Relationships between Composition and Density of Tobermorite

Department of Civil and Environmental Engineering, Northwestern University, ... and Engineering Laboratory, National Institute of Standards and Techno...
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J. Phys. Chem. C 2010, 114, 7594–7601

Relationships between Composition and Density of Tobermorite, Jennite, and Nanoscale CaO-SiO2-H2O Jeffrey J. Thomas,*,† Hamlin M. Jennings,‡ and Andrew J. Allen§ Department of CiVil and EnVironmental Engineering, Northwestern UniVersity, EVanston, Illinois 60208, Department of Materials Science and Engineering, Northwestern UniVersity, EVanston, Illinois 60208, and Materials Science and Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 ReceiVed: NoVember 11, 2009; ReVised Manuscript ReceiVed: March 8, 2010

Relationships between composition, mass density, and atomic packing density for CaO-SiO2-H2O (C-S-H), the main hydration product of cement, and its mineral analogues tobermorite and jennite, are examined. A graphical approach, similar to a phase diagram, is used to display the variation in density as a function of water content. In order to provide insight into atomic packing density differences between these phases, hypothetical phase transitions are performed by adding the stoichiometrically correct amount of CaO and H2O to convert one phase into another, and then the molar volumes before and after the transformation are compared. These calculations indicate that C-S-H formed from cement hydrated under normal conditions has a considerably higher atomic packing density than both tobermorite and jennite. This is attributed to both the atomic structure of C-S-H and to its nanoparticulate morphology. The solid density values for C-S-H are used to predict the amount of chemical shrinkage that should occur in a pure tricalcium silicate or dicalcium silicate paste, and these calculations are in good qualitative agreement with published experimental measurements for cement paste. New experimental measurements for the composition and mass density of C-S-H in cement paste cured at elevated temperatures, dried and resaturated, and hydrated with silica fume are presented and interpreted using the same approach. An important finding is that curing at 80 °C leads to a C-S-H phase with a lower atomic packing density, a finding in agreement with experimental observations of less chemical shrinkage at elevated temperatures. Introduction The main binding phase in hydrated cement, CaO-SiO2H2O (C-S-H), is generally believed to consist of nanometerscale solid particles that are packed, or agglomerated, into randomly oriented structures containing internal gel pores.1-5 The size and shape of these particles and the nature of the contacts between them are currently a topic of significant interest. This nanogranular, or colloidal, aspect of cement paste is leading to new insights into structure-property relationships and has significant promise for quantitative modeling.6-8 Models of the nanostructure of C-S-H in neat cement paste are now mature enough to be used in micromechanical modeling of properties.4,9 While nanometer-scale models for neat cement paste are maturing, to date only limited attention has been paid to blended cement systems containing reactive admixtures such as silica fume, blast furnace slag, and fly ash that can significantly change the composition and structure2 of the C-S-H phase that forms. For example, development of the quantitative structural model of C-S-H known as the colloid model10 relied on extensive published data for the density and specific surface area of normally hydrated cement paste as a function of water content, but comparable data for blended cement systems do not yet exist. * To whom correspondence should be addressed. E-mail: jthomas@ northwestern.edu. † Department of Civil and Environmental Engineering, Northwestern University. ‡ Departments of Civil and Environmental Engineering and Materials Science and Engineering, Northwestern University. § National Institute of Standards and Technology.

In general, much remains to be learned about both the atomic structure and the nanostructure of disordered C-S-H phases formed by hydration. Tobermorite and jennite are both layered minerals composed of Ca-O sheets ribbed with silicate chains that repeat every 3 tetrahedral units (i.e., dreierketten). In this respect, they are similar to clay minerals, except that the silica moieties are infinite chains instead of sheets. The interlayer space of both tobermorite and jennite contains Ca ions as well as water molecules. An important difference is that jennite has both Ca-OH and Ca-O bonds in the main calcium layer, while tobermorite contains only Ca-O. On heating, irreversible structural changes occur to these minerals that are accompanied by a loss of interlayer water and a decrease in the layer spacing, which for tobermorite ranges from 1.4 to 0.9 nm. The structure of the C-S-H phase that forms in a standard cement paste is generally believed to be either a solid solution of tobermorite and calcium hydroxide (e.g., refs 11 and 12), or a fine-scale mixture of tobermorite and jennite.13 Richardson and Groves14 proposed a generalized model in which the location of Ca-OH units in the structure are not specified. The C-S-H that forms in neat cement or tricalcium silicate paste has an average CaO/SiO2 molar ratio (C/S) of about 1.7, whereas 1.4 nm tobermorite and jennite have C/S values of 0.83 and 1.5, respectively. In all models, the high C/S and the highly disordered structure of C-S-H are explained by removal of bridging silicate tetrahedra and other modifications.15,16 Richardson has reviewed structural data for calcium silicate hydrate phases17 and models for C-S-H gel.2,17

10.1021/jp910733x  2010 American Chemical Society Published on Web 04/14/2010

C-S-H and Its Mineral Analogues Molecular modeling is now providing a powerful and versatile avenue for exploring the structure and properties of C-S-H. Dolado et al.18 used molecular dynamics to study the polymerization of silicic acid in the presence of calcium, confirming that the disorganized silicate structure of C-S-H is directly related to its high calcium content. Manzano et al.19 used force field atomistic methods to predict the elastic properties of C-S-H phases. Pellenq et al.20 recently developed a molecular model for C-S-H in cement paste. Using dry tobermorite as a starting point, they modified the structure to account for the higher C/S and shorter silicate chains observed in C-S-H. By minimizing energy, the position of each atom in the structure was determined, resulting in a model from which various observations such as NMR, IR, and X-ray spectra can be calculated and compared to experimental measurements. The mass density of a phase provides an important constraint on models because it depends on the packing density of atoms, and thus on the atomic structure. The chemical composition and density of solid C-S-H in neat OPC or C3S paste have been measured precisely using a novel small-angle scattering method,21 resulting in a density value that is higher than that of tobermorite or jennite, despite a greater water content. Molecular modeling results give reasonable agreement with this value,20 with some caveats that will be discussed later. This suggests that the atomic packing density of C-S-H, and thus its structure, may be more different from these mineral phases than has previously been assumed. This paper addresses the problem of comparing the atomic packing density of calcium silicate hydrate phases with different compositions. A graphical approach, similar to a phase diagram, is used to display the variation in density with water content among these phases. In order to provide insight into atomic packing density differences between these phases, hypothetical phase transitions are performed by adding the stoichiometrically correct amount of CaO and H2O to convert one phase into another, and then the molar volumes before and after the transformation are compared. This approach is also used to interpret new data for the composition and density of C-S-H formed in cement samples hydrated at elevated temperatures, after drying and resaturating, and in the presence of silica fume. Methods Specimen Preparation. One paste was made with white portland cement (WPC, US Gypsum) with 30% mass silica fume (Elkem Microsilica) replacement. This paste was mixed at a water:solids mass ratio of 0.5 and hydrated under water for 8 years. Assuming complete reaction of both the cement and silica fume, the calculated CaO/SiO2 molar ratio (C/S) of the C-S-H hydration product is 1.0. Three other pastes were made with triclinic tricalcium silicate (3CaO.SiO2, denoted C3S) powder (Construction Technology Laboratories, Skokie, IL). All were made with a water:solids mass ratio of 0.5 and were hydrated for 1 d under sealed, moist conditions and then hydrated under limewater for the remainder of the specified time. Paste “80C” was hydrated for 14 d at 80 °C and then for 14 d at room temperature. Paste “40C” was hydrated for 4 d at 40 °C and then for 5 d at room temperature. Paste “dry-resat” was hydrated for 14 d at room temperature, then vacuum-dried with a rotary pump for 5 d, and then resaturated in limewater for 7 d. A few days prior to the SANS measurements, five thin (0.6 mm thick) coupons were cut from each block of paste using a water-lubricated wafering saw. One coupon of each paste type was submerged in methanol for 72 h before the SANS

J. Phys. Chem. C, Vol. 114, No. 17, 2010 7595 TABLE 1: Published Composition and Density Information for C-S-H Phases phase

formula

density (g/cm3)

ref

1.1 nm tobermorite 1.4 nm tobermorite metajennite jennite C-S-H (dry) C-S-H (wet)

(CaO)0.75(SiO2)(H2O)0.92 (CaO)0.83(SiO2)(H2O)1.33 (CaO)1.5(SiO2)(H2O)1.17 (CaO)1.5(SiO2)(H2O)1.83 (CaO)1.7(SiO2)(H2O)1.2 (CaO)1.7(SiO2)(H2O)1.8

2.48 2.23 2.62 2.33 2.86 2.60

15, 26 22 23, 24 23, 24 25 21

measurements. The methanol was refreshed twice to ensure complete replacement of H2O in the pore system with methanol. SANS Measurements. SANS measurements were performed at the NIST Center for Neutron Research using the NIST/NSF NG3 SANS instrument. The neutron wavelength was 0.8 nm. For the results discussed here, the instrument was configured to obtain data in the magnitude range of scattering vector of 0.12 < q < 3.0 nm-1, where q ) [4π sin(φ/2)]/λ, φ is the angle of scatter, and λ is the neutron wavelength. Scattering from hydrated paste in this q -range is dominated by the interface between the solid C-S-H nanoparticles and the pore fluid. In general, the upper limit in q for obtaining data from hydrating cement is about 2 nm-1, due to the decrease in SANS intensity with increasing q. The methanol-exchanged and water-saturated coupons were measured at the start of the experiment, using a 6.35 mm diameter neutron beam. They were then exchanged with a deuterated fluid. The methanol coupons were submerged in pure d3-methanol (CD3OH). The four water-saturated coupons from each paste type were submerged in water containing 31%, 55%, 81%, and 100% D2O (heavy water) by volume. The minimum exchange times were 24 h for the water-saturated coupons and 48 h for the methanol-saturated coupons. Each coupon was then measured a second time, taking care to expose the same location to the neutron beam. The SANS analysis of water content and mass density is based on the ratio of the SANS intensities obtained on the same coupons before and after exchange.21 Results and Discussion Relationships between Density and Water Content. A goal of this paper is to explore relationships between the density and chemical composition of C-S-H and its closest mineral analogues, tobermorite and jennite. Composition and density data for 1.1 nm tobermorite, 1.4 nm tobermorite, metajennite, and jennite are listed in Table 1. Conversion of 1.4 nm tobermorite into 1.1 nm tobermorite requires heating to 60-120 °C and causes the linear silicate chains facing each other on adjacent layers to condense into double chains.22 As a result there is less room in the interlayer space for calcium cations and water molecules. Jennite can be converted to metajennite by heating to 70-90 °C, causing a similar condensation of the silicate structure. The density of metajennite has not been published explicitly, but a value of 2.62 g/cm3 can be calculated from the X-ray parameters given originally by Gard and Taylor23 and from the exact structure published later by Bonaccorsi et al.24 A wide range of density values for C-S-H obtained under different conditions using different techniques have been reported, as summarized by Taylor.15 Here we consider only density values associated with the solid C-S-H phase, excluding any interparticle porosity. Brunauer and Greenberg25 measured the density, after vacuum drying at room temperature, of C-S-H formed by the hydration of C3S or C2S pastes. Their technique of water pycnometry generates a high density value

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Thomas et al. than the H2O component, unless the atomic structures are very different the density should increase with the ratio (CaO + SiO2)/H2O, so for a fixed H/S value the density should increase with the C/S. However, the large difference in density between jennite and C-S-H at a given H/S, coupled with the relatively small difference in the C/S of these phases, raises questions as to the similarity of their structures. Transition from One Phase to Another. To explore the implications of the density differences illustrated in Figure 1, it is necessary to account in some way for the compositional differences between the phases. Since all compositions in Figure 1 are referred to one mole of SiO2, a transition from one phase composition to another can always be performed by adding (or subtracting) the necessary amounts of CaO and H2O

Figure 1. Diagram illustrating relationships between density and water content for tobermorite, jennite, and nanoscale C-S-H. Information based on measurements in the literature for plotted points is given in Table 1. The solid C-S-H phase line is calculated by assuming that interlayer water with density 1.1 g/cm3 enters and leaves reversibly, as described in the text. The dotted lines with arrows connecting the jennite and tobermorite phases illustrate the irreversible changes in density and water content that occur during the conversion.

and associated low water content, listed as C-S-H “dry” in Table 1. More recently,21 the density of solid C-S-H formed in C3S or OPC paste was measured without drying using the SANS technique described earlier, and this value is listed as C-S-H “wet” in 1. Note, however, that the latter value represents the composition of the solid C-S-H phase in contact with the pore fluid, and as such does not include any water adsorbed onto the particles or trapped between them, so the water content is significantly lower than the bulk saturated value for C-S-H. To visualize the relationships between the phase data listed in Table 1, a phase diagram approach is used. The variables of interest are the mass density (of the solid phase excluding pores), the C/S, and the H2O/SiO2 molar ratio (H/S). Since only two variables can be easily viewed on a graph and since the C/S is nearly constant for tobermorite and jennite, the density and H/S are used as the plot axes (see Figure 1). The conversion of 1.4 nm tobermorite to 1.1 nm tobermorite, and the conversion of jennite to metajennite, require irreversible structural changes. On the other hand, the water content of the C-S-H gel can in principle be reversibly moved between the dry and wet values by drying and resaturating at room temperature. Thus for C-S-H it is appropriate to connect the data points with a continuous phase line. The line shown in Figure 1 represents the calculated C-S-H density as water with density d ) 1.1 g/cm3 is added or subtracted. This line passes very close to both data points for C-S-H, indicating good self-consistency between these published values and this assumed density value for interlayer water. The 1.1 g/cm3 density is a reasonable value for the effective density of water given that the water being considered here occupies nanometer-scale spaces within the C-S-H structure and therefore is influenced by the solid surface. Extension of the C-S-H phase line out to H/S ) 2.1 assumes that the first monolayer of water adsorbed on the outside of the C-S-H nanoparticles also has density 1.1 g/cm3. At greater H/S values, additional water will reside in the gel pores and have a density close to 1 g/cm3. Figure 1 indicates that when comparisons are made at similar H/S values, C-S-H is denser than jennite, which in turn is denser than tobermorite. This is not surprising: since the CaO and SiO2 components of these phases are both much denser

CxR-S-HyR + (xβ - xR)CaO + (yβ - yR)H2O ) Cxβ-S-Hyβ

(1) where subscript R indicates the initial phase composition and subscript β indicates the final phase composition. The left and right sides of eq 1 both contain the same number and type of atoms, i.e., xβ moles of CaO, 1 mol of SiO2, and yβ moles of H2O. In this case, comparing the total volume occupied by the phases on each side of eq 1 provides information about their atomic packing density. In particular, it allows a relative comparison of the atomic packing density within the R and β calcium silicate hydrate phases. For any phase composition plotted in Figure 1, the molar volume (MV) can be calculated directly from the composition and density according to MV ) MW/d, where MW is the gram molecular weight and d is the mass density. Since calcium silicate hydrate phases contain both Ca-O and Ca-OH bonds,17,27 the molar volume values for the added CaO and H2O in eq 1 should be consistent with those of the phases calcium oxide (CaO, MV ) 16.79 cm3) and calcium hydroxide (Ca(OH)2, MV ) 33.07 cm3). To convert 1 mol of CaO into 1 mol of Ca(OH)2 requires adding 1 mol of H2O with molar volume 16.28 cm3, which corresponds to an H2O density of 1.1 g/cm3. This is the same water density used to generate the C-S-H phase line in Figure 1. These molar volume values for CaO and H2O will therefore be used for calculations based on eq 1. Table 2 lists calculations based on eq 1 for tobermorite and jennite minerals. As an example of the calculations required to produce the values in Table 2, consider the transition from 1.4 tobermorite to jennite. Referring to eq 1, the difference in CaO content, xβ - xR, is 0.67 mols, and the volume occupied by this CaO is VCaO ) (0.67)(16.79) ) 11.25 cm3. Similarly, the difference in water content, xβ - xR, is 0.5 mols, which occupies 8.14 cm3. Thus the total volume occupied by a simple mixture of tobermorite, CaO, and H2O with the same overall composition as 1 mol of jennite is 58.55 + 11.25 + 8.14 ) 77.94 cm3. This value is greater than the molar volume of jennite (76.03 cm3), so the volume change associated with the transition is negative: ∆V ) -2.46%. The volume change from 1.1 nm tobermorite to metajennite is also negative. Thus it can be concluded that the atomic packing density of jennite is greater than that of tobermorite. Similarly, the transitions from 1.4 to 1.1 nm tobermorite and from jennite to metajennite (indicated by dotted arrows in Figure 1) also have negative volume changes. This indicates that the conversion of single silicate chains (in 1.4 nm tobermorite and jennite) into double silicate chains (in 1.1 nm tobermorite and metajennite) increases the atomic packing density.

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TABLE 2: Comparison of the Molar Volumes (cm3) of Tobermorite and Jennite Phases at Equivalent Compositionsa R-phase

β-phase

MVR

VCaOb

VH2Oc

MVmix

MVβ

∆V (%)

1.4 nm tobermorite 1.1 nm tobermorite 1.4 nm tobermorite jennite

jennite metajennite 1.1 nm tobermorite metajennite

58.55 47.87 58.55 76.03

11.25 12.59 –1.34 0

8.14 4.07 –6.68 –10.75

77.94 64.53 50.54 65.29

76.03 63.08 47.87 63.08

–2.46 –2.25 –5.28 –3.38

a See eq 1 and associated discussion in text. b Volume of CaO representing the difference in the R and β phase compositions, assuming MVCaO ) 16.79. c Volume of H2O representing the difference in the R and β phase compositions, assuming MVH2O ) 16.28.

TABLE 3: Comparison of the Molar Volume (cm3) of “Wet” C-S-H (See Table 1) with the Molar Volumes of Phase Mixtures with the Same Compositiona R-phase 1.4 nm tobermorite 1.1 nm tobermorite jennite metajennite afwillite

MVR

VCaOb VH2Oc MVmix MVCSH ∆V (%)

58.55 14.61 7.65 47.87 15.95 14.33 76.03 3.36 –0.49 63.08 3.36 10.26 64.73 3.36 4.88

80.81 78.14 78.90 76.69 72.97

72.13 72.13 72.13 72.13 72.13

–10.74 –7.70 –8.58 –5.95 –1.15

a See eq 1 and associated discussion in the text. b Volume of CaO representing the difference in the R-phase C-S-H compositions, assuming MVCaO ) 16.79. c Volume of H2O representing the difference in the R-phase C-S-H compositions, assuming MVH2O ) 16.28.

Similar calculations for transitions to C-S-H formed by cement hydration are listed in Table 3. For convenience, we use the C-S-H “wet” composition listed in Table 1 as the β-phase composition. In principle, any point on the C-S-H phase line shown in Figure 1 can be used, since the density of water used to generate the phase line (d ) 1.1) is the same as that assumed for the phase transitions defined by eq 1. In all cases, the molar volume of the phase mixture with the same composition as “wet” C-S-H is significantly higher than that of C-S-H. Although C-S-H has single silicate chains and is thus structurally more similar to 1.4 nm tobermorite and jennite than it is to 1.1 nm tobermorite and metajennite, the difference in atomic packing density is greater for the former phases. The comparison between jennite and C-S-H gel is particularly telling, because their compositions are very similar. Jennite contains less CaO and has almost the same H2O content as the “wet” C-S-H composition, yet its molar volume is greater. From this analysis it can be concluded that the C-S-H phase that forms in cement paste cannot be considered a simple mixture of any combination of tobermorite, jennite, and Ca(OH)2, because the atoms in C-S-H are packed more closely together. The stable calcium silicate hydrate phase in the presence of water and calcium hydroxide at standard temperature and pressure is the mineral afwillite, (CaO)1.5(SiO2)(H2O)1.5,15 and hydration of aqueous suspensions of C3S in a ball mill results in afwillite.28 Afwillite is a nesosilicate, a silicate subclass for which all silicate tetrahedra are monomeric, that is, unbonded to other silicate tetrahedra. As a class, nesosilicates have stronger bonds and higher densities than more polymerized silicate minerals. Indeed, the density of afwillite (2.646 g/cm329) is significantly higher than that of the compositionally similar, but highly polymerized, jennite. The transition from afwillite to C-S-H (see Table 3) indicates that the atomic packing density of C-S-H is much closer to that of afwillite than to jennite. While this further illustrates the possibility of significant atomic packing density variation among calcium silicate hydrates, it is well established that the atomic structure of C-S-H has little similarity to that of afwillite (containing, for example, no silica monomers), and many similarities to tobermorite and jennite.15

New Measurements of C-S-H Density and H/S Using SANS. The SANS intensity at a given q depends on the amount of structure in the associated size range, but is also directly proportional to the square of the difference in the neutron scattering length densities (F) of the scattering phases, or the scattering contrast factor. If the chemical composition and mass density of both phases are known, then the F values, and thus the contrast factor, can be calculated precisely. This is generally not the case for hydrated C-S-H phases, since the composition and density of the solid phase, exclusive of any nanoporosity, is not known a priori.21 To calculate the mass density and H/S of the C-S-H phase in hardened pastes requires calculating both the H2O/D2O contrast matchpoint for the C-S-H phase and FCSH. The contrast matchpoint for a particular solid phase is defined as the D2O content of the pore fluid, expressed as a volume fraction, for which the scattering length densities of the pore fluid and solid phase are equal, and thus the scattering contrast is zero. Because the F of H2O is quite low, and that of D2O is quite high, nearly all solid phases have such a matchpoint. For a two-phase system consisting of a solid phase and water, a plot of scattering contrast vs D2O content will be a parabola with a minimum of zero contrast at the matchpoint. For a hydrated paste, the scattering is dominated by the C-S-H-water interface, but there is also a small contribution from nanoscale calcium hydroxide, which does not undergo H/D exchange. As a result the relationship is parabolic but the minimum intensity is nonzero. These data can be fit as the sum of two parabolas representing Ca(OH)2 (with its known contrast matchpoint (MP) at 31% D2O) and C-S-H.21,30 The fit parameters are the fractional intensity contribution from Ca(OH)2 (fCH) and the C-S-H matchpoint. Figure 2 shows such a plot for the dried and resaturated C3S paste. Results for the four paste types reported here are listed in Table 4. At the H2O/D2O contrast matchpoint, the F of the solid phase is by definition equal to that of the fluid, and is therefore known. However, in an H2O/D2O mixture, all of the hydrogen in the solid C-S-H exchanges with deuterium,21 so the resulting F value is for C-S-H/D. This occurs because the D+ and ODions in the fluid can exchange with the interlayer water. To determine FCSH (as is required for calculating density and H/S) requires exchanging with a fluid that does not cause H/D exchange within the interlayers. Deuterated methanol with composition CD3OH is such a fluid, as the deuterated alcohol molecules cannot penetrate the interlayer space.21 In this case, the scattering length density of the solid hydration product, Fsolid, can be determined from

Fsolid )

FCD3OH√R + FCH3OH 1 + √R

(2)

where R is the ratio of the SANS intensities in pure CH3OH and pure CD3OH for the same specimen. Figure 3 shows the calculated

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Thomas et al. contribution from nanoscale Ca(OH)2, determined from the H2O/ D2O contrast matchpoint fits. For the samples measured here, the differences between Fsolid and FCSH are less than 1%. Resulting values of FCSH are listed in Table 4. The value of FCSD, that is, the scattering length density of C-S-H that has fully exchanged with D2O, can be calculated from FCSH and the H2O/D2O contrast matchpoint, M

FCSD ) FD2O -

(1 - M) (FCSH - FH2O) M

(3)

The neutron scattering length density of any phase depends only on its atomic composition, its mass density, and the tabulated neutron scattering lengths of its constituent atoms. For C-S-H

FCSH ) Figure 2. Contrast curve for C3S paste hydrated for 14 d then dried and resaturated. The solid circles are the intensity of the high-q scattering at various D2O contents, normalized by the intensity in pure H2O, with measured standard deviation uncertainties shown as vertical bars. The line is a fit obtained by modeling the data as the sum of two parabolas associated with the scattering from Ca(OH)2-water and C-S-H-water, and the locations of the two corresponding matchpoint (MP) values are shown. Fit results are given in Table 4.

TABLE 4: Parameters Obtained from the SANS Measurements paste type C3S 40C C3S 80C C3S dry-resat WPC-fume

C-S-H FCSH matchpointa fCH (%) (× 1014 m-2) C-S-H H/S 0.799(14) 0.726(10) 0.833(15) 0.756(7)

b

0.27(25) 0.15(22) 1.45(20) 0c

2.931(20) 2.562(7) 2.820(14) 2.658(6)

1.33(6) 1.55(4) 1.54(5) 1.18(2)

(C/S)bCaO + bSiO2 + (H/S)bH2O (C/S)MWCaO + MWSiO2 + (H/S)MWH2O

NAdCSH

(4) and

FCSD )

(C/S)bCaO + bSiO2 + (H/S)bD2O (C/S)MWCaO + MWSiO2 + (H/S)MWH2O

NAdCSH

(5) dCSH (g/cm3) 2.783(26) 2.514(16) 2.762(23) 2.504(11)

a Volume fraction of D2O in the pore fluid. b Numbers in parentheses are the fit standard deviation uncertainties in least significant digits. c Fixed at zero.

Figure 3. Scattering length density of the nanoscale solid hydration product, Fsolid, for the C3S 80C paste, plotted with computed standard deviation uncertainties. At each q value, Fsolid is calculated from the ratio of the SANS intensities in CH3OH and in CD3OH according to eq 2. At higher q-values (corresponding to finer microstructural features) the calculated values become statistically constant as indicated by the horizontal line.

value of Fsolid as a function of q for one of the specimens. At higher q-values the Fsolid value is constant. The value of FCSH is then obtained by correcting Fsolid to account for fCH, the intensity

where bCaO is the neutron scattering length of CaO, etc., and NA is Avogadro’s number. The form of eq 5 utilizes the fact that substituting D for H in a phase affects the molecular weight and mass density equally. Assuming the C/S is known, the only unknowns in eqs 4 and 5 are H/S and dCSH, the water content and mass density of solid C-S-H, and thus these can be solved for. The resulting values are listed in Table 4, and are also plotted in Figure 4 along with the previously described phase lines from Figure 1. The value for C3S and OPC pastes hydrated under typical curing conditions obtained originally by the SANS method is also plotted (labeled “wet”). From Figure 4 it can be seen that the C-S-H data for the 40 °C paste falls on the previously established phase line for C-S-H, but with a lower H/S value. This implies that a moderately elevated curing temperature decreases the amount of H2O in the solid C-S-H without greatly affecting the atomic packing density. A very different result is obtained for the 80 °C paste: this value falls well below the C-S-H phase line, suggesting that there are structural differences associated with this higher curing temperature. The molar volume comparisons for transitions from the 80 °C data point to C-S-H “wet” and to jennite are listed in Table 5. Those calculations indicate that the atomic packing density of C-S-H formed by hydration at 80 °C is lower than that of C-S-H formed at ambient hydration temperatures but slightly greater than that of jennite. For the C3S paste that was dried and resaturated at room temperature, the plotted value falls fairly close to (but slightly above) the established C-S-H phase line, but at a lower H/S value than the previously established C-S-H “wet” value. This indicates that, as with the paste cured at 40 °C, there is less water in the solid C-S-H, suggesting that not all of the water removed from the interlayers by drying returns on resaturation. The separation of this value from the C-S-H phase line is close to the overall resolution of this experimental method, and may not be significant.

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Figure 4. Density versus water content for C-S-H phases. The lines and the point labeled “wet” were described previously with Figure 1. The points described in the legend are new experimental measurements, along with their computed standard deviation uncertainties (see Table 4). Points m1 and m2 are values from the molecular model of Pellenq et al.20 before (m1) and after (m2) final relaxation of the structure.

TABLE 5: Comparison of the Molar Volume (cm3) of C-S-H in the C3S 80C and WPC+Fume Samples (See Table 3) with Those of “Wet” C-S-H and Jennitea R-phase

β-phase

C-S-H (C3S 80C) C–S–H (C3S 80C) C–S–H (WPCfume) C–S–H (WPCfume)

C-S-H “wet” jennite C–S–H “wet” jennite

MVR VCaOb VH2Oc MVmix MVβ ∆V (%) 72.92 0 4.07 76.99 72.92 –3.36 4.56 74.12 54.88 11.75 10.09 76.72 54.88 8.40 10.58 73.85

72.13 76.03 72.13 76.03

–6.74 2.51 –6.37 2.86

paste in contact with a reservoir of water, the uptake of water into the specimen provides a direct measure of the chemical shrinkage over time.31 It is useful to compare calculated values of chemical shrinkage for C3S or C2S hydration with measured values for cement hydration. The complete hydration of C3S or C2S can be written as

a See eq 1 and associated discussion in the text. b Volume of CaO representing the difference in the R and β phase compositions, assuming MVCaO ) 16.79. c Volume of H2O representing the difference in the R and β phase compositions, assuming MVH2O ) 16.28.

The data for the mature paste made with WPC and silica fume falls the farthest from the C-S-H phase line of all of the new samples. However, this must be interpreted in light of its lower C/S value of 1.0. Table 5 lists the molar volume comparisons for the transitions from this result to C-S-H “wet” and to jennite. Those calculations indicate that for C-S-H formed at the lower C/S of 1.0 the atomic packing density is lower than that of C-S-H formed at C/S ) 1.7 but slightly greater than that of jennite, and is quite similar to that of the C3S specimen hydrated at 80 °C. The formation of C-S-H gel at elevated curing temperatures and at lower C/S values are both known to increase the degree of polymerization of the silicate chains. The present finding that both of these conditions also decrease the atomic packing density of the C-S-H supports the hypothesis that the depolymerized silicate structure of C-S-H is at least partly responsible for its high atomic packing density. Implications for Chemical Shrinkage. A property of cement paste and concrete directly influenced by the C-S-H density is chemical shrinkage, which is the decrease in the total volume of the paste components as hydration proceeds. The primary effect of chemical shrinkage is migration of water within the pore system, with larger pores emptying to supply water for the hydration reactions. If this water is not replenished, then this pore emptying results in internal capillary stresses. For a

C3S + 5.3H2O f C1.7-S-H4 + 1.3Ca(OH)2

(6)

C2S + 4.3H2O f C1.7-S-H4 + 0.3Ca(OH)2

(7)

Note that the higher C-S-H water content (H/S ) 4) used in eqs 6 and 7 corresponds to the bulk phase including water trapped in the gel pore spaces between the nanoparticles. Calculating the chemical shrinkage resulting from eqs 6 and 7 requires calculating the difference in the total volume occupied by the phases on each side of the equations, which is very similar to the phase transition calculations based on eq 1. Chemical shrinkage values are typically reported as the volume change per 100 g of cement.31,32 In that form, the chemical shrinkage, c, resulting from eqs 6 or 7 is given as

c)

(

)

MWH2O MWCSH MWCH 100 MWcem +A -B MWcem dcem dH2O dCSH dCH (8)

where subscript cem indicates the cement mineral phase (C3S or C2S) and A and B are the stoichiometric coefficients for H2O and Ca(OH)2, respectively, in eqs 6 and 7. The bulk density of C-S-H can be calculated by adding water with density d ) 1 g/cm3 to the water-rich limit of the C-S-H phase line (H/S ) 2.1, d ) 2.51 g/cm3) shown in Figure 1. This results in a bulk density of 2.05 g/cm3 for (CaO)1.7(SiO2)(H2O)4.

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TABLE 6: Comparison of the Molar Volume (cm3) of the Molecular Dynamics Model of C-S-H from Pellenq et al.20 with Those of 1.4 nm Tobermorite and “Wet” C-S-Ha R-phase

β-phase

MVR

VCaOb

VH2Oc

MVmix

MVβ

∆V (%)

1.4 nm tobermorite 1.4 nm tobermorite MD (initial) MD (relaxed)

MD (initial) MD (relaxed) C–S–H “wet” C–S–H “wet”

58.55 58.55 71.92 75.15

13.77 13.77 0.84 0.84

6.84 6.84 0.81 0.81

79.16 79.16 73.58 76.81

71.92 75.15 72.13 72.13

–9.14 –5.06 –1.97 –6.09

a See eq 1 and associated discussion in the text. b Volume of CaO representing the difference in the R and β phase compositions, assuming MVCaO ) 16.79. c Volume of H2O representing the difference in the R and β phase compositions, assuming MVH2O ) 16.28.

For the chemically impure form of C3S found in cement, the density is about 3.15 g/cm3,15 and the calculated chemical shrinkage based on eqs 6 and 8 is 6.02 cm3/100 g. A similar calculation based on eqs 7 and 8 for the impure form of C2S found in cement (d ) 3.33 g/cm315) gives a chemical shrinkage of 4.67 cm3/100 g. The lower chemical shrinkage value for C2S implies that this mineral phase has a higher atomic packing density than C3S. These calculated values are in good qualitative agreement with the experimental values reported for the chemical shrinkage (after complete hydration at 20 °C) of ordinary portland cement (5.2 cm3/100 g)31 and white portland cement (5.7 cm3/100 g),32 which contain primarily C3S and C2S along with smaller amounts of other cement minerals. For a hypothetical reaction in which C3S and water are converted into the stoichiometrically appropriate amounts of jennite and calcium hydroxide, the calculated chemical shrinkage according to eq 8 is 2.97 cm3/100 g, just half the value for hydration of C3S to form C-S-H. This suggests that about half of the chemical shrinkage that occurs in cement paste can be attributed to the unusually high atomic packing density of C-S-H, with the remainder arising from the conversion of bulk water into structural water with a density greater than 1 g/cm3. Interestingly, Geiker and Knudsen31 found that the chemical shrinkage of OPC paste after complete hydration decreased with increasing curing temperature in roughly linear fashion, from 5.2 cm3/100 g at 20 °C to 3.4 cm3/100 g at 50 °C. Based on the present results, the calculated bulk density of the C-S-H phase (H/S ) 4) formed at 40 °C is 2.03 g/cm3, and that of C-S-H formed at 80 °C is 1.95 g/cm3. Using these values in eq 8 results in calculated chemical shrinkage values, for full hydration of C3S, of 5.63 cm3/100 g at 40 °C and 3.67 cm3/100 g 80 °C, in good qualitative agreement with the trend noted above. Zhang et al.33 recently reported a similar trend of decreasing chemical shrinkage of oil well cement (containing primarily C3S and C2S) at a given degree of hydration with increased curing temperature. In that study the trend continued to 10 °C, suggesting that the atomic packing density of C-S-H formed at subambient temperature could be even higher than that of C-S-H “wet”. Further experiments measuring the density of C-S-H as a function of curing temperature and establishing the relationship with chemical shrinkage are planned. Microscopic investigations34,35 have shown previously that the C-S-H phase appears denser when the curing temperature is elevated, and we have previously shown9 that the packing density of the primary nanometer-scale C-S-H particles increases with the curing temperature. Those observations do not contradict the present finding that curing at 80 °C generates a lower solid density, or atomic packing density, of the particles themselves. They also do not contradict the experimental trend of less chemical shrinkage at higher temperatures noted above, as chemical shrinkage depends on the atomic packing density but is independent of the particle packing density. We also point out, to avoid confusion, that the terms low density (LD) C-S-H and high-density (HD) C-S-H introduced by one of us10 refer to the particle packing density only.

General Discussion. The main finding of this paper is that the atomic packing density within the C-S-H phase formed by the hydration of cement minerals is significantly higher than that of tobermorite and jennite, mineral phases that are widely cited as structural analogues of C-S-H. While no conclusive reason for this is given here, some brief discussion and speculation are warranted. C-S-H differs from tobermorite and jennite in its higher calcium content, in its disorganized structure containing relatively short silicate chains, and in its nanoparticulate morphology. Pellenq et al.20 recently developed a molecular model of C-S-H based on 1.1 nm tobermorite but modified extensively to account for the higher calcium content and structural disorder, such as shorter silicate chains, observed for C-S-H. Once these modifications were complete, water was adsorbed, resulting in a composition of (CaO)1.65(SiO2)(H2O)1.75 and a density (prior to final relaxation of the structure) of 2.56 g/cm3, in reasonable agreement with the neutron scattering result and the C-S-H phase line (see Figure 4). This supports the hypothesis that structural disorder in C-S-H is at least partially responsible for its high atomic packing density. However, when the model structure was allowed to relax, the density decreased to 2.45 g/cm3.20 Table 6 compares the molar volumes calculated from both densities reported from the molecular model to those of 1.4 nm tobermorite and C-S-H “wet”. Those calculations confirm that while the structural modifications incorporated by the molecular model do indeed increase the overall atomic packing density above that of tobermorite, the atomic positions predicted by the model, particularly after relaxation, are not as tightly packed as in C-S-H gel. The nanoparticulate structure of C-S-H (with a characteristic size of about 5 nm) is an important aspect of its overall physical structure independent of its composition and chemical structure. It is well-known that many properties of nanoparticulate materials, including solubility, chemical reactivity, and density, can vary significantly from the bulk state, due to the high proportion of atoms that are at or near a free surface.36,37 In general, average lattice spacings in nanoparticulate materials are observed to decrease with decreasing particle size. While this phenomenon, known as nanosolid densification, can be attributed generally to contraction of bonds with a lower coordination number near the surface, the exact mechanisms are controversial.36 We hypothesize that such nanosolid effects play an important role not only in the high measured density, but also in the long term stability of the C-S-H nanoparticles in cement and concrete, which are not observed to coarsen even after many years.38 Summary Relationships between the composition and density of the minerals tobermorite and jennite and of the C-S-H phase formed by hydration of cement under various conditions are explored. A graphical approach, similar to a phase diagram, is used to display the variation in density with water content. In

C-S-H and Its Mineral Analogues order to provide insight into atomic packing density differences between these phases, hypothetical phase transitions are performed by adding the stoichiometrically correct amount of CaO and H2O to convert one phase into another, and then the molar volumes before and after the transformation are compared. Those calculations indicate that jennite has a slightly higher atomic packing density than tobermorite, while C-S-H has a significantly higher atomic packing density than either mineral phase. The unusually high atomic packing density of the C-S-H phase, which has important implications for modeling of C-S-H and cement paste, is attributed to both its highly defective atomic structure, including relatively short silicate chains, and to nanosolid effects related to its nanoparticulate morphology. New results for the composition and density of C-S-H hydrated under various conditions are reported (see Table 4). For C3S hydrated at 40 °C and for C3S hydrated at room temperature and then vacuum-dried and resaturated, the solid C-S-H has a lower water content and higher density than the previously established value for normal hydration conditions, but in both cases the results fall near the C-S-H phase line. For C3S hydrated at 80 °C, and for white portland cement paste blended with silica fume (with an overall C/S of 1.0), the C-S-H has a lower atomic packing density similar to that of jennite. This may be due to a higher degree of polymerization of the C-S-H formed in these pastes. Calculations of the amount of chemical shrinkage during C3S and C2S hydration using the C-S-H density values reported and discussed here are in good qualitative agreement with published experimental measurements for chemical shrinkage of cement paste, including the observation that chemical shrinkage decreases with higher hydration temperatures. About half of the chemical shrinkage that occurs in a cement paste hydrated near 20 °C can be attributed to the unusually high atomic packing density of the C-S-H phase. Acknowledgment. Support from the Infrastructure Technology Institute is gratefully acknowledged. This work utilized neutron scattering facilities supported in part by the National Science Foundation under Agreement No. DMR-0454672. References and Notes (1) Allen, A. J.; Oberthur, R. C.; Pearson, D.; Schofield, P.; Wilding, C. R. Phil. Mag. B 1987, 56, 263–268. (2) Richardson, I. G. Cem. Concr. Res. 1999, 29, 1131–1147. (3) Nonat, A. Cem. Concr. Res. 2004, 34, 1521–1528. (4) Constantinides, G.; Ulm, F.-J. J. Mech. Phys. Solids 2007, 55, 64– 90.

J. Phys. Chem. C, Vol. 114, No. 17, 2010 7601 (5) Thomas, J. J.; Jennings, H. M.; Chen, J. J. J. Phys. Chem. C 2009, 113, 4327–4334. (6) Thomas, J. J.; Jennings, H. M. Cem. Concr. Res. 2006, 36, 30–38. (7) Jennings, H. M. Cem. Concr. Res. 2008, 38, 275–289. (8) Jennings, H. M.; Bullard, J. W.; Thomas, J. J.; Andrade, J. E.; Chen, J. J.; Scherer, G. W. J. AdV. Concr. Tech. 2008, 6, 1–25. (9) Jennings, H. M.; Thomas, J. J.; Gevrenov, J. S.; Constantinides, G.; Ulm, F.-J. Cem. Concr. Res. 2007, 37, 329–336. (10) Jennings, H. M. Cem. Concr. Res. 2000, 30, 101–116. (11) Kantro, D. L.; Brunauer, S.; Weise, C. H. J. Phys. Chem. 1962, 66, 1804–1809. (12) Fujii, K.; Kondo, W. J. Am. Ceram. Soc. 1983, 66, C220–C221. (13) Taylor, H. F. W. J. Am. Ceram. Soc. 1986, 69, 464–467. (14) Richardson, I. G.; Groves, G. W. Cem. Concr. Res. 1992, 22, 1001– 1010. (15) Taylor, H. F. W. Cement Chemistry; Thomas Telford: London, 1997. (16) Chen, J. J.; Thomas, J. J.; Taylor, H. F. W.; Jennings, H. M. Cem. Concr. Res. 2004, 34, 1499–1519. (17) Richardson, I. Cem. Concr. Res. 2008, 38, 137–158. (18) Dolado, J. S.; Griebel, M.; Hamaekers, J. J. Am. Ceram. Soc. 2007, 90, 3938–3942. (19) Manzano, H.; Dolado, J. S.; Ayuela, A. Acta Mater. 2009, 57, 1666– 1674. (20) Pellenq, R. J.-M.; Kushima, A.; Shahsavari, R.; Van Vliet, K. J.; Beuhler, M. J.; Yip, S.; Ulm, F.-J. Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 16102–16107. (21) Allen, A. J.; Thomas, J. J.; Jennings, H. M. Nat. Mater. 2007, 6, 311–316. (22) Bonaccorsi, E.; Merlino, S.; Kampf, A. J. Am. Ceram. Soc. 2005, 88, 505–512. (23) Gard, J. A.; Taylor, H. F. W.; Cliff, G.; Lorimer, G. W. Am. Mineral. 1977, 62, 365–368. (24) Bonaccorsi, E.; Merlino, S.; Taylor, H. Cem. Concr. Res. 2004, 34, 1481–1488. (25) Brunauer, S.; Greenberg, S. A. The Hydration of Tricalcium Silicate and beta-Dicalcium Silicate at Room Temperature; National Bureau of Standards: Washington, DC, 1960. (26) Merlino, S.; Bonaccorsi, E.; Armbruster, T. Eur. J. Mineral. 2001, 13, 577–590. (27) Thomas, J.; Neumann, D.; Chen, J.; Jennings, H. Chem. Mater. 2003, 15, 3813–3817. (28) Brunauer, S.; Copeland, L. E.; Bragg, R. H. J. Phys. Chem. 1956, 60, 112–120. (29) Malik, K. M. A.; Jeffery, J. W. Acta Crystallogr. 1976, B32, 475– 480. (30) Thomas, J. J.; Chen, J. J.; Allen, A. J.; Jennings, H. M. Cem. Concr. Res. 2004, 34, 2297–2307. (31) Geiker, M.; Knudsen, T. Cem. Concr. Res. 1982, 12, 603–610. (32) Geiker, M. Ph.D. thesis, Technical University of Denmark, 1983. (33) Zhang, J.; Weissinger, E.; Peethamparan, S.; Scherer, G. Cem. Concr. Res., in press. (34) Kjellsen, K.; Detwiler, R.; Gjorv, O. Cem. Concr. Res. 1990, 20, 308–311. (35) Famy, C.; Scrivener, K.; Atkinson, A.; Brough, A. Cem. Concr. Res. 2002, 32, 269–278. (36) Sun, C. Q. Prog. Solid State Chem. 2007, 35, 1–159. (37) Waychunas, G. A.; Hengzhong, Z. Elements 2008, 4, 381–387. (38) Thomas, J. J.; Allen, A. J.; Jennings, H. M. J. Am. Ceram. Soc. 2008, 91, 3362–3369.

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