Formation and characterization of calcium silicate ... · Formation and characterization of calcium silicate hydrate–hexadecyltrimethylammonium nanostructure James J. Beaudoin,a)

Formation and characterization of calcium silicatehydrate–hexadecyltrimethylammonium nanostructure

James J. Beaudoin,a) Harouna Dramé, Laila Raki, and Rouhollah AlizadehInstitute for Research in Construction, National Research Council of Canada,Ottawa, Ontario K1A 0R6, Canada

(Received 25 March 2008; accepted 14 July 2008)

Results of an investigation of the interaction potential of synthetic and pre-treatedcalcium silicate hydrate (C-S-H) [with hexadecyltrimethylammonium (HDTMA)] arereported. The effective and strong interaction of these molecules with the C-S-Hsurface was shown using 13C and 29Si cross polarization magic angle spinning (CPMAS) nuclear magnetic resonance, x-ray diffraction, thermogravimetric analysis,scanning electron microscopy, and Fourier transform infrared spectroscopy analysis.The HDTMA–C-S-H interaction is influenced by the poorly crystallized layeredstructure of C-S-H. An indefinite number of layers and an irregular arrangement areconfirmed by the SEM images. The position and shape of the 002 reflection of C-S-Hare affected by drying procedures, chemical pre-treatment, and reaction temperature.Recovery of the initial 002 peak position after severe drying and rewetting withdistilled water or interaction with HDTMA is incomplete but accompanied by anincrease in intensity. It is inferred that the stability of C-S-H binders in concrete canbe affected by a variation in nanostructure resulting from engineering variables such ascuring temperature and use of chemical admixtures.

I. INTRODUCTION

The volume stability of hydrated Portland cement andconcrete is central to durability issues concerning expo-sure to aggressive media.1 Disruptive expansions oftenoccur in concert with deleterious reactions between hy-drated cement phases (or cement minerals themselves)and ions in the pore solution present as a result of varioustransport processes.2 Mechanisms of expansion due toreactions with specific ions, e.g., sulfate and chloride,have been extensively discussed in the literature.3,4

Recently, clay scientists have had considerable interestin the volume stability and formation of organomineralderivatives because they combine the structural, physicaland chemical properties of both the inorganic host ma-terial and the organic guest species at a nanometer scale.5

Generally polymer/layered silicate (PLS) nanocompos-ites have attracted great interest, both in industry and inacademia because they often exhibit remarkable im-provement in material properties when compared withvirgin polymer or conventional micro- and macro-composites. These improvements can include high elasticmoduli,6 increased strength and heat resistance,7 de-creased gas permeability8 and flammability,9 and in-

creased resistance to biodegradation.10 Methods basedon the modification of pre-existing inorganic structuresoffer considerable potential for the design of newnanocomposites of interest to the construction and build-ing material industries. The intercalation of organic mol-ecules is well established in clay mineralogy5,11–14 andrelated layered structures such as calcium aluminate hy-drates.15,16

There is a paucity of information available on the be-havior of synthetic calcium silicate hydrate (C-S-H)modified with surfactants and polymers. The nature ofC-S-H is determined by many factors, including the com-position of the cement, the water to cement ratio (w/c),the curing temperature, the degree of hydration, and thepresence of chemical and mineral admixtures. Significantvariation in its composition, nanostructure, and morphol-ogy can occur.17 The objective of this work was to studythe behavior of modified quasi-crystalline synthesizedC-S-H to understand the potential impact of its compo-sition, nanostructure, and morphology on the durabilityof concrete structures. Results of a systematic study ofthe modification of C-S-H structure by surface interac-tion of hexadecyltrimethylammonium (HDTMA) mol-ecules in its structure are reported. The modifications arecharacterized using several techniques.

The present study is designed to determine to whatextent the surface interaction processes could affect thedurability of the concrete structures. This is relevant as

a)Address all correspondence to this author.e-mail: [email protected]

DOI: 10.1557/JMR.2008.0342

J. Mater. Res., Vol. 23, No. 10, Oct 2008 © 2008 Materials Research Society2804

anionic polymers are used as plasticizers and set-modifying admixtures in concrete technology. Theresults of a study to assess the relationship between thebehavior of hydrated calcium silicate and clay, based onthe similarity of their structural assembly and upon modi-fication with alkyl ammonium salts and polymers of dif-ferent molecular weight, are reported. The method ofC-S-H synthesis, effect of Na exchange and surfactant oninteractions with polymers, nature of organic guest mol-ecules and their molecular weight and size, as well as themechanism of ion interaction and specific adsorption atsurfaces, will be discussed. The 13C cross polarization(CP) and 29Si CP magic angle spinning (MAS) nuclearmagnetic resonance (NMR) spectroscopy, x-ray dif-fraction (XRD), thermal gravimetric analysis (TGA),Fourier transform infrared spectroscopy (FTIR), scan-ning electron microscopy (SEM), and energy disper-sive x-ray (EDX) techniques were used to characterizeand follow changes and behavior of the C-S-H modifi-cations.

C-S-H phases are the major reaction products (50–70% by mass) and primary binding phases in hydratedPortland cement.17,18 Recent models of hydrated Port-land cement nanostructure attest to the layered nature ofthe silicate phases. These include those of Richardson,Jennings, Feldman and Sereda, and Taylor.17,19–21 Cal-cium silicate hydrate structure is often referred to that oftobermorite.17,22,23 It has similarities to the clay mineralsin crystal structure assembly.5,23 The composite 2:1sheets are made up of a distorted calcium hydroxide sheetflanked on both sides by parallel rows of wollastonite-type chains having composition Ca4Si6O18.23 The re-maining or interlayer calcium atoms and water moleculesreside between them. Generally, two types of calcium areconsidered: “nonlabile” Ca linked to silica chains and“labile” Ca linked to Si–OH (silanol) groups. C-S-H hasa surface pH dependent charge due to the existence ofsilanol sites carried by bridging silica tetrahedra or by theend chain tetrahedra.17,18,22,24 Other characteristics inwhich C-S-H resembles clay are (to some degree) thevariability of basal spacing with water content, wide vari-ability in degree of crystallinity, and variability in C:Sratio from 0.83 to 1.75.5

II. EXPERIMENTAL

A. Materials

All chemicals used were of reagent-grade quality andwere not further purified unless otherwise specified.

1. C-S-H synthesis

C-S-H is the principal hydration product and primarybinding phase in hydrated Portland cement. C-S-H was

synthesized by mixing of CaO and reactive SiO2 in stoi-choimetric proportions in distilled water (preferably de-carbonated) under N2 to avoid exposure to the atmo-spheric CO2. The preparation of CaO consisted of heat-ing pure CaCO3 at 900 °C for at least 3 h and cooling ina N2 atmosphere. Amorphous silica (Cabosil) was heatedat 110 °C for 3 h and mixed intimately with the CaO (in500 ml high density polypropylene bottles) to obtain aC/S molar ratio of 1.6. The required amount of demin-eralized decarbonated water was added and the systemwas flushed with N2 before sealing. The reaction wasconducted at room temperature with the bottles rotatedfor up to 14 days. Longer hydration periods were notstudied in this investigation. The above synthesis proce-dure involved subjecting the slurry mixture to high speedshearing for 5 min before placing the bottles on rotatingrollers. The high speed shearing was at 6100 rpm/minusing a Silverson Laboratory Mixer (Waterside,Chesham Bucks, UK). The C-S-H was filtered under N2

and dried under vacuum for 12 h without heating. Ex-treme care was taken during the drying procedure toensure the reproducibility of the results and stability ofthe material.

Four batches of C-S-H with a C/S ratio of 1.6 wereprepared as described above but at different dryingtemperatures. They were labeled C-S-H01, C-S-H02,C-S-H03 (A and B), and C-S-H04. C-S-H01 was vacuumdried at 65 °C, C-S-H02 at 70–75 °C, C-S-H03A at 47–50 °C, C-S-H03B at 50–60 °C and C-S-H04 at 25 °C, allfor 12 h. Previous work by the authors on the volumestability of this C-S-H indicated that length change afterimmersion in distilled water was significantly reducedwhen vacuum drying occurred at temperatures above50 °C.

2. Pre-treatment of Pre-formed C-S-H

Selected C-S-H preparations were chemically pre-treated to assess their potential to accommodate organicintercalates. The treatment involved the following cationexchange and intercalation procedures.

3. Na–C-S-H

Preparation of Na–C-S-H from the originally preparedC-S-H material was carried out by reacting the C-S-Hwith NaCl solutions (20 g/l and saturated) for 24 h fol-lowed by filtration and washing with distilled water untilthe halide (Cl) was not detected with AgNO3. The ma-terials were dried under vacuum at 50–60 °C and labeledNa–C-S-H03B1 and Na–C-S-H03B2 to differentiate theconcentration of the NaCl solution. In an alternate ex-periment, an aqueous solution of NaCl (20 g/l) was useddirectly during C-S-H synthesis. The mixture was sub-mitted to high-speed shear at 6100 rpm for 5 min andreacted for 6 days. The sample obtained was dried at50–60 °C and labeled Na–C-S-H hs6d.

J.J. Beaudoin et al.: Formation and characterization of calcium silicate hydrate–hexadecyltrimethylammonium nanostructure

J. Mater. Res., Vol. 23, No. 10, Oct 2008 2805

4. HDTMA–C-S-H

Preparation of hexadecyltrimethylammonium(HDTMA) treated C-S-H designated HDTMA–C-S-Husing the reference C-S-H and Na–C-S-H as a precursorwas carried out by reacting C-S-H with an excess ofaqueous 0.01M HDTMA–Br (MW 354.46) solution for24 h followed by washing with distilled water until thehalide (Br) was not detected with AgNO3. The materialwas then vacuum dried for 12 h and designatedHDTMA–C-S-H or HDTMA–Na–C-S-H. In an alternateexperiment an aqueous solution of HDTMA was useddirectly during C-S-H synthesis (using high speed shearmixing at 6100 rpm/5 min) with different reaction times.The samples obtained were designated HDTMA–C-S-Hhs15min, HDTMA–C-S-H hs24h, HDTMA–C-S-Hhs3d, HDTMA–C-S-H hs6d (hs � high shear, d �days).

5. Summary: Specimen designations

A summary of the specimen designations is providedin Table I for easy reference.

B. Analytical procedures

1. NMR13C CP MAS NMR (50.33 MHz), and 29Si CP MAS

NMR (39.76 MHz) spectra were obtained using a BrukerASX-200 instrument (Madison, WI) with NMR magicangle spinning rates ranging between 3 and 6 kHz. 13CCP MAS NMR spectra were referenced to hexamethyl-benzene at � � 14.9 ppm; the 29Si CP MAS spectra werereferenced to tetramethylsilane at � � 0.0 ppm.

2. Powder XRD

Powder XRD were performed with a Phillips PW3710based diffractometer (at 45 kV and 40 mA; Almelo, The

Netherlands) using Cu K� radiation. Powder XRD pat-terns were recorded using step scanning with a step sizeof 0.02° 2� at 0.5° 2�/min in the interval 1.41° to 35° 2�,1.41° to 15° 2�, or 1.41° to 4° 2� when a low-angledetailed pattern was needed. A background subtractioncorrection (empty sample holder scan) was performed onall XRD patterns.

3. SEM

A Hitachi S-4800-FEG high-resolution (1 nm; Pleas-anton, CA) scanning electron microscope was used formicrostructural investigation of the various C-S-H prepa-rations.

4. Thermal analysis

The study of the thermal stability of all the sampleswas conducted using thermogravimetric analysis/differential thermal analysis (TGA/DTA) measurementsobtained with a Polymer Labs STA 1500H instrument(Amherst, MA) at a nitrogen flow rate � 25 cc/min anda heating rate � 20 °C/min from 30 to 1000 °C.

5. FTIR

FTIR spectra were obtained using a Bomem-Michelson MB 100 FTIR spectrometer (Zurich, Switzer-land) with 30–50 averaged scans at 4 cm−1 resolution.The samples were prepared as KBr pellets.

6. Carbon content determination

The amount of carbon in the C-S-H–HDTMA materialwas determined from the mass loss in a TGA instrumentat 600 °C. The atmosphere was changed from nitrogen(after a 1 h hold time) to air. This mass loss is a result of

TABLE I. Designations for C-S-H preparations.

Designation Description

C-S-H01 Vacuum dried at 65–70 °C for 12 hC-S-H02 Vacuum dried at 70–75 °C for 12 hC-S-H03A Vacuum dried at 47–50 °C for 12 hC-S-H03B Vacuum dried at 50–60 °C for 12 hC-S-H04 Vacuum dried at 25 °C for 12 hNa–C-S-HO3B1 C-S-H immersed in NaCl solution (20 g/l) for 24 h and vacuum dried for 12 h at 50–60 °CNa–C-S-HO3B2 C-S-H immersed in NaCl solution (saturated) for 24 h and vacuum dried at 50–60 °C for 12 hNa–C-S-H hs6d NaCl solution (20 g/l) used during synthesis of C-S-H. High-speed shearing (hs) for 5 min was used (see text) with a

6-day reaction timeHDTMA–C-S-H C-S-H immersed in 0.01M HDTMA–Br for 24 h and vacuum dried for 12 h at 25 °CHDTMA–Na–C-S-H Na–C-S-H 03B1 or B2 immersed in 0.01M HDTMA-Br for 24 h and vacuum dried for 12 hHDTMA–C-S-H hs 15 min HDTMA solution used during synthesis of C-S-H. High-speed shearing (hs) for 5 min was used with a 15 min reaction

timeHDTMA–C-S-H hs 24 h Same as above with 24 h reaction timeHDTMA–C-S-H hs 3d Same as above with 3 days reaction timeHDTMA–C-S-H hs 6 days Same as above with 6 days reaction time


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the oxidation of the carbon in the organic component ofthe material. The carbon content of the material was cal-culated to be 0.33% of the total mass assuming that allthe C and H in the HDTMA molecule (with the chemicalformula of C19H42BrN) has reacted with the oxygen. Anestimate for the HDTMA content is 0.53% of the totalmass.

III. RESULTS AND DISCUSSION

The results of the various characterization techniques(13C CP MAS NMR and 29Si CP MAS NMR, XRD,FTIR, and TGA) applied to the various C-S-H prepara-tions and test regimes are presented in the followingsections.

A. NMR Evidence for interaction of HDTMAmolecules with C-S-H surfaces

13C CP MAS NMR spectra of HDTMA-C-S-H(Fig. 1) preparations were obtained to investigate struc-tural properties of the organic component in this organo-calcium silicate hydrate. 29Si CP MAS NMR spectrawere also obtained to assess the nature of modificationsto the silicate-based C-S-H nanostructure due to the in-teractions with HDTMA molecules. Figure 1(a) providesevidence of the effective presence of HDTMA moleculesin the C-S-H matrix. The only significant 13C resonancesignal of HDTMA appears at 29.32 ppm correspondingto the three methyl groups. For the dipolar dephasingtechnique, the ratio of IDD/I0, where IDD and I0 are thepeak intensities of the 13C resonance obtained respec-tively with and without dipolar dephasing conditions, is asemi quantitative measure of the dynamic state of themolecular group.25–27 If a molecular group is rigidlyfixed, the ratio IDD/I0 will be decreased or disappear. Theoverall signal decay for carbons strongly coupled to pro-tons, such as methylene carbons, has been shown to bebest described by the following equation28:

IDD = I0e��2��2T23�� ,

where � is the dipolar dephasing delay time and T2 is thetransverse relaxation time constant.

A dipolar dephasing (DD) experiment (13C CP DDMAS) was performed for HDTMA–C-S-H [Fig. 1(b)]where the 13C signal was allowed to dephase for 40 �sunder cross polarization. It showed that 98% of the signalintensity was maintained compared to the simple CPMAS conditions [Fig. 1(a)]. This confirms that themethyl groups of HDTMA are functional and located atthe surface of the hydrate, presumably from rapid rota-tion about the C3 axis.

29Si CP MAS NMR spectra of C-S-H, Na–C-S-H, andHDTMA–Na-C-S-H are presented in Fig. 2. In all cases,the spectra are dominated by a doublet (Q1, Q2) locatedrespectively at (−79.3, −85.2 ppm), (−79.7, −85.9 ppm),and (−79.3, −84.8 ppm). These two resonances are char-acteristic of silicate dreierketten feature of the C-S-Hstructure. The Q2/Q1 ratio, which determines the meanchain length for the silicate entities present in the struc-ture significantly increases with the Na [Fig. 2(b)] andHDTMA [Fig. 2(c)] treatment, the latter having the larg-est value. This observation implies that the polymeriza-tion state of silicates is increased by the HDTMA and Nainteractions with C-S-H. This is analogous to an increasein the Q2 contribution when Ca/Si ratio decreases.27,28 AtC/S ratios >1.0, the silicate chains consist primarily ofdimers with defect sites at locations of missing bridgingtetrahedra. It is possible that the polymer can bond to theoxygens associated with the Q1 silicon i.e., –Si–O–[polymer]. These types of external bonding lead to shiftsin the Q1 peaks. It is theoretically possible that these Q1

peaks can shift to Q2 sites. This would account for theincrease in Q2/Q1 ratio in the spectra of the treatedsamples. Evidence for these type of surface interactions

FIG. 1. 13C CP MAS NMR spectra of HDTMA-C-S-H: (a) regular CPMAS spectra and (b) dipolar dephasing spectra.

FIG. 2. 29Si CP MAS NMR spectra of (a) C-S-H04, (b) Na-C-S-H04,and (c) HDTMA-C-S-H04. C-S-H designations are described in detailin Table I.



have been obtained for C-S-H modified with polyvinyl-alcohol (PVA) and silylated polymers.29,30

It is instructive to examine the work of Viallis et al.with respect to the interaction of Na and C-S-H.28 Theirresults suggest that Na has an affinity for the C-S-Hsurface. They report 29Si NMR data with the Q2/Q1 ratiomuch smaller than observed in the present work. Thiscould be due to differences in sample preparation meth-ods (NaCl was added to their C-S-H preparations after3 weeks) including the chloride concentration. The re-sults in this study indicate a significant increase in theQ2/Q1 ratio of the Na–C-S-H preparation (degree of sili-cate polymerization). This could be a result of decalcifi-cation of the C-S-H following immersion in NaCl solu-tion and the subsequent washing with distilled water.Evidence based on x-ray diffraction methods support thisview and will be presented in the following section onXRD.

A grafting mechanism operative at sites of missingsilica tetrahedra has been reported for silylated poly-mers.30 It is known that atoms in the vicinity of existing–O–Si–O–bonds can attract some of the electron cloud ofthe silicon resulting in a detectable chemical shift. Theseshifts depend on the strength of the new atoms in af-fecting the electrons. A shift for the first silicon atomwill occur in the following cases: –O–Si–O–H, –O–Si–O–Na, –O–Si–O–Si–, and –O–Si–O–[polymer]. Thechemical shift is different most of the time, but it istheoretically possible to have two different attachmentsthat result in a similar chemical shift. In other words, thechemical shift of silicon in the vicinity of the polymercan be similar to that obtained with a silicon bond tothe oxygen and mimic the latter labeled Q2. It wouldappear likely that organic molecules can be involved indifferent types of interaction with C-S-H including ad-sorption or grafting at sites of missing silica tetrahedra. Aschematic illustration of the C-S-H structure showingpossible sites for polymer adsorption or grafts is pro-vided in Fig. 3.

B. X-ray investigation ofC-S-H–HDTMA nanostructure

The nanostructures of the reference C-S-H and C-S-H-based HDTMA materials were investigated usingXRD analysis. The possibility of HDMTA moleculespartially penetrating the interlayer regions of C-S-H wasof particular interest. The x-ray pattern of the quasicrys-talline reference C-S-H (C/S � 1.6) is characterized bywell-defined peaks at approximately 0.306, 0.280, and0.182 nm. In the absence of guest molecules, a (002)basal reflection at 1.25 nm is clearly detected. The pat-tern obtained is similar to that reported in the literature31

for C-S-H (I). The 002 peak position will be used as areference peak for the HDTMA modified samples. Theeffect of drying at 70 �C on the two low angle peaks(d � 1.25 and 4.50 nm) [curve (a), reference] and theirrecovery after being in contact with distilled water for19 days and aqueous HDTMA for 24 h [curves (b) and(c) shown in Fig. 4]. The recovery of the peaks is sig-nificant. The intensity of the peaks is significantly greaterthan that of the reference. The recovery rate was alsomuch greater for the C-S-H treated with aqueousHDTMA solution. Drying of C-S-H at 70 �C inducescollapse of interlayer space.26 The volume change ofC-S-H is also significantly reduced.32 Although the002 peak has not shifted following treatment with dis-tilled water and HDTMA solution after the initial col-lapse of the C-S-H structure at 70 �C, it can be inferred(although not conclusively) that some intercalation of thepolymer may be possible, given the more rapid rate ofrecovery of the d-spacing in presence of the polymer.This, however, is not strong evidence. It has been shownin previous work that the volume stability of C-S-H im-mersed in distilled water is similar if drying temperaturesdo not exceed 50 °C.26 At more elevated temperatures,the volume change is significantly reduced. Hard dryinginduces collapse of interlayer space. More persuasiveevidence for the possibility of partial intercalation may

FIG. 3. Schematic illustration of C-S-H nanostructure showing possible sites for polymer graft.



result from examination of the basal spacing shift forC-S-H at temperatures of 50 �C. The predominantmechanism contributing to the formation of the C-S-H-HDTMA nanostructure is, however, likely not intercala-tion but possibly surface adsorption of organic moleculesat defect sites on the C-S-H surface as discussed in theprevious section.

1. HDTMA interaction during C-S-H synthesis:High-speed shear mixing

An attempt to modify the C-S-H by interactionwith HDTMA directly during synthesis of C-S-H wasperformed using a high-speed shear mixing method(6100 rpm for 5 min) and reacting for up to 6 days withconstant agitation and no heating. The material wasdesignated HDTMA-C-S-H hs6d. The evolution of the(002) peak does not fully develop until 6 days (Fig. 5).There is a small shift to a higher value of the basalspacing for these preparations compared to the synthesis

procedure used previously. This is not conclusive evi-dence for partial intercalation but would be expected ifthis mechanism is operative.

SEM images of the in situ formation of HDTMA-treated C-S-H(I) are shown in Fig. 6 for reaction times of15 min, 24 h, 3d [Figs. 6(a)–6(c)]. A platy microstructurebegins to form after 3 days reaction.

2. HDTMA interaction: Effect of Napre-treated C-S-H

The effect of HDTMA treatment of pre-formed C-S-H(dried at temperatures below 50 °C) is shown in Fig. 7.The reference C-S-H03A [curve (a)] has a (002) reflec-tion at d � 1.22 nm, typical of C-S-H dried in thistemperature range (i.e., 25–50 °C). The HDTMA-treatedC-S-H (HDTMA–C-S-H03A) dried for 48 h actuallyexhibits a shift in the basal spacing to a higher value thanthe reference, i.e., d � 1.38 nm. This shift would beexpected if intercalation were to occur [curve (b)] asswelling would likely occur due to the presence of theorganic molecule. Prolonged drying (48 h) of Na-treatedC-S-H (Na–C-S-H03A) results in a shift of the basalspacing to d � 1.01 nm indicative of structural collapse[curve (c)]. Peak broadening is also observed. TheHDTMA–Na-treated C-S-H has a similar basal spacing(d � 1.06 nm) but peak broadening is observed, indicat-ing some interaction of this material with HDTMA[curve (d)]. The diffraction pattern for the HDTMA–C-S-H (in situ reacted) system [curve (e)] has a patternsimilar to the Na-treated C-S-H reacted with HDTMA[curve (d)]. The basal spacing is similar, with only aslight difference in the peak broadening. It appears that alimited amount of intercalation of HDTMA in the C-S-Hinterlayer of both these preparations may be possible. Forexample, the difference in basal spacing values betweenNa–C-S-H03A (taken as a reference) and HDTMA–C-S-H03B (curve b) is �d � 0.37 nm. It is clear that it isimportant to define a reference (002) peak before makingany statement of possible intercalation. Each drying con-dition can lead to a shift in the (002) peak.

X-ray patterns (4° < 2� < 85°) for C-S-H04 (Ca/Si �1.6), Na–C-S-H04 and HDTMA–Na–C-S-H04 are pre-sented in Fig. 8. There is a small amount of portlandite(CH in Fig. 8) in the reference sample. The Na andHDTMA–C-S-H samples contain no portlandite. Decal-cification of the latter samples is, however, indicated bythe presence of calcite (CC in Fig. 8). There is an indi-cation of some calcite (d � 0.316nm) in the Na–C-S-Hsample. The shoulder on this peak is assigned to C-S-H.It has an intensity significantly greater than the corre-sponding peak for the control C-S-H. Further, it is notedthat the HDTMA treatment of the Na–C-S-H appears toincrease the amount of decalcification as more intensecalcite peaks are apparent. Decalcification of C-S-H gen-erally opens up the structure. For example, it has been

FIG. 4. Effect of C-S-H drying condition on the position of the002 XRD peak and its recovery following immersion in distilled water(19d) and HDTMA solution (24 h). Curves: (a) C-S-H02 vacuum dried12 h at 70 °C, (b) C-S-H02-DW immersed in DW 19d and vacuumdried 12 h at 25 °C, and (c) HDTMA-C-S-H02 immersed in aqueousHDTMA for 24 h and vacuum dried at 25 °C. C-S-H designations aredescribed in detail in Table I.

FIG. 5. Evolution of the XRD (002 peak development during synthe-sis of in situ HDTMA treated C-S-H employing high speed shearmixing (6100 rpm for 5 min). Hydration time: 15 min to 6 days.



demonstrated to significantly increase the rate of heliuminflow into the structure.33 If intercalation of HDTMAoccurs, it may be facilitated by the decalcification proc-ess.

3. Low-angle XRD basal spacing analysis

A feature of all the previous XRD patterns is the pres-ence of an additional peak at the lower angle between1.41° and 4° 2� (d ≅ 4.50 nm). Analysis [after the back-ground subtraction, curve (a)] of the evolution of the lowangle peak after Na and HDTMA treatments (Fig. 9)indicates similarities (in the increase in intensity, broad-ening, and shifting) as observed previously with the(002) peak. C-S-H in hydrated Portland cement productsis a poorly crystalline-layered silicate.21 Microstructuralexamination of the synthetic C-S-H analogues may pro-vide evidence to support strategies for modification ofC-S-H in Portland cement-based materials. The micro-structures for synthetic C-S-H and HDTMA modifiedC-S-H are shown in Figs. 10 and 11. HDTMA modifiedC-S-H has a microstructure consisting of a uniformlydispersed array of much smaller particles than those of

FIG. 6. Scanning electron micrographs of in situ HDTMA treatedC-S-H (HDTMA-C-S-Hhs) prepared using high-speed shear mixing(6100 rpm for 5 min). Slurry was hydrated for (a) 15 min, (b) 24 h,(c) 3 days, and (d) 6 days.

FIG. 7. Effect of HDTMA treatment of C-S-H and Na–C-S-H on theXRD 002 peak position. XRD curves: (a) C-S-H03A (vacuum dried12 h at 47-50 °C), (b) HDTMA-C-S-H03A (immersed in aqueousHDTMA solution and vacuum dried 48 h at 25 °C), (c) Na-C-S-H03A(C-S-H03A immersed in NaCl solution for 24 h and vacuum dried12 h at 47–50 °C), (d) HDTMA-Na-C-S-H03A (immersed in aqueousHDTMA solution and vacuum dried 48 h at 25 °C), and HDTMA-C-S-H hs bd vacuum dried at 25 °C). C-S-H designations are describedin detail in Table I. Numbers in brackets indicate 2� angle and d-spacing (nm).

FIG. 8. XRD patterns (4� < 2� < 85�) for C-S-H04 (Ca/Si � 1.6),Na-C-S-H04, and HDTMA–Na–C-S-H04 [CC � CaCO3; CH �Ca(OH)2].



the reference C-S-H. An array of smaller particles maybe conducive to more efficient packing and bonding ar-rangements promoting improved engineering perfor-mance. These micrographs show that a simple organicmolecule e.g., HDTMA, is able to dramatically modifythe texture of the material. The implication of this withrespect to control of the growth and organization of C-S-H particles in cementitious materials remains to beclarified. Changes to the accessibility of helium gas intothe structure will be explored to assess the potential forreduced ingress of destructive ions. The Na exchangedstructures (not shown) are more dispersed and platy-like.The C-S-H synthesized in situ in presence of NaCl so-lution (Fig. 12) also has a platy-like microstructure. Thepresence of water in the structure renders the surfaceslightly hydrophilic, and the treatment with HDTMAshould favor interaction with this molecule. It was con-cluded above that a partial intercalation of C-S-H withHDTMA may be possible. This is a manifestation ofirregular layer stratification of the C-S-H as evidenced inthe SEM images (Figs. 10–12). Similar observations ofirregular layer stratification with serrated edges havebeen made for clays intercalated with organic molecules.The presence of water, its removal upon drying, and thereaction temperature are critical aspects of the interactionprocess due to the structural changes that take place withreorganization of layers or chemical bond formation.Water molecules are most likely bonded to Ca2+ ions inthe interlayer. Loss of interlayer water results in forma-tion of Ca–O–Si bonds, and this next nearest neighboreffect may cause some changes in the bond angles anddistances in the silica tetrahedra.34

4. Summary

XRD study has shown how the temperature can affectthe structure of C-S-H and its interaction with HDTMAmolecules. It has provided some evidence that the inter-action of HDTMA molecules with C-S-H may includeintercalation. This mechanism, however, needs furthervalidation. The temperature effects are in agreement withthe study of Cong et al.,35 which shows the inappropri-ateness of oven drying C-S-H at 110 °C because at thistemperature, the layered structure collapses and the po-lymerization of silicate chains changes. In this study, ithas been shown that vacuum drying over 50 °C and pro-longed vacuum drying at ambient temperatures have asimilar effect on the C-S-H structure. It is suggested thatif possible mild vacuum drying without any source ofheat should be used (depending on the amount of wetsample being treated) to preserve the structure of thehydrate.

FIG. 10. SEM micrographs of C-S-H04 (C-S-H vacuum dried at25 °C for 12 h).

FIG. 9. Evolution of low-angle XRD peak (about 2.15°2�) for Na,HDTMA, and PEG polymer treated C-S-H. XRD curves: (a) back-ground, (b) C-S-H03B (vacuum dried at 60-65 °C for 12 h), (c) Na–C-S-H03B (vacuum dried at 50–60 °C), (d) HDTMA–Na–C-S-H03B(vacuum dried at 25 °C for 12 h). C-S-H designations are described indetail in Table I.



C. FTIR analysis

The infrared absorption spectra of Na–C-S-H andHDTMA–Na–C-S-H are given in Fig. 13 to further char-acterize the effects of the pre-treatment of C-S-H onpolymer interaction. The Na–C-S-H spectrum [curve (a)]shows absorption bands at 675, 877, 981, 1452, 1625,and 3460 cm−1. Additional bands are observed at 1818,2881, and 2954 cm−1 for HDTMA–Na–C-S-H [curve(b)]. The 3460 cm−1 broad band is attributed to hydro-gen-bonded surface hydroxyl stretching vibrations ofH2O molecules. The 1452 and 877 cm−1 bands are re-lated to carbonation of C-S-H at higher Ca/Si ratio36 asevidenced also by the 13C CP MAS. The weak broadband at 1625 cm−1 corresponds to the bending of the OHgroups in C-S-H and water molecules in the interlayer.The 675 and 981 cm−1 bands are attributed to bendingand asymmetric and symmetric stretching vibrations ofSiO4, respectively.37,38 The bands in the range 400–

500 cm−1 are due to deformation of SiO4 tetrahedra. Nofree OH stretching absorption bands at 3600–3640 cm−1,which could be attributed to less strongly hydrogen-bonded interlayer water molecules and any calcium hy-droxide sites, were observed in all samples.36,37

The additional bands at 1818, 2881, and 2954 cm−1

observed in treated Na–C-S-H samples with HDTMAconfirm the presence of organic molecules in the C-S-Hmatrix. They correspond to CH2 and CH stretching fre-quencies of the alkyl chain in HDTMA39 as confirmed bythe 13C CP MAS NMR spectra. The 1818 cm−1 band isevidence of the possible presence of a carbonyl group.

Despite confirming that the organic molecules are as-sociated with the nanostructure of C-S-H, the FTIR spec-tra shows that only a small amount of HDTMA is re-tained by the solid. This observation supports the viewthat intercalation of HDTMA is minor if it occurs at all.

D. Thermal analysis

1. TGA

TGA provided additional evidence of the character ofthe polymer interaction with the C-S-H preparations.

The TGA curves of C-S-H and modified C-S-Hsamples (Fig. 14) are similar in character up to 200 °Cand different above this temperature. The curve for thereference C-S-H (Fig. 14) is similar to that reported in theliterature,38,39 except that there are no significant weightlosses in the temperature range 450–490 °C typically as-sociated with portlandite decomposition. The referenceC-S-H continuously loses weight with increasing tem-perature up to about 700 °C. There are 3 stages in theweight loss–temperature curve. The first stage begins atabout 100 °C (the specimen was vacuum dried for14 days prior to testing). This is due to loss of adsorbed,interlayer, and some compositional water. The second

FIG. 11. SEM micrographs of HDTMA treated C-S-H (HDTMA-C-S-H04). See Table I for details.

FIG. 12. SEM micrograph of C-S-H synthesized in a saturated NaClsolution i.e., in situ formation using a high-speed shear mixing pro-cedure.



change beginning at about 430 °C (a region of smallweight loss) is probably due to a loss of surface Si–OHgroups. A small amount of free lime (if present) wouldalso decompose in this region. The third stage beginningat about 680 °C is due to the loss of compositional waterand possible decomposition of any calcium carbonatethat may have formed.

The HDTMA modified samples have greater weightlosses than the reference C-S-H. The difference at theend of the first stage was 4.5% (at 400 °C) for bothHDTMA–C-S-H, HDTMA–Na–C-S-H and can be as-signed to the decomposition of surface adsorbedHDTMA. The difference (relative to the reference) at theend of the second stage (685 °C) was 6.0, 8.5, 9.5, and12.5% weight loss. The differences at the end of the thirdstage (1000 °C) were 10 and 7.5% of weight loss.

2. DTGA

The three major stages of weight loss are clearly seenin the DTGA curves (Fig. 15). The peaks for the refer-ence C-S-H are typically at 160, 430, and 675 °C. Thereis also a shoulder at 400 °C indicating there may be two

decompositions occurring in this stage. A peak at about550 °C is present in all the treated C-S-H and is likelydue to the decomposition of HDTMA complexes. Thesecond peak at 430 °C (for the reference) is significantlyreduced (almost disappears) with HDTMA treatment, al-though low-temperature effects (at 200–300 °C) are ob-served with HDTMA treatment of Na–C-S-H. The sec-ond major peak at 430 °C is recovered. It is evident thatthe intercalation of HDTMA into the interlamellar spaceof C-S-H has dramatically altered the decomposition se-quence of C-S-H. This is seen in the reduced tempera-tures of C-S-H dehydroxylation as well as the appearanceof additional weight loss at 383, 555, 800, and 904 °C.Structural reorganization occurs above 830 °C and up to914 °C for the analyzed samples. The structural collapsefollowed by the complete dehydroxylation of the C-S-Hitself appears to cause the residual interlayer carbon-aceous material to be trapped within a meta–C-S-H likematrix, forming a carbon–calcium silicate nanocompos-ite material. Some of this carbonaceous material may bereleased once the material undergoes this structural reor-ganization between 800 and 1000 °C. Combustion in air

FIG. 13. FTIR patterns for (a) Na–C-S-H03B and (b) HDTMA–Na–C-S-H03B.

FIG. 14. TGA curves illustrating the effects of Na and HDTMA pretreatment of C-S-H. C-S-H designations are described in detail in Table I.



occurs relatively fast, and a weight loss not normallyassociated with a structural reorganization is observed.12

IV. CONCLUSIONS

The present study has shown how the temperatureand drying conditions affect the structure and ability ofC-S-H to interact with HDTMA molecules. The resultsshow that vacuum drying over 50 °C and even prolongedvacuum drying at 25 °C lead to a collapse of the inter-layer space of the C-S-H structure. It is suggested thatmild vacuum drying without any source of heat be usedto preserve the structure of the hydrate. The study hasalso shown that depending on the drying conditions, in-tercalation of HDTMA into the C-S-H interlayer spacecannot be completely ruled out. It needs additional vali-dation to confirm its role. The importance of defining a002 peak reference position (XRD) before making anyassessment of possible interlayer penetration is recom-mended.

The present study has provided evidence (based onTGA) that the interaction of C-S-H surfaces withHDTMA molecules can dramatically alter the decompo-sition behavior of C-S-H. This was reflected in the re-duced temperatures of C-S-H (I) dehydroxylation as wellas the appearances of additional weight losses at 383,555, 800, and 904 °C. Structural reorganization occursabove 830 °C and up to 914 °C for analyzed sampleswith weight loss thought to be the result of the release ofcarbonaceous materials trapped within a meta-C-S-H-like matrix. All data are consistent with the structuralintegrity of C-S-H being maintained after organic modi-fication. It can be inferred from this study that the curingtemperature, degree of hydration, and presence of chemi-cal and mineral admixtures in Portland cement-basedmaterials can significantly influence the nature of theC-S-H composition, nanostructure, and morphology. Anunderstanding of these processes could have meaningful

impact on the long-term durability of concrete structures.Further, it is suggested that tailoring the nanostructure ofC-S-H-based materials offers a potential route for therealization of durability strategies.

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