Tel.: +81 422 37 3750; fax: +81 422 37 3871. E-mail address: saikianj@yahoo.co.uk (N. Saikia). chloride may be bonded in the CSH gel or as a formation of com- plex calcium oxychloride, Friedel’s salt (Ca 2 Al(OH) 6 Cl·2H 2 O) [4,5]. It is reported earlier that the MK-Portland cement paste can bind considerable amount of dissolved chloride present in pore water with relatively low reduction in pH due to the formation of stable Friedel’s salt. MK removes chloride and hydroxide ions from solution and also maintains a low [Cl - /OH - ] ratio, which is important from concrete corrosion point of view [6,7]. Again one major problem on the utilization of high chloride bearing wastes like municipal solid waste incineration (MSWI) ash as pozzolanic material or as a raw material for production of cement is the presence of high amounts of chloride salts [8]. However the amount of chloride can be reduced by mixing these mate- rials or by blending produced waste based cement with other pozzolanic materials [9]. In this context, MK can be used to prepare this type of pozzolana or as a pozzolanic additive in waste derived cement, which can improve the chloride binding behaviour of cement. The knowledge of chloride binding behaviour of cement based system containing MK or other similar types of pozzolana will therefore help to understand the chloride binding behaviour of these pozzolanic materials in concrete as well as help to develop some new class of pozzolanic materials for utilization 0040-6031/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.tca.2006.02.012