Hardening mechanisms of an alkaline-activated class F fly ash Zhaohui Xie a , Yunping Xi b, * a China Building Materials Academy, Beijing, China b Department of Civil, Environmental and Architectural Engineering, University of Colorado, Campus Box 428, Boulder, CO 80309, USA Received 12 April 1999; accepted 12 June 2001 Abstract The hardening mechanism of a paste composed of a low calcium fly ash and alkali was investigated. It was found that a fraction of fly ash reacted with water-glass and formed amorphous or low-ordered crystalline compounds of the type of Na 2 O–Al 2 O 3 – SiO 2 , after the paste was cured at 60°C for 24 h. For the water-glass with a modulus of 1.64, the strength of the paste is mainly attributed to the gel-like reaction products that bind the particles of fly ash together. When the modulus is decreased to 1.0, crystalline sodium silicate is formed in the matrix, which helps to achieve high strengths. D 2001 Elsevier Science Ltd. All rights reserved. Keywords: Alkali-activated cement; Fly ash 1. Introduction There have been extensive efforts to convert fly ash into useful binding materials by means of alkaline activation. Two different processes have been developed so far [1–4]. One is a direct method, in which fly ash is mixed with certain activators and then the mixture is cured under a certain temperature to make solid materials. The other is an indirect method, in which the mixture of fly ash and activators is converted into cement clinker first, and then the cement can be used to make concrete. Comparatively, the first approach is much easier to apply mainly due to its simple processing technique and low treatment tempera- tures. In practice, no matter which method is used, the selection of proper activators is the most important part of the technology. The alkaline activators that have been used for activat- ing fly ash include Portland cement, lime, NaOH, NaCO 3 , and water-glass (sodium silicate solution). The effect of activation strongly depends on the physical–chemical nature of the fly ash and the type of activator. In general, fly ash is an acidic material containing acidic oxides such as Al 2 O 3 , SiO 2 , and Fe 2 O 3 , thus it possesses a potential to react with alkalis. It is believed that in the hardened mixture of fly ash and alkaline activators, there are gel- like compounds formed as well as hydrated phases with different degrees of crystallinity, e.g., thomosonite, hydro- nepheline, natrolite, zeolite and even hydrated silicates like C-S-H (II) gel [5,6]. Partly because of a wide variety of compositions of fly ash and the types of activators used, and partly because of the difficulty in direct observation of reaction products, the mineralogical composition of the reaction products in alkali-activated fly ash has not been studied in detail, and therefore, the hardening mechanisms of the pastes made of fly ash and activators have not been understood very well. In the previous work [3], it was found that using class F fly ash and water-glass, a high strength paste can be made with compressive strengths up to 8000 psi after curing at 60°C for 24 h. The control parameters that are particularly important for the paste have been identified, namely, the ratio of total activation chemicals to fly ash, the molar ratio of silica dioxide to sodium oxide (SiO 2 /Na 2 O, also known as the modulus of water-glass). In addition to the paste made of the fly ash and the activator, fine and coarse aggregates have also been added to the mixture to make concrete. In this case, it was found that the water to binder ratio (binder is the sum of water-glass and fly ash) is very important. Other control parameters are similar to those for Portland cement concrete, such as binder to aggregate ratio and coarse to fine aggregate ratio. * Corresponding author. Tel.: +1-303-492-8991; fax: +1-303-492- 7317. E-mail address: xiy@bechtel.colorado.edu (Y. Xi). Cement and Concrete Research 31 (2001) 1245 – 1249 0008-8846/01/$ – see front matter D 2001 Elsevier Science Ltd. All rights reserved. PII:S0008-8846(01)00571-3