Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Direct Transmission Electron Microscopy Visualization of the Cement Reaction by Colloidal Aggregation of Fumed Silica Hisao Satoh,*,† Yuki Kimura,*,‡ and Erika Furukawa§ †
Naka Energy Research Laboratory, Mitsubishi Materials Corporation, 1002-14 Mukoyama, Naka, Ibaraki 311-0102, Japan Institute of Low Temperature Science, Hokkaido University, Kita-19, Nishi-8, Kita-ku, Sapporo, Hokkaido 060-0819, Japan § Department of Earth Science, Graduate School of Science, Tohoku University, 6-3 Aramaki Aza Aoba, Aoba-ku, Sendai, Miyagi 980-8578, Japan ‡
S Supporting Information *
ABSTRACT: Unlike other solidified materials, concrete has an internal structure that contains small pores that vary in size from several nanometers to the submicrometer range. These pores govern the strength of the material and hence the lifetime of concrete buildings. The size of the pores is governed by the presence of calcium silicate hydrate (CaO·2SiO2·4H2O, C−S−H). To identify how the pore size is determined during the concretion process, we used transmission electron microscopy to examine the early solidification of a simplified reaction system consisting of silica, portlandite [Ca(OH)2, CH], and water. The silica particles, used to suppress the degradation of concrete, expanded and, consequently, embedded the pores through hydration before C−S−H was formed, contrary to the predicted decrease in pore size by C−S−H formation after simple dissolution of silica. Visualization of this type of solidification process should permit improvement in the mechanical strength of concrete.
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INTRODUCTION Conventional concrete materials may degrade as a result of volume expansion due to unavoidable alkali−silica reactions. The concrete reaction of ordinary Portland cement (OPC) is initiated by the production of solutions of alkalies such as potassium hydroxide (KOH), sodium hydroxide (NaOH), or portlandite [Ca(OH)2, CH] through hydration of clinker materials present in the cement, and it proceeds by subsequent attack by the alkaline solution on silicate structures present in the concrete. This reaction produces alkali silicate gels [e.g., (K,Na)2SiO3] that equilibrate with calcium ions (Ca2+) to form precipitates of calcium silicate hydrate (CaO·2SiO2·4H2O, C− S−H).1 However, when alkali-metal ions are present, the alkali silicate gel can subsequently re-form. The increase in volume resulting from the formation of this gel then raises the pressure in the concrete, resulting in deformation. This degradation of a concrete can be suppressed by the addition of fumed silica (SiO2),2 which consists of amorphous nanoparticles. Fumed silica is used industrially as a filler for concrete materials because of the high fluidity of its colloidal suspension and its reactivity. In industry, fumed silica is obtained as a byproduct from the production of elemental Si or SiC. In the presence of portlandite, fumed silica is hydrated, with consumption of hydroxide ion (OH−), resulting in the formation orthosilicic acid (H4SiO4); this saturates C−S−H more effectively than do other silicates and consequently suppresses the production of alkali silicate gel.3−5 The C−S−H phase is mainly responsible for controlling the mechanical strength of cured OPC6 and thereby the lifetime of buildings. A © XXXX American Chemical Society
recent detailed study revealed that C−S−H consists structurally of a hydration layer of CH and a tobermorite-like silicate sheet.7 The time scale for the production and precipitation of C−S− H during the cement reaction is crucial in controlling the mechanical strength of cured OPC, particularly in relation to the pore spacing of cast concrete. The methods that are generally used for direct observation of the hydration reaction of cement clinker are atomic force microscopy (for surface reactions)8 and optical microscopy techniques such as laser confocal microscopy.9 However, the latter techniques are limited in their ability to observe submicrometer-scale particles, such as fumed silica or fly ash, because their resolution is limited by the wavelength of visible light. The use of fumed silica, an effective pozzolanic material, and CH provides a simplified model for the pozzolanic reaction and for the mechanisms operating in the solidifying material. Here, we extend our recent in situ transmission electron microscopy (TEM) technique using a liquid cell10,11 to examine a concrete consisting of silica and CH to obtain direct evidence regarding the early solidification process in the aqueous reaction system.
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RESULTS AND DISCUSSION After conventional observation of unreacted fumed silica particles under vacuum (run VAC-1; Figure S1, Supporting Received: Revised: Accepted: Published: A
October 4, 2017 December 8, 2017 December 12, 2017 December 12, 2017 DOI: 10.1021/acs.iecr.7b04137 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
portlandite and the absence of C−S−H precipitation. After 60 min, many islands of an unknown phase appeared on the silica surface (Figure 1B), and a small amount of C−S−H first became visible after 90 min; the amount of C−S−H became significant after 180 min (Figure 1C). C−S−H veils and strings then covered the spheres as aggregates (Figure 1E,F). The islands that appeared early (shown by white arrowheads in Figure 1B) cannot consist of dried-up salt such as portlandite, but might possibly be thin films of C−S−H with a C/S ratio of
