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Title: A multi-technique investigation of the nanoporosity of cement paste

Abstract

The nanometer-scale structure of cement paste, which is dominated by the colloidal-scale porosity within the C-S-H gel phase, has a controlling effect on concrete properties but is difficult to study due to its delicate structure and lack of long-range order. Here we present results from three experimental techniques that are particularly suited to analyzing disordered nanoporous materials: small-angle neutron scattering (SANS), weight and length changes during equilibrium drying, and nanoindentation. Particular attention is paid to differences between pastes of different ages and cured at different temperatures. The SANS and equilibrium drying results indicate that hydration of cement paste at 20 deg. C forms a low-density (LD) C-S-H gel structure with a range of gel pore sizes and a relatively low packing fraction of solid particles. This fine structure may persist indefinitely under saturated conditions. However, if the paste is dried or is cured at elevated temperatures (60 deg. C or greater) the structure collapses toward a denser (less porous) and more stable configuration with fewer large gel pores, resulting in a greater amount of capillary porosity. Nanoindentation measurements of pastes cured at different temperatures demonstrate in all cases the existence of two C-S-H structures with different characteristic values of themore » indentation modulus. The average value of the modulus of the LD C-S-H is the same for all pastes tested to date, and a micromechanical analysis indicates that this value corresponds to the denser and more stable configuration of LD C-S-H. The experimental results presented here are interpreted in terms of a previously proposed quantitative 'colloid' model of C-S-H gel, resulting in an improved understanding of the microstructural changes associated with drying and heat curing.« less

Authors:
 [1];  [2];  [3];  [4];  [5];  [5]
  1. Department of Civil and Environmental Engineering, Northwestern University, Evanston, IL 60208 (United States) and Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208 (United States). E-mail: h-jennings@northwestern.edu
  2. Department of Civil and Environmental Engineering, Northwestern University, Evanston, IL 60208 (United States). E-mail: jthomas@northwestern.edu
  3. Department of Civil and Environmental Engineering, Northwestern University, Evanston, IL 60208 (United States)
  4. (United States)
  5. Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139 (United States)
Publication Date:
OSTI Identifier:
20995369
Resource Type:
Journal Article
Resource Relation:
Journal Name: Cement and Concrete Research; Journal Volume: 37; Journal Issue: 3; Conference: International Conference on cementitious materials as model porous media: Nanostructure and transport processes, Centro Monte Verita (Switzerland), 17-22 Jul 2005; Other Information: DOI: 10.1016/j.cemconres.2006.03.021; PII: S0008-8846(06)00084-6; Copyright (c) 2006 Elsevier Science B.V., Amsterdam, The Netherlands, All rights reserved; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; CALCIUM SILICATES; CEMENTS; CONCRETES; DRYING; EQUILIBRIUM; FINE STRUCTURE; GELS; HYDRATES; HYDRATION; MICROSTRUCTURE; NANOSTRUCTURES; NEUTRON DIFFRACTION; POROSITY; POROUS MATERIALS; SMALL ANGLE SCATTERING; TEMPERATURE RANGE 0273-0400 K

Citation Formats

Jennings, Hamlin M., Thomas, Jeffrey J., Gevrenov, Julia S., Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208, Constantinides, Georgios, and Ulm, Franz-Josef. A multi-technique investigation of the nanoporosity of cement paste. United States: N. p., 2007. Web. doi:10.1016/j.cemconres.2006.03.021.
Jennings, Hamlin M., Thomas, Jeffrey J., Gevrenov, Julia S., Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208, Constantinides, Georgios, & Ulm, Franz-Josef. A multi-technique investigation of the nanoporosity of cement paste. United States. doi:10.1016/j.cemconres.2006.03.021.
Jennings, Hamlin M., Thomas, Jeffrey J., Gevrenov, Julia S., Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208, Constantinides, Georgios, and Ulm, Franz-Josef. Thu . "A multi-technique investigation of the nanoporosity of cement paste". United States. doi:10.1016/j.cemconres.2006.03.021.
@article{osti_20995369,
title = {A multi-technique investigation of the nanoporosity of cement paste},
author = {Jennings, Hamlin M. and Thomas, Jeffrey J. and Gevrenov, Julia S. and Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208 and Constantinides, Georgios and Ulm, Franz-Josef},
abstractNote = {The nanometer-scale structure of cement paste, which is dominated by the colloidal-scale porosity within the C-S-H gel phase, has a controlling effect on concrete properties but is difficult to study due to its delicate structure and lack of long-range order. Here we present results from three experimental techniques that are particularly suited to analyzing disordered nanoporous materials: small-angle neutron scattering (SANS), weight and length changes during equilibrium drying, and nanoindentation. Particular attention is paid to differences between pastes of different ages and cured at different temperatures. The SANS and equilibrium drying results indicate that hydration of cement paste at 20 deg. C forms a low-density (LD) C-S-H gel structure with a range of gel pore sizes and a relatively low packing fraction of solid particles. This fine structure may persist indefinitely under saturated conditions. However, if the paste is dried or is cured at elevated temperatures (60 deg. C or greater) the structure collapses toward a denser (less porous) and more stable configuration with fewer large gel pores, resulting in a greater amount of capillary porosity. Nanoindentation measurements of pastes cured at different temperatures demonstrate in all cases the existence of two C-S-H structures with different characteristic values of the indentation modulus. The average value of the modulus of the LD C-S-H is the same for all pastes tested to date, and a micromechanical analysis indicates that this value corresponds to the denser and more stable configuration of LD C-S-H. The experimental results presented here are interpreted in terms of a previously proposed quantitative 'colloid' model of C-S-H gel, resulting in an improved understanding of the microstructural changes associated with drying and heat curing.},
doi = {10.1016/j.cemconres.2006.03.021},
journal = {Cement and Concrete Research},
number = 3,
volume = 37,
place = {United States},
year = {Thu Mar 15 00:00:00 EDT 2007},
month = {Thu Mar 15 00:00:00 EDT 2007}
}
  • Several open questions related to the experimental protocol and processing of data acquired by the nano-indentation (NI) technique are investigated. The volume fractions of mechanically different phases obtained from statistical NI (SNI) analysis are shown to be different from those obtained by back-scattered electron (BSE) image analysis and X-ray diffraction (XRD) method on the same paste. Judging from transmission electron microscope (TEM) images, the representative volume element of low-density calcium-silicate hydrates (C-S-H) can be considered to be around 500 nm, whereas for high-density C-S-H it is about 100 nm. This raises the question how the appropriate penetration depth for NImore » experiments should be selected. Changing the maximum load from 1 mN to 5 mN, the effect of penetration depth on the experimental results is studied. As an alternative to the SNI method, a 'manual' indentation method is proposed, which combines information from BSE and atomic-force microscopy (AFM), coupled to the NI machine. The AFM allows to precisely indent a high-density C-S-H rim around unhydrated clinkers in cement paste. Yet the results from that technique still show a big scatter.« less
  • We propose a new method to estimate the initial cement content, water content and free water/cement ratio (w/c) of hardened cement-based materials made with Portland cements that have unknown mixture proportions and degree of hydration. This method first quantifies the composition of the hardened cement paste, i.e. the volumetric fractions of capillary pores, hydration products and unreacted cement, using high-resolution field emission scanning electron microscopy (FE-SEM) in the backscattered electron (BSE) mode and image analysis. From the obtained data and the volumetric increase of solids during cement hydration, we compute the initial free water content and cement content, hence themore » free w/c ratio. The same method can also be used to calculate the degree of hydration. The proposed method has the advantage that it is quantitative and does not require comparison with calibration graphs or reference samples made with the same materials and cured to the same degree of hydration as the tested sample. This paper reports the development, assumptions and limitations of the proposed method, and preliminary results from Portland cement pastes with a range of w/c ratios (0.25-0.50) and curing ages (3-90 days). We also discuss the extension of the technique to mortars and concretes, and samples made with blended cements.« less
  • The distribution of paste-void spacing in cement-based materials is an important feature related to the freeze-thaw durability of these materials, but its reliable estimation remains an unresolved problem. Herein, we evaluate the capability of X-ray computed tomography (CT) for reliable quantification of the distribution of paste-void spacing. Using X-ray CT images of three mortar specimens having different air-entrainment characteristics, we calculate the distributions of paste-void spacing of the specimens by applying previously suggested methods for deriving the exact spacing of air-void systems. This methodology is assessed by comparing the 95th percentile of the cumulative distribution function of the paste-void spacingmore » with spacing factors computed by applying the linear-traverse method to 3D air-void system and reconstructing equivalent air-void distribution in 3D. Results show that the distributions of equivalent void diameter and paste-void spacing follow lognormal and normal distributions, respectively, and the ratios between the 95th percentile paste-void spacing value and the spacing factors reside within the ranges reported by previous numerical studies. This experimental finding indicates that the distribution of paste-void spacing quantified using X-ray CT has the potential to be the basis for a statistical assessment of the freeze-thaw durability of cement-based materials. - Highlights: Black-Right-Pointing-Pointer The paste-void spacing in 3D can be quantified by X-ray CT. Black-Right-Pointing-Pointer The distribution of the paste-void spacing follows normal distribution. Black-Right-Pointing-Pointer The spacing factor and 95th percentile of CDF of paste-void spacing are correlated.« less