Evidence of Solid Water Bridges Found in Hydrating TricalciumSilicate Paste

Bernie O’Hare,z,z Rachel A. Steinle,z Hayden Black,z,J Pearl Kaplan,z,** Michael W. Grutzeck,y andAlan J. Benesiw,z

zDepartment of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802yMaterials Research Laboratory, The Pennsylvania State University, University Park, Pennsylvania 16802

The D2O hydration of pure tricalcium silicate (Ca3SiO5) wasexamined at 231C with parallel measurements using deuteriumNMR spectral analysis, deuterium NMR T1 relaxation mea-surements at several magnetic fields, isothermal calorimetry,scanning electron microscopy, and Vicat needle tests. The re-sults show that the maximum rate of decrease in the deuteriumT1 relaxation time of the D2O resonance, the maximum rate ofincrease in heat production, and the maximum rate of decreasein Vicat needle penetration occur simultaneously within exper-imental error. Together with results from other recent deuteriumNMR studies in our laboratory, these observations support thenew hypothesis that the formation of solid water bridges betweenclosely adjacent developing hydrate surfaces causes initial setting.As a confirmatory experiment, it was found that microwave heat-ing immediately after setting, i.e. immediately after zero Vicatneedle penetration, softened the paste sufficiently to once againallow some needle penetration, and furthermore, that upon cool-ing, the paste reset. This process could be repeated several times.

I. Introduction

TRICALCIUM silicate (Ca3SiO5), often abbreviated in cementscience as C3S, is the main clinker phase found in anhydrous

Portland cement as well as the primary source of calcium andsilica needed to form calcium silicate hydrate (C–S–H), the ma-jor strength-producing phase that forms in hydrating Portlandcement and concrete. The addition of water to C3S initiates anumber of kinetically complex and still poorly understoodhydration reactions that ultimately result in the productionof two hydrated solid phases, i.e. high compressive strengthcalcium silicate hydrate (C–S–H) and calcium hydroxide(Ca(OH)2) (CH). Because C–S–H is the primary source ofcompressive strength in hardened cement,1 it seems logical topropose that the intergrowth and entanglement of developingC–S–H foils is responsible for both the initial setting and thelong-term hardening of cement.

Conventional tools including Vicat needle, X-ray diffraction,isothermal calorimetry, microwave heating, and scanning elec-tron microscopy (SEM) were used to study the kinetics of settingand hardening of pure C3S hydrated with either H2O or D2O at231711C. In parallel with these measurements, deuteriumNMRwas used to obtain static 2H NMR quadrupole echo powder

spectra and T1 relaxation times at three different magnetic fieldsfor the same preparation of D2O-hydrated C3S in sealed cap-sules as a function of time after the addition of D2O. Our ex-perimental data indicate that the initial setting of hydrating C3Smay not be caused by intergrowth and entanglement of C–S–H.Instead, our data support the hypothesis that the solid surfacesare covered with two or three layers of solid-state waterthroughout the hydration reactions, and that when the adjacentsurfaces grow to within four to six water layers, the solid waterlayers fuse to form a solid water bridge between them. After asufficient number of solid water bridges form between closelyadjacent C–S–H foils, the cement sets.

II. Background and Theory

The aim of this work was to focus on the nature and role ofwater and its products during the setting and hardening process.We have developed programs that can be used to quantitativelycalculate deuterium quadrupolar dynamic powder lineshapes,T1, and T2 values for specific jump geometries, jump rates, orrotational diffusion coefficients.2,3 Using these programs, it hasbeen possible to devise reasonable models of O–2H bond motionthat match the experimental deuterum NMR data over a widerange of temperatures and magnetic fields for 2H2O in poroussolids. The match between experimental and theoretical spectraand relaxation times was used to show that the O–2H bondmotion consists of a mixture of C2 and tetrahedral symmetryjumps in 2H2O-synthesized kanemite and in 2H2O-hydratedZeolite A, and that the 2H2O in these materials was thereforesolid even well above ambient temperatures.2 We have also pro-posed in earlier work that solid-state water was present inhydrated cement.4 The objective in the work presented herewas to measure deuterium NMR T1 relaxation times as a meansof tracking the development of solid-state water in the systemC3S–D2O and correlate it with corresponding Vicat needle,calorimetric, and morphological data.

2H NMR offers distinct advantages in comparison with 1HNMR for the characterization of angular dynamics because it isdominated by the single nucleus 2H quadrupolar interaction.The electric field gradient tensor in 2H2O has nearly axial sym-metry (Z5 0.1) about the O–2H covalent bond axis and yields aqcc of 215 kHz.5 The spectrum and relaxation times for 2H2Ocan therefore be used to monitor the time dependence of theangle made by the molecular O–2H covalent bond with themagnetic field.6 The situation for 1H NMR, where the intramo-lecular 1H–1H dipolar interactions, intermolecular 1H–1H dipo-lar interactions, and heteronuclear (X–1H) dipolar interactionsmust be considered, is much more complicated, and for thisreason was not used in these studies.wwA repertoire of deuteriumNMR techniques are sensitive to O–2H motions with frequen-

L. Struble—contributing editor

Based in part on the thesis submitted by Bernie O’Hare for the Ph.D. degree in Chem-istry, The Pennsylvania State University, University Park, PA, 2009.

zPresent address: Bruker-Biospin Corporation, Billerica, MA 01821, U.S.A.JPresent address: Department of Chemistry, The University of North Carolina at

Chapel Hill, Chapel Hill, NC 27599, U.S.A.**Present address: Department of Environmental Sciences and Engineering, The

University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, U.S.A.wAuthor to whom correspondence should be addressed. e-mail: alan@chem.psu.edu

Manuscript No. 28210. Received June 18, 2010; approved September 24, 2010

wwH NMR relaxation studies, including field cycling NMR measurements, yield usefulinformation about motion of water, but the information is more difficult to connect tospecific types of motion because of the large number of pairwise dipolar interactions thatmust be considered.

Journal

J. Am. Ceram. Soc., 94 [4] 1250–1255 (2011)

DOI: 10.1111/j.1551-2916.2010.04207.x

r 2010 The American Ceramic Society

1250

cies (nmotion) of 1� 10�2onmotiono1� 1015 s�1.7,8 These encom-pass most of the range of frequencies of reorientational andtranslational motions of water and other molecules in condensedphases.

Almost all experimentally based knowledge about the de-tailed angular dynamics of water has been derived from deute-rium NMR experiments. For bulk liquid and supercooled liquidwater, the 2H T1 and T2 relaxation times show that the O–2Hbonds experience isotropic rotational diffusion. At 292 K andatmospheric pressure, the rotational rate constant for liquid2H2O is 2.8� 1011 s�1 (5 6 Drot, where Drot is the rotationaldiffusion coefficient),9 while for supercooled liquid water at pres-sures 42000 atm and temperatures between 188 and 210 K, therotational rate constants are on the order of 108–109 s�1.8,10

Although it occurs in some solids with spherically symmetricmolecules,11 isotropic rotational diffusion is typical of liquidscomposed of small molecules. In contrast, for pure solid Ice Ih atatmospheric pressure, 2H NMR lineshape analysis and stimu-lated echo experiments show that the O–2H bonds experiencetetrahedral jumps (jump rateB104 s�1 several 1C below freezing)due to the diffusion of Bjerrum defects through the ice lattice.12

On a slower time scale, there is also a seven-site reorientationmediated by proton transfer and interstitial translational diffu-sion.5 For most crystalline hydrates (CaSO4 � 2H2O for example),variable temperature 2H NMR lineshape analysis shows that theO–2H bonds experience rapid C2 symmetry jumps around thebisector of the HOH bond angle with jump rates � 106 s�1 atambient temperatures.13,14 In a few cases, spectral analysis forcrystalline hydrates shows that the water molecules are rigid onthe ‘‘NMR timescale’’ (njumps � qcc), except for librations thatreduce the effective 2H qcc. 2H NMR stimulated echo experi-ments on tetrahydrofuran clathrate hydrate show that the O–2Hbonds of the water molecules experience tetrahedral jumps(ascribed to Bjerrum defects) on a distorted tetrahedral latticeas well as a slower randomization process ascribed to a combi-nation of Bjerrum and ionic defects.15 Dynamics characterizedby rigidity or jumps rather than rotational diffusion providesatomic-level evidence of the solid state.

Recent 2H NMR studies in our laboratories have found thatsolid-state water exhibits both tetrahedral jumps (like ice Ih) andC2 symmetry jumps (like crystalline hydrates) at room temper-ature and higher in the layered silicate kanemite (NaH-Si2O5 � 3H2O) synthesized with 2H2O and in the nearlyspherical porous cavities of 2H2O-hydrated Na1-Zeolite A(Na12Al12Si12O48 � 27H2O).2,16 In these samples, where the in-terlayer spacing and maximum pore diameter are 10.3 and 14.3A, respectively, all of the internal water was shown to be in thesolid state, hydrogen bonded to and extending several layersaway from the negatively charged silicate or aluminosilicatesolid surfaces, or coordinated to Na1 cations adjacent to thenegative surface. In both of these hydrated solids, at ambientand higher temperatures, there is fast exchange between thesolid-state water molecules that experience tetrahedral jumps oftheir O–2H covalent bonds and the solid-state water moleculescoordinated to Na1 that experience C2 symmetry jumps of theirO–2H bonds about the 2H–O–2H bond angle bisector. C2 sym-metry jump rates were below 109 s�1, far below the isotropicdiffusion rotational rate constants observed for liquid water atambient temperature and pressure. The tetrahedral jump rateswere several orders of magnitude slower than the C2 jumps inboth cases. The mixture by fast exchange of these two popula-tions of O–D deuterium atoms causes an exceptionally broadtemperature minimum in the T1 vs jump rate and T1 vs temper-ature data. This model was found to match the unusual tem-perature dependence of the experimental deuterium relaxationtimes as well as the low-temperature powder spectra for thesehydrated minerals over a very wide range of temperatures.

The presence of solid-state water adjacent to silicate surfaceshas also been shown previously with other experimental tech-niques17–19 Attenuated total reflection infrared spectroscopystudies by Asay and Kim indicate that the first three layers ofwater on fresh silicate surfaces are in the solid state.20

The adsorbed water in C–S–H is often referred to as inter-layer or interstitial water. The surfaces of the C–S–H foils incontact with the aqueous phase consist of silicate units similar inatomic arrangement to those in kanemite. 29Si solid-state NMRshows that the silicate units are primarily Q2, i.e. that most sil-icate tetrahedra are covalently bonded to two silicate units,yielding linear (or cyclic) oligomers and polymers. The silicatechains form a sheet-like tobermorite structure sandwichingCaOx polyhedra.21 The oxygen atoms of the silicate units pro-vide hydrogen bonding sites for water, while Ca21 cations pro-vide coordination sites for water in a manner similar to Na1

ions in kanemite and Zeolite A. Oxygen atoms in water,hydroxide ions, silicate Si–O–Si, and silicate Si–O� ions are allexpected to coordinate to the Ca21 ions in an aqueous solution(i.e., those that are not in the solid Ca(OH)2 phase). The sim-ilarity to kanemite is supported by the observation that C2 sym-metry jumps are known to dominate the deuterium NMRspectrum of D2O-synthesized gypsum, CaSO4 � 2D2O (where,for convenience, we have introduced D to represent 2H).13,14

The other nonaqueous solid phase is Ca(OH)2, and stronghydrogen bonds augmented by residual ion dipole forces arealso expected. Strong hydrogen bonds to the solid C–S–H andCa(OH)2 surfaces and the coordination of water to cationicCa21 would therefore be expected to stabilize solid-state waterstructures in their vicinities. We assume that the extent of solid-state water is comparable to what we observed for Zeolite A andkanemite, and to what was observed by Asay and Kim for wateron fresh silicate surfaces.2,21 Beyond three water diameters froma nonaqueous solid surface or Ca21 ion, the water moleculeswould therefore be expected to be in the liquid state at ambienttemperatures, although we speculate that solid-state water maybe able to extend even farther than three layers in some cases. Asrelatively large pores are known to exist in hydrated cement, asignificant fraction of the water would be expected to remain inthe liquid state. The presence of both solid-and liquid-statewater is confirmed in the experiments described below.

III. Experimental Methods

Tricalcium silicate was synthesized from reagent-grade silicicacid and CaCO3 (r0.002% Fe by weight). These reagents wereheated to 10001C in a Pt crucible to dehydrate and decarboxy-late them, and then mixed in stoichiometric proportions and ballmilled in absolute ethyl alcohol with alumina/zirconium ballsuntil homogeneous (typically 24 h). The mixture was pouredinto a Pyrex dish, allowing the alcohol to evaporate at roomtemperature. The resulting powder was pressed into 1-in.-diam-eter pellets, stacked on a zirconia plate, and fired repeatedly(after grinding between firings) in a SiC furnace at 15001C orabove. The sample was X-rayed and found to be single-phasetricalcium silicate (M1 polymorph). No evidence of CaO wasobserved.

The sample was ground by hand in an agate mortar and thenball milled dry with zirconium balls. The Blaine surface area ofthe powder was measured as 3400 cm2/g, similar to that of Port-land cement. The powder served as the C3S starting material. Itwas mixed with D2O to yield a D2O/C3S ratio of 0.40 by weight.Part of the well-mixed paste was used for calorimetry, part forVicat needle testing, and part was placed in NMR sample tubesand sealed with epoxy for deuterium NMR analysis as a func-tion of hydration time.

The spectrometers used for deuterium NMR experimentswere a Chemagnetics CMX-300 (Varian NMR Systems, FortCollins, CO) (6.98 T, 45.65 MHz), a Chemagnetics Infinity 500(11.74 T, 76.73 MHz), a Bruker DRX-400 (Bruker-BiospinCorporation, Billerica, MA) (9.41 T, 61.50 MHz), a BrukerDPX-300 (7.05 T, 46.08 MHz), a Bruker AMX2-500 (11.75 T,76.77 MHz), and a Bruker DRX-600 (14.10 T, 92.15 MHz). TheT1 value of the sharp D2O peak was determined on the liquid-state spectrometers using the inversion recovery pulse sequence,px�tvariable�(p/2)f1�Acquirefref, with f15x, y,�x,�y and

April 2011 Evidence of Solid Water Bridges in Hydrating Tricalcium Silicate 1251

fref5x, y,�x,�y (p/2�90 ms and p�180 ms) at 46.08 MHz,61.50 MHz, and 76.77 MHz at 231711C. Deuterium solid-statepowder spectra were obtained at 231711C on a solid-statespectrometer at 45.65 MHz with the quadrupole echo pulsesequence, (p/2)x�t1�(p/2)7y�t2�Acquirex with Cyclops phasecycling added to all pulse phases and the receiver phase(p/25 2.0 ms, t15 30 ms, t2 5 25 ms, spectral width5 1 MHz).This is the standard phase cycling for the quadrupole echoexperiment and eliminates artifacts from FID breakthrough.The inversion recovery quadrupole echo experiment, px�tvari-able�(p/2)x�t1�(p/2)7y�t2�Acquirex (with Cyclops phasecycling added), was used to obtain T1 values on the solid-statespectrometers. T1 values obtained at the same magnetic field forthe sharp central peak were the same within experimental error(ca. 75%) whether they were obtained on the liquid-statespectrometers with the inversion recovery sequence and ‘‘soft’’pulses or with the quadrupole echo inversion recovery sequenceon the solid-state spectrometers with ‘‘hard’’ pulses.

Corresponding paste samples made from the same prepara-tion of C3S with D2O (H2O/C3S5 0.40 by weight) were testedusing a modified Vicat needle procedure that penetrated into asmall paste-filled mold (2 in. smallest diameter and 2 in. tall).The mold was covered with plastic wrap to prevent evaporationbetween measurements to simulate the sealed NMR samples.Vicat needle tests were also carried out on paste samples madewith H2O (H2O/C3S5 0.36 by weight).

The heat evolution from 1.85 g of the same D2O hydratedpaste was monitored continuously using a Thermonetics iso-thermal conduction calorimeter in a thermally isolated Styro-foam enclosure placed in a room with a well-regulatedtemperature of 231711C. After thorough mixing, paste sampleswere placed in a sealed 1 in.2 cubic stainless-steel sample holderthat fit snugly into the calorimeter cup. Data were collectedusing a computer data logger. Calorimetric tests were alsocarried out on paste samples made with H2O.

In addition, small subsamples of H2O-hydrated C3S were al-lowed to hydrate at 231C as a function of time in tightly sealedglass vials. Using Vicat needle penetration to monitor the set-ting, samples (w/s5 0.36, which is the same stoichiometric ratioas that used for the D2O-hydrated samples) were freeze dried(first immersed in liquid N2 and then freeze dried at low pres-sure) at various stages of hydration and used to examine mi-crostructural characteristics with SEM.

IV. Results

The Vicat needle tests (Fig. 1) for H2O-hydrated and D2O-hy-drated C3S show that setting takes approximately three timeslonger for D2O-hydrated C3S than for H2O-hydrated C3S. Thesame phenomenon has been reported previously.22 This is a

large kinetic isotope effect with a significance that remains to beelucidated, but clearly demonstrates a major role of water insetting.

Figure 2 shows the Vicat needle data for H2O-hydrated C3Salong with the times (indicated with stars) when samples of thepaste were removed for freeze drying in preparation for SEM.

For both H2O- and D2O-hydrated C3S samples, Vicat needlepenetration decreased as the sample set. Setting is consideredcomplete when the needle no longer penetrates the sample. Forexample, it took B400 min for the sample in Fig. 2 to set. Insamples that had only just achieved complete setting, full orpartial penetration could be reestablished easily by heating thepaste in a microwave oven for 30–60 s. Cooling the sample for afew minutes stopped needle penetration as setting againoccurred, reproducibly. This process could be repeated severaltimes for samples that had not been set completely for morethan about an hour, suggesting that the ‘‘freezing and melting’’of water bridges plays a crucial role in initial setting. It is pro-posed that the microwave heating of the sample introducedenough local energy to ‘‘melt’’ the solid-state water bridgessufficiently to allow the sample to become soft enough for nee-dle penetration.

Scanning electron micrographs of some of the selected H2O-hydrated C3S samples described earlier were examined in orderto observe the progressive development of surface features thatcould be correlated with the other kinetic data. In summary, theimages did confirm a sharp increase in the production of foil-likeC–S–H hydrate after about 280 min. The production of hydratefoils and the corresponding increase in surface area acceleratesvery rapidly thereafter.

The deuterium T1 relaxation times of the D2O resonance andheat evolution of the D2O-hydrated C3S are shown as a functionof time along with the Vicat needle data in Fig. 3.

The data in Fig. 3 show that the maximum rate of increasein heat output occurs when the Vicat needle penetration isdecreasing rapidly and the needle penetrates about halfwaythrough the sample. Recalling that the paste is first mixed ex-ternally and then placed in the calorimeter cavity, it is suggestedthat the initial decrease in the heat flux observed in Fig. 3 (10–15min) is an artifact of placing the paste-filled calorimeter cup inthe slightly warmer insulated calorimeter cavity. It is noteworthythat the maximum heat evolution rate is observed after thecement has set completely (B1200 min). After reaching its apexat B2000 min, the rate of heat evolution asymptoticallydecreases toward zero, as hydration continues to occur forweeks, months, and even years thereafter. The heat outputrate data shown in Fig. 3 start at 50 min after mixing and end10045 min after mixing. Integration of the entire heat output

Vicat Needle Penetration (mm)

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Fig. 1. Vicat needle penetration vs time at 231C for C3S/H2O(w/s50.36) vs C3S/D2O (w/s5 0.40).

Vicat H2O 23 C

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Fig. 2. Vicat needle penetration at 231C as a function of time for H2O-hydrated C3S with sampling points indicated by open stars, when sam-ples were removed into glass vials and were immersed in liquid N2 tostop hydration. The samples were subsequently freeze dried for exam-ination by scanning electron microscopy. Note: the first sample wastaken from the starting C3S before the addition of water.

1252 Journal of the American Ceramic Society—O’Hare et al. Vol. 94, No. 4

rate with time yields the heat evolved over this period: 1624 J(this value includes the negative contribution of the initialapparent heat absorption).

The deuterium T1 values (Fig. 3) start to decline immediatelyafter mixing, and precede any observable setting by the Vicatneedle method. The deuterium T1 values, like the heat output,continue to decrease long after setting is complete. Also, there isan increasing and reproducible effect of the magnetic fieldstrength on the deuterium T1 values, with the shortest T1’s ob-served at the lowest field and the longest at the highest field. Aninteresting empirical observation is that setting is about halfwaycomplete when the deuterium T1 value declines to about 200 msat this temperature.

The deuterium quadrupole echo spectrum of D2O-hydratedC3S 3 months after mixing is shown in Fig. 4.

In Fig. 4, the broad Pake powder pattern with outer horns at798.3 kHz (qcc5 262 kHz, Z5 0) is assigned to rigid (on the Dtime scale) solid Ca(OD)2 (CH) or Portlandite.4 The slow an-gular dynamics of this rigid solid also produce long deuteriumT1 values, hence the necessity for the long relaxation delay forthe spectrum shown in Fig. 4. The sharp central peak representsD2O. It is the T1observed of the sharp D2O resonance that isplotted in Fig. 3 for the hydrating paste. At 231C, the deuteriumT1observed for the 3-month-old sample shown in Fig. 4 is 16.7 msat 46 MHz.

V. Discussion

The accumulation of solid-state water with time is indicated bythe decreasing deuterium T1 value of the central D2O peak,Fig. 4, and by its magnetic field dependent deuterium T1 values,Fig. 3. In bulk liquid D2O at 231C, the deuterium T1liquid5 400ms and is independent of the magnetic field.23 In D2O-synthe-sized kanemite and D2O-hydrated Zeolite A at 231C, where allof the D2O is solid state, the T1solid values are slightly differentfor the two materials and are also magnetic field dependent: at46 MHz, T1solid5 4.6 ms for kanemite and T1solid5 7.3 ms forZeolite-A and at 76.8 MHz, T1solid5 6.4 ms for kanemite andT1solid5 9.7 ms for Zeolite A.zz These values correspond to jumprates ranging from 103 to 106 s�1 for tetrahedral and 106–109 s�1

for C2 symmetry jumps, many orders of magnitude below therotational rate constants observed for liquid water at atmo-spheric pressure (1011–1012 s�1). We propose that the solid–wa-ter interfaces of hydrating C3S are similar to those that wereobserved for D2O in kanemite and Zeolite-A (Table I), withsolid-state water extending two to three layers away from Ca21

ions, silicate oxygen atoms, and Ca(OH)2 surfaces.Some of the experimental inversion recovery data used to cal-

culate the T1 values shown in Fig. 3 were fit with the Kohlrausch

stretched exponential function, IðtÞ ¼ aþ ðb� e �t=T1Þbð Þ. Theseanalyses yield b4.95 in all cases, consistent with a nearly mono-exponential relaxation process and a narrow range of T1 relax-ation times within the sample throughout the setting process.24

This finding is consistent with the well-known biphasic fast ex-change model.25

Invoking the fast exchange model and our solid water hy-pothesis, the experimentally observed T1observed values fromFig. 3 represent the fast exchange between residual liquid waterand solid-state water T1 values

2:

1

T1

� �observed

¼ fliquid1

T1

� �liquid

þ fsolid1

T1

� �solid

(1)

where fliquid1fsolid5 1.The experimental deuterium T1observed values obtained at

231C at all magnetic fields were then fit to Eq. (1) using the as-sumptions that the total fraction of water must be 1, that theT1liquid at all magnetic fields is 400 ms, and that at any given timeof hydration, the fliquid and fsolid values must be fixed and inde-pendent of the magnetic field. With the additional relationshipfliquid5 (1–fsolid), each experimental T1 value at a specific mag-netic field requires the calculation of two unknowns: T1solid andfsolid. This is underdetermined. However, our findings for T1solid

in a variety of D2O-hydrated materials, namely kanemite, Zeo-lite A, freeze dried D2O-hydrated starch, and freeze dried D2O-hydrated cellulose at room temperature, all have very similarvalues (Table I).yy Assuming that similar values occur in D2O-hydrated C3S, one can estimate the fraction of solid water as afunction of time, Fig. 5. For example, if we assume that theT1solid5 4.6 ms at 231C as is observed for kanemite at 45.65MHz, the fsolid value for the D2O in Fig. 4 (T1observed 5 16.7 ms)is 0.266. Using the same assumption for the field-dependent datafrom Fig. 3 yields Fig. 5.

If this assumption holds, Fig. 5 shows that even after 3 monthsof hydration, more than 70% of the water in the hardened ce-ment is still in the liquid state. There are apparently many water-filled pores with dimensions greater than six layers of water (B15A). It should be kept in mind, however, that none of the loosely

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Fig. 3. Vicat needle penetration (solid triangles), heat output (opensquares), and deuterium NMR T1 values at three magnetic fields (11.75T, 76.77 MHz: x, 9.41 T, 61.50 MHz: solid squares, and 7.05 T, 46.08MHz T: solid diamonds) for sealed samples held at 231C as a function oftime after mixing Ca3SiO5 with D2O (0.40 g D2O per g Ca3SiO5).

300 200 100 0 –100 –200 –300KHz

Fig. 4. 2H (D) quadrupole echo spectrum of D2O-hydrated C3S at 231C3 months after mixing ((p/2)x�t1�(p/2)7y�t2�Acquirex, with addi-tional Cyclops phase cycling of all pulses and the receiver), withp/25 2.0 ms, t1 5 30 ms, t1 5 25 ms, relaxation delay of 60 s, full spec-tral width5 1 MHz, phased FID’s left shifted to the echo maximum, 3kHz of exponential apodization).

zzIn both of these hydrated solids, the dynamics of the O–D bond vector is limited toslower tetrahedral jumps on an ice-like lattice of hydrogen-bonding sites in addition to fasterC2 symmetry jumps for those water molecules directly coordinated to Na1. Fast exchangebetween the water molecules experiencing the two types of solid-state jump motion yieldsthe observed T1solid using the fast exchange model.

ð1=T1Þsolid ¼ fC2ð1=T1ÞC2 þ ftetð1=T1Þtet

yyTable I includes unpublished results for D2O hydrated, freeze-dried starch, andcellulose.

April 2011 Evidence of Solid Water Bridges in Hydrating Tricalcium Silicate 1253

bound liquid-state water can evaporate because the samples aresealed. Unsealed samples would be expected to lose much of theirloosely bound liquid-state water by evaporation.

Overall, the picture that emerges is that the surfaces of thenonaqueous solid phases in hydrated tricalcium silicate pastesare coated with a solid-state aqueous phase (water) two to threewater layers thick. Water farther than about three layers from aC–S–H or CH surface is in the liquid state. As C–S–H and CHphases grow into the liquid-filled spaces between the C3S grains,their solid water-covered surfaces grow close enough togetherthat they are eventually separated by only four to six water lay-ers. At this point, the solid water regions make contact and fuseto form solid water bridges that ‘‘glue’’ the grains together. Thesolid-state water ‘‘glue’’ is created by hydrogen bonding betweenwater molecules, by hydrogen bonding between water moleculesand silicate oxygen atoms, by ion–dipole forces between Ca21

and water oxygen lone pair electrons, and by the electric fieldgradient created by the negative surface charge and the positiveCa21 ions. When the number of interparticle solid-state watercontacts is sufficiently large that all of the particles are con-nected, the cement is said to be completely set, i.e. the Vicatneedle can no longer penetrate the sample. At zero Vicat needlepenetration (i.e., complete setting), the sample appears to con-tain roughly 10% solid water and 90% liquid water (assumingthe T1solid values in Table I).

The alternative hypothesis ascribes setting to intergrowth andinterpenetration of nonaqueous solid phases, particularly C–S–H. Although the latter hypothesis is not refuted by the datapresented here, it is troublesome for several reasons. First, liq-uid-state D2O has a field-independent T1 of 400 ms correspond-ing to a rotational rate constant of 2.8� 1011 s�1, whereas ourdata show field-dependent T1 values indicating rotational rate

constants or jump rates below 109 s�1. Only solid-state D2Oexperiencing jumps or supercooled liquid D2O at pressuresabove 2000 atm and temperatures on the order of 200 K couldexplain these observations. As the experiments were carried outat 1 atm and 296 K, the latter explanation is improbable. More-over, the ability of a paste to go through repeated cycles ofrheological measurements and each time exhibit Binghambehavior: a nearly consistent yield stress, followed by nearNewtonian behavior, argues against the intergrowth and inter-penetration hypothesis (M. W. Grutzeck, unpublished results).Struble and Lei26 presented rheological data for the setting ofcement paste, which led them to conclude ‘‘The failure strainincreased abruptly at the initial set, indicating a change in thenature of the forces that hold the particles together. Based onthis abrupt increase in failure strain and on the SEM evidence ofhydration during the induction period, it is concluded that theincrease in yield stress was due to the accumulation of gel be-tween cement grains, which increases the force by which thegrains are held together.’’ From this point of view, the solidwater bridges of our hypothesis fit well because they are basedon noncovalent, relatively weak hydrogen and ion dipole bondsthat do not hold the paste together strongly. The solid waterbridges provide some rigidity, but are not of high strength andwould be expected to break above a certain yield stress, beyondwhich the paste would exhibit fluid behavior. Further supportfor the solid water bridge hypothesis is provided by the obser-vation that ‘‘just set’’ pastes revert to fluid behavior withmicrowave heating. Although arguments to explain these ob-servations can also be made to fit the intergrowth/interpenetra-tion model of setting, the simpler explanation is provided by thesolid water bridge hypothesis. Therefore, we would now inter-pret Struble and Leit’s findings as evidence of increasing solidwater bridges with the reduction in interparticle spaces ashydration proceeds. Initial set occurs when a critical numberof bridges have formed to confer rigidity deemed as initial set.

The amount of heat given off during setting is much greater(1624 J for the 1.85 g calorimetry sample between 50 and 10045min) than the heat of fusion from the approximately 10% of theD2O that becomes solid during setting (ca. 19 J).27 Heat is alsoevolved during the formation of the C–S–H and Ca(OH)2 solidphases; hence, the assignment of the respective enthalpy changesis ambiguous. Stadelmann et al.28 used krypton adsorption mea-surements to study the surface area of hydrating C3S with time.They observed a rapid increase in the surface area of the C–S–Hduring the acceleratory period (the period of rapidly decreasingVicat penetration, rapidly increasing heat output, and rapidlydecreasing deuterium T1 values). This finding lends substance tothe idea that the heat evolution curve is due in large part to C–S–H formation. The accumulation of solid-state water occurs asmore solid silicate surfaces are formed, and the molecular filmsof water on their fresh surfaces are converted to the solid state.

VI. Conclusions

Systematic analysis of tricalcium silicate hydration shows thatsetting, the maximum rate of change in heat evolution, and themaximum rate of change in D2O T1 relaxation times occur si-multaneously within experimental error. The data are consistentwith the hypothesis that solid surfaces are covered with two orthree layers of solid-state water throughout the hydration reac-tions, and that when the adjacent surfaces grow to within four tosix water layers, the solid water layers fuse to form solid waterbridges. As very high surface area C–S–H and lower surface areaCa(OH)2 phases grow into the water-filled spaces between thereacting C3S grains, the distances separating the C–S–H foilsdecrease. Because they are immersed in an aqueous solutionthroughout the setting process, first contacts between growinghydrated particles must be preceded by ‘‘water contact.’’ Thesolid water bridges comprise the initial network contacts thatform during and after the setting process. When there are suffi-cient solid-state water bridges, the cement begins to exhibit what

0

0.2

0.4

0.6

0.8

1

1.2

0 20000 40000 60000 80000 100000 120000Time (min)

Fra

ctio

n S

olid

an

d L

iqu

id

Fraction SolidFraction Liquid

Fig. 5. Fractions of liquid- and solid-state water in D2O-hydrated C3Sas a function of time at 231C after mixing obtained from the experi-mental deuterium T1 data and Eq. (1), assuming T1liquid5 400 ms,T1solid5 4.6 ms at 46 MHz and 6.4 ms at 77 MHz (as observed forkanemite at room temperature).

Table I. Room-Temperature Deuterium T1 Values for theWater Resonance in D2O-Hydrated Materials

Material

Deuterium T1

value,

46 MHz (ms)

Deuterium T1

value,

77 MHz (ms)

D2O-synthesized kanemite 4.6 6.4D2O-hydrated zeolite A 7.3 9.7Freeze-dried D2O-hydratedstarch

5 10

Freeze-dried D2O-hydratedcellulose

5 10

D2O 400 400

1254 Journal of the American Ceramic Society—O’Hare et al. Vol. 94, No. 4

one refers to as setting as measured by Vicat needle penetration.The heat evolved during the setting process is primarily due tothe formation of stable C–S–H and Ca(OH)2 hydrates, but thereis also a small contribution from the heat of fusion of thesolid-state water on C–S–H and Ca(OH)2 surfaces. Even aftercomplete hardening for sealed samples, a significant amount ofliquid-state water remains in pores larger than 15 A due to theweaker forces farther from the solid surfaces.

In solid water bridges, the water molecules are hydrogenbonded to other water molecules in the adjacent layer or viatheir H atoms to oxygen in –Si–O–Si–, –Si–O�, Ca(OH)2, and/or via their O atoms to Ca21 cations, Ca(OH)2, and perhaps Si–OH groups. The dynamic solid water network and residual liq-uid-state water more than three layers from the nonaqueoussolid surface allow for Ca21 and SiO4

4� to diffuse, and providethese reactants access for the formation of more C–S–H foilsand fibrils with time. Finally, it is important to note that solidwater bridges are not likely to be the source of strength for fullycured and hardened cement. They may contribute, but it is morelikely that interpenetration, fusion, and entanglement of non-aqueous solid phases provide most of the strength in fully hard-ened cement.

Acknowledgments

We thank Ed Rakiewicz for suggesting investigations of cement with deuteriumNMR, John Phair for insights about kanemite, Burkhard Geil for his generoushelp with our jump dynamics model, Seong Kim for insightful discussions aboutsolid-state water at interfaces, and Shoshanna Pokras for assisting with some ofthe experiments.

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