Synthesis of belite cement from lignite fly ash K. Pimraksa a, * , S. Hanjitsuwan a , P. Chindaprasirt b a Department of Industrial Chemistry, Faculty of Science, Chiang Mai University, 50200, Thailand b Department of Civil Engineering, Faculty of Engineering, Khon Kaen University, 40002, Thailand Received 1 October 2008; received in revised form 11 December 2008; accepted 3 February 2009 Available online 25 February 2009 Abstract Synthesis of belite cement from lignite fly ash is studied as it can be produced using low temperature between 750 and 1200 8C leading to energy saving and low carbon dioxide emission. Two synthesis methods viz., clinkerization and hydrothermal processes assisted by calcinations are studied. Lignite fly ash is used as a main starting material. For the clinkerization process, the firing temperatures, types of additives and calcium oxide/silicon dioxide ratios (Ca/Si) are studied. In this process, the reaction between fly ash and calcium carbonate produces gehlenite (2CaOÁAl 2 O 3 ÁSiO 2 ) which is undesirable due to its poor hydraulic property. A slightly higher belite (2CaOÁSiO 2 ) phase is obtained using sulfate ion as a dopant and using high Ca/Si ratio. The strength of gehlenite bearing belite cement is, however, rather poor. For the hydrothermal– calcination process, the alkaline concentrations and calcining temperatures are studied. The final products are belite phase and mayenite (12CaOÁ7Al 2 O 3 ) which are desirable as they possess hydraulic properties. The reasonable 28-day compressive strength of the belite cement mortar of 9.5 MPa is obtained. The hydrothermal process assisted by calcination is, therefore, suitable for use in the synthesis of belite cement from lignite fly ash. # 2009 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: A. Calcination; C. Strength; Lignite fly ash; Solid-state reaction; Dissolution-precipitation 1. Introduction The Portland cement (PC) manufacturing industry has been facing the pressure to reduce energy consumption and CO 2 emissions [1]. Some estimates put the CO 2 emissions from cement industry as high as 5% of global emissions [2]. Cement with high proportion of belite (C 2 S) is receiving more attention as it can be manufactured using lower LSF (lime saturation factor), lower carbonate content and lower burning temperature resulting in less CO 2 emission in comparison to PC. The belite cement phase of dicalcium silicate does not contribute to the early strength owing to its slow hydraulic activity [3]. However, the long-term strength can reach the same level or even higher than that of PC. The early strength is improved by adding calcium sulfoaluminate (C 4 A 3 S*) to form ettringite (C 6 AS 3 *H 32 ) or inorganic salt accelerators such as CaCl 2 , Ca(NO 3 ) 2 and K 2 CO 3 [4]. Belite phase is generally produced by clinkerization and hydrothermal processes. The clinkerization is controlled by solid-state reaction at high temperature over 800 8C [5]. Belite is formed when CaCO 3 dissociates to free CaO around 750–850 8C and afterwards reacts with free SiO 2 . The reactivity and particle size of starting materials and firing temperature are very crucial for the synthesis. In general the system of CaO–SiO 2 easily provides belite phases. Belite has five allotropic forms: a, a 0 H , a 0 L , b and g [6]. The b-form is metastable under 500 8C and has hydraulic property while g-form is hydraulically inactive. The others are stabilized at room temperature with the addition of foreign ions in the crystal structure. The reactive belite phase can be obtained when a faster cooling and some cationic and anionic dopants such barium, boron and sulfate ions are used in the synthesis. For example, BaCl 2 is used to maintain belite in a reactive form of b-C 2 S at room temperature. In addition to stabilizing reactive belite phase, sulfate compounds are also used to attack AlO 4 5À group from aluminosilicate phases [7,8]. Hydrothermal process is controlled by dissolution-precipi- tation. Hydrothermal processing is widely used for C–S–H and calcium aluminate hydrate (C–A–H) preparations. Fly ash contains glassy phase and high silica content with pozzolanic property. It can dissolve in lime solution and precipitate to form calcium silicate hydrate (C–S–H). The obtained products www.elsevier.com/locate/ceramint Available online at www.sciencedirect.com Ceramics International 35 (2009) 2415–2425 * Corresponding author. Tel.: +66 53 943404; fax: +66 53 892262. E-mail address: kpimrakp@science.cmu.ac.th (K. Pimraksa). 0272-8842/$34.00 # 2009 Elsevier Ltd and Techna Group S.r.l. All rights reserved. doi:10.1016/j.ceramint.2009.02.006