Effects of decalcification on the microstructure and surface area of cement and tricalcium silicate pastes Jeffrey J. Thomas a, * , Jeffrey J. Chen b , Andrew J. Allen c , Hamlin M. Jennings a,b a Department of Civil and Environmental Engineering, Northwestern University, Evanston, IL 60208, USA b Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208, USA c Materials Science and Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA Received 12 November 2003; accepted 8 April 2004 Abstract Thin coupons of white portland cement (WPC) and tricalcium silicate paste were decalcified by leaching in concentrated ammonium nitrate solutions, resulting in calcium-to-silicon molar ratios (C/S) ranging from 3.0 (control) down to 0.3. The microstructure and surface area were measured using both small-angle neutron scattering (SANS) and nitrogen gas sorption. The intensity in the SANS data regime corresponding to the volume fractal C-S-H gel phase increased significantly on leaching, and the total surface area per unit specimen volume measured by SANS doubled on leaching from C/S = 3.0 to near C/S = 1.0. The nitrogen BET surface area of the WPC pastes, expressed in the same units, increased on decalcification as well, although not as sharply. The primary cause of these changes is a transformation of the high-density ‘‘inner product’’ C-S-H gel, which normally has a low specific surface area as measured by SANS and nitrogen gas sorption, into a morphology with a high specific surface area. The volume fractal exponent corresponding to the C-S- H gel phase decreased with decalcification from 2.3 to 2.0, indicating that the equiaxed 5 nm C-S-H globule building blocks that form the volume fractal microstructure of normal, unleached cement paste are transformed by decalcification into sheetlike structures of increasing thickness. D 2004 Elsevier Ltd. All rights reserved. Keywords: Calcium silicate hydrate (C-S-H); Microstructure; Small-angle neutron scattering; Surface area; Degradation 1. Introduction Decalcification of cement paste is closely associated with various types of concrete deterioration. Important examples include leaching by exposure to flowing fresh water or weak acids, which removes calcium from the paste altogether, and external sulfate attack, which reor- ganizes the cement paste in such a way that the calcium in portlandite [Ca(OH) 2 = CH 1 ] and in C-S-H gel is transferred to gypsum and ettringite. The addition of mineral admixtures, such as silica fume and blast furnace slag, to cement paste causes a similar reorganization, with a lower Ca form of C-S-H gel forming at the expense of CH and the typical high-Ca form of C-S-H gel. In recent years, the kinetics of calcium leaching [1,2] and the effects of decalcification on the mechanical properties of cement paste [3–6], the solubility of C-S-H phases [7], and the chemical structure of C-S-H [8–10] have been studied and modeled. What is missing at present is a full understanding of the effects of decalcification, or changes in the calcium-to-silicon ratio (C/S), on the morphology [11] and nanometer-level structure of C-S-H. This paper reports results of small-angle neutron scattering (SANS) and nitrogen gas sorption experiments on tricalcium silicate (C 3 S) and white portland cement (WPC) pastes leached to different C/S. An experimental difficulty with leaching studies is the very long time required to leach in fresh water. Leaching cement pastes in concentrated solutions of NH 4 NO 3 can reduce leaching times from weeks or months to hours. The increased rate of calcium dissolution in NH 4 NO 3 can be attributed to a significant increase in the equilibrium calcium solubility. For example, Heukamp et al. [5] have estimated that the equilibrium calcium concentration of a solution saturated in CH increases from 0.022 mol/l in 0008-8846/$ – see front matter D 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.cemconres.2004.04.007 * Corresponding author. E-mail address: jthomas@northwestern.edu (J.J. Thomas). 1 Cement chemistry notation: C = CaO, S = SiO 2 , H=H 2 O, A=Al 2 O 3 , F=Fe 2 O 3 . Cement and Concrete Research 34 (2004) 2297 – 2307