Influence of particle size on the early hydration of slag particle activated by Ca(OH) 2 solution Zhijun Tan a,⇑ , Geert De Schutter a , Guang Ye a,b , Yun Gao a , Lieven Machiels c a Magnel Laboratory for Concrete Research, Department of Structural Engineering, Ghent University, Technologiepark-Zwijnaarde 904, B-9052 Ghent, Belgium b Microlab, Faculty of Civil Engineering and Geosciences, Delft University of Technology, 2628 CN Delft, The Netherlands c Centre for High Temperature Processes and Sustainable Materials Management, Department Metallurgy and Materials Engineering, KU Leuven, Kasteelpark Arenberg 44, B-3001 Leuven, Belgium highlights  Hydration and chemical compositions of slag were measured according to different size fractions.  The calculation of hydration speed was conducted at particle level and based on measured particle size distribution.  The hydration speed of slag particle is denoted as the rate of increase of the hydrating layer thickness.  Higher k value of slag particle arises from its chemical composition. article info Article history: Received 2 October 2013 Accepted 20 November 2013 Keywords: Blast-furnace slag Particle size distribution Reactivity Hydration degree abstract This paper investigates the influence of slag particle size on its hydration speed at particle level in the early age. Slags were separated with sieves into groups of different size fractions, considering a wide range of sizes. The chemical compositions of each group were analyzed by X-ray Fluorescence (XRF). Activated by 15% Ca(OH) 2 (by mass) at water/powder ratio 1:1, the hydration heat evolution was recorded by isothermal calorimetry up to 84 h and converted to hydration degree. Based on the hydration degrees and particle size distributions, the rate of increase of hydrating layer thickness of each single slag particle (k value) was calculated. Results reveal that k values of coarse particles are higher than that of fine particles. Coarse particles contain higher content of CaO but relatively lower content of MgO, Al 2 O 3 and SiO 2 , resulting in higher reactivity index of (CaO + Al 2 O 3 + MgO)/SiO 2 . Ó 2013 Published by Elsevier Ltd. 1. Introduction Ground granulated blast-furnace slag (GGBFS) is widely used as construction material due to its latent cementitious property when mixing with lime, alkali-hydroxides or Portland cement. To im- prove the use of the slag in cement industry, a better understand- ing is needed of its hydration process. Modeling the hydration of cementitious materials improves this understanding [1]. It is of paramount importance to know the reactivity of slag when simu- lating the hydration process of slag. The reactivity of slag is usually estimated by the overall hydration rate or overall kinetics of slag hydration. Investigation [2] shows that the overall reactivity of slag is generally influenced by its glass content chemistry and particle size distribution (PSD). Additionally, process conditions also deter- mine its reactivity, such as the process curing temperature, water/ solids ratio and the ratio of slag to cement [3]. From the standpoint of overall kinetics, it is obvious that slag made up of fine particles reacts faster than that of coarser particles, since the former has significantly larger specific surface area than the latter. The hydration of a single slag particle takes place by moving the reaction front inwards, thus the rate of increase of the hydrating layer thickness of a single particle is worthwhile to know, which is applied as input by van Breugel [4] in HYMOSTRUC to simulate the hydration of Portland cement. However, less attention has been paid to the influence of slag particle size on this parameter at particle level. Results of Sato et al. [5] and Chen et al. [6] showed that the thickness of the hydrating layer of a slag particle is not significantly influenced by its particle size. It should be noted that the particle sizes of the slag used in their experiments were in a narrow range, viz. from 3 to 13 lm, while the particle size of slag used in cement industry var- ies between micrometer and several hundred micrometers. There- fore, it is necessary to investigate the influence of slag particle size on its rate of increase of the hydrating layer thickness at a wider size range. 0950-0618/$ - see front matter Ó 2013 Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.conbuildmat.2013.11.073 ⇑ Corresponding author. Tel.: +32 488471548. E-mail address: zhijun.tan@ugent.be (Z. Tan). Construction and Building Materials 52 (2014) 488–493 Contents lists available at ScienceDirect Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat