The influence of sodium and potassium hydroxide on alite hydration: Experiments and simulations Aditya Kumar a, ⁎, Gaurav Sant b, c , Cedric Patapy a , Caterina Gianocca a , Karen L. Scrivener a a Laboratory of Construction Materials, Ecole Polytechnique Fédérale de Lausanne, Ecublens, Lausanne, Switzerland b Laboratory for the Chemistry of Construction Materials, Department of Civil and Environmental Engineering, University of California, Los Angeles, CA, USA c California Nanosystems Institute (CNSI), University of California, Los Angeles, CA, USA abstract article info Article history: Received 7 April 2012 Accepted 23 July 2012 Keywords: Hydration (A) Alkalis (D) Simulations The basic nature of alkali hydroxides (NaOH, KOH) when added to mixing water, increases the pH in proportion to the level of salt addition. For alite (impure tricalcium silicate; MIII-Ca 3 SiO 5 ) hydration, this pH increase accel- erates the rate of hydration and reduces the duration of the induction, acceleration and deceleration regimes. This study evaluates alite hydration in solutions of varying compositions and alkalinities (0.1 M, 0.2 M and 0.5 M NaOH and KOH) in context of their heat release behavior and analysis of the solid/liquid phases. The modeling platform, μic, is used to simulate, describe and discriminate the impact of the pore solution chemistry and reaction product formation parameters on alite hydration (Bishnoi and Scrivener, 2009 [1]). Numerical predictions of the solid and liquid phase compositions and the heat release response show good agreement with experimental determinations. The simulations indicate that the effects up to the end of the induction period follow directly from a change in the pore solution composition under a solution controlled dissolution mecha- nism, which leads to the faster precipitation of portlandite. The changes in the main heat evolution peak appear to be related to an increase in the nucleation density of C–S–H in alkali hydroxide solutions. Examination under the SEM did not indicate significant difference in C–S–H morphology and composition in the presence of NaOH/ KOH. © 2012 Elsevier Ltd. All rights reserved. 1. Introduction and background Past research indicates that alkalis enhance the rate of hydration of alite (as assessed using isothermal calorimetry) [2–5]. It has been reported that while alkali additions enhance the degree of reaction at short time scales (first few days), at longer time scales (28 days) the degree of hydration is similar [2]. Acceleration by alkalis has been attributed to enhancements in the rate of dissolution and precipitation of the reactant/product phases in relation to the chemistry of the contacting solution (i.e., the Ca 2+ , Na + ,K + , Si 4+ , OH - abundance) and the common-ion effect [6–10] or alterations in the composition or morphology of the hydration products [11–14]. However, most of the studies conducted in the past have been qualitative in nature. In this study experimental observations of alite hydration in the presence of NaOH and KOH in the mixing water are compared with computer simulations. Simula- tions are carried out using the μic modeling platform, incorporating recent theories of hydration [1,15–17]. 2. Materials and methods Two batches of alite (MIII-Ca 3 SiO 5 [18]) were synthesized as described by Costoya [19]. The precursor minerals (i.e., CaCO 3 , SiO 2 , Al 2 O 3 and MgO) were mixed in distilled water to prepare a slurry which was mixed for 24 h in a ball mill with Zirconium balls. The mixture was then oven dried at 110 °C for 5 days. Pellets were pressed, and then burnt in a furnace as follows: 25 °C to 1550 °C in 8 h; 12 h at 1550 °C; air quenching from 1550 °C. Finally, the burnt pellets were ground using a ring grinder followed by sieving to generate two different particle size distributions (PSDs) namely Batch A and Batch B. The particle size distributions (PSDs) are shown in Fig. 1. On the basis of the particle size analysis, the specific surface area of Batch A and Batch B was determined to be 276.0 m 2 /kg and 192.3 m 2 /kg respectively; assuming a spherical particle geometry. Batch A was used for calorimetric and thermogravimetric analyses (TGA) while Batch B was used for pore solution analyses. Next, scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM-EDS) was carried out on paste samples from both batches. As explained below, the two alite batches hydrate equivalently, when differences in their PSDs are taken into account. Alite pastes were prepared at a constant water-to-solids ratio (w/s) of 0.35. The solids were combined with de-ionized water or the appro- priate aqueous solution of NaOH or KOH (AR-grade) and mixed using Cement and Concrete Research 42 (2012) 1513–1523 ⁎ Corresponding author. E-mail addresses: a.kumar@epfl.ch (A. Kumar), gsant@ucla.edu (G. Sant). 0008-8846/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.cemconres.2012.07.003 Contents lists available at SciVerse ScienceDirect Cement and Concrete Research journal homepage: http://ees.elsevier.com/CEMCON/default.asp