nanomechanical

Article pubs.acs.org/JPCC

Effect of the Ca/Si Molar Ratio on the Micro/nanomechanical Properties of Synthetic C‑S‑H Measured by Nanoindentation Fernando Pelisser,*,† Philippe Jean Paul Gleize,‡ and Alexandre Mikowski§ †

Civil Engineering Department, Universidade do Extremo Sul Catarinense, 88806-000, Criciúma, Santa Catarina, Brazil Civil Engineering Department, Federal University of Santa Catarina, CxP 476, 88040-900, Florianópolis, SC, Brazil § Centro de Engenharia da Mobilidade, Federal University of Santa Catarina, 89218-000, Joinville, SC, Brazil ‡

ABSTRACT: Calcium silicate hydrate (C-S-H), the main product in Portland cement hydration, influences the physical and mechanical properties of most cementitious materials. However, there are no structural models that currently relate chemical composition, nanostructure, and microstructure with the physicochemical and mechanical properties. In this work, the indentation technique was used to evaluate the micro/ nanomechanical properties of synthetic C-S-H with different Ca/Si (CaO/SiO2) molar ratios. C-S-H was also characterized by X-ray diffractometry (XRD), Fourier transform infrared spectroscopy (FT-IR), and X-ray fluorescence (XRF). Analysis of the results verified that the elastic modulus and hardness increased when the Ca/Si molar ratio of C-S-H decreased, achieving elastic modulus values of 27 and 20 GPa for Ca/Si ratios of 0.7 and 2.1, respectively, corroborating calculations based on the force field method of Manzano et al.1 Our results also determined that micro- and nanoporosity significantly influence the outcome. The research results are limited to synthesized C-SH, but clarify the potential of the Ca/Si ratio to modify the mechanical properties, while permitting investigation of C-S-H without the presence of other phases of hydrated Portland cement.

1. INTRODUCTION It is well-known that calcium silicate hydrate (C-S-H), the main product in Portland cement hydration, has a strong influence on the physical and mechanical properties of most cementitious materials. Despite the great socioeconomic importance of Portland cement-based materials and the existence of several proposed models for describing the structure of C-S-H (the reader can find a review of most of these models in Table 2 of ref 25), so far, no existing model relates its chemical composition, microstructure, and nanostructure with the physicochemical and mechanical properties.2 Without this knowledge, it is not possible to understand and control the behavior of cement-based materials, such as creep and shrinkage.3 This difficulty is closely related to the intrinsic complexity of C-S-H structure: (i) C-S-H has a structural multiscale organization formed by agglomerated particles or nanocrystallites composed of stacked lamellae formed by a double central layer of CaO octahedra inserted between two layers of SiO4 tetrahedra, which are kinked with a periodicity of three tetrahedral; these chains are called dreierketten.4 Thus, C-S-H is an amorphous, nanoporous composite, with a very large surface area.5 (ii) The feature as described above provides C-S-H with a strong affinity for water, which interacts differently with the surfaces of C-S-H according to the degree of confinement in the structure; in the nanopores, water © 2012 American Chemical Society

is absorbed on the nanocrystallite surface and inserted in the intralayer space.6 (iii) C-S-H is a nonstoichiometric compound. For hydrated Portland cement pastes, the average Ca/Si molar ratio of C-S-H is around 1.7; however, large local variations in composition occur, between 0.6 and 2.3 or greater.7−9 The C-S-H present in hardened C3S or neat Portland cement pastes generally has a mean Ca/Si ratio of about 1.75,7 with a range of values within a given paste from 1.2 to 2.1; if the paste contains a supplementary cementing material, then the mean value is further reduced, in some cases to less than 1.7 There are at least 30 crystalline minerals that are similar in composition to C-S-H.10 However, even though their overall chemical composition is similar, they differ in atomic arrangement, Ca/Si ratio, and the number of OH and H2O groups.10 It is widely accepted that C-S-H has a layered structure that is most akin to that of tobermorite and jennite minerals. Given their similar chemical compositions and crystal structure, tobermorite and jennite minerals have been suggested as possible model crystals for the C-S-H structure.11 In this research, the micro/nanomechanical characteristics of different Ca/Si molar ratios in C-S-H synthesis were accessed by nanoindentation, to more clearly evaluate the relation Received: March 7, 2012 Revised: July 11, 2012 Published: July 18, 2012 17219

dx.doi.org/10.1021/jp302240c | J. Phys. Chem. C 2012, 116, 17219−17227

The Journal of Physical Chemistry C

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Table 1. Summary of Some C-S-H Mechanical Properties Characteristics Reported in the Literature reference Constantinides et al. (2003)

14

Constantinides and Ulm (2007)15 Acker (2001)16 Mondal and Shah (2007, 2008)a17,18

Hughes and Trtic (2004)19 Zhu et al. (2007)20 Jennings et al. (2007)21 Vandamme et al. (2009)22

C-S-H

Ca/Si

elastic modulus (GPa)

hardness (GPa)

density (kg/m3)

porosity (%)

LD HD LD HD LD HD LS MS HS LD HD LD HD LD HD LD HD

− − − − 1 − − − 2.36 1.85 − − − − − −

21.7 29.7 18.2 29.1 20.0 31.6 22.89 31.16 41.45 25.74 22.97 23.4 31.4 18.1 31.0 22 32

0.8 1.0 0.45 0.83 0.8 0.9 0.93 1.22 1.75 0.88 0.88 0.73 1.27 − − 0.6 0.9

− − − − − − − − − − − − − 1700 2000 − −

− − − − − − − − − − − − − 36 26 − −

The authors classified C-S-H into three categories related to the distance from an unhydrated particle: 5 μm (HS: high stiffness); 10 μm (MS: medium stiffness); 20 μm (LS: low stiffness). The MS and LS C-S-H correspond respectively to the HD and LD C-S-H of other studies. a

only one type statistical analysis (bimodal distribution) to interpret the results of the nanoindentation technique, because of the strong influence of the roughness of cement pastes, and they recommended providing images of the individual indentation sites or of the indentation surface to examine the results more precisely. In cement paste, some authors have reported that HD C-S-H usually presents an average Ca/Si (CaO/SiO2) molar ratio higher than that of LD C-S-H,16,12 while others have reported inverse results.19,24 This inconsistency could be due to C-S-H entanglement with other phases,7,19 such as calcium hydroxide.6 On the other hand, considering the models that describe C-S-H structure, it is well-known that the length of the silicate chains decreases when the Ca/Si molar ratio increases,9,25 leading to variations in C-S-H structure, particularly the nature of intralamellar interactions and between nanocrystallites, and that an increase in the Ca/Si ratio reduces the interplanar distance, promoting increased strength and density.26,27 In synthesized C-S-H, one study28 verified the effect of the Ca/Si molar ratio (0.6, 0.8, 1.0, 1.2, 1.5, and 1.6) on the Q2/Q1 (types of connections between SiO4 tetrahedra) ratio and, consequently, on the mean length of silicate chains of C-S-H between dreierketten units, observing an increase in length as the ratio decreased, principally for a Ca/Si ratio less than 1.0. Therefore, it is likely that the mechanical properties vary according to the Ca/Si ratio. Using force field atomistic methods to model the C-S-H structure, another study showed that the C-S-H elastic modulus increases with the average length of silicate chains, when the Ca/Si molar ratio decreases.1 This observation is apparently inconsistent with the trend that HD C-S-H presents a higher Ca/Si ratio and displays a greater elastic modulus and hardness compared with that for LD C-SH, with a low Ca/Si ratio. However, the authors also stated1,23,26 that the packing factor or intrinsic porosity of the C-S-H gel is another important factor that should be considered in assessing the elastic properties, as shown by Jennings et al.21 in Table 1. The results of Thomas et al.27 also contributed to this theory, because the relations between the composition and density of tobermorite, jennite, and C-S-H show that the greater the H2O/SiO2 molar ratio, the greater the

between chemical composition and mechanical properties and contribute to studies regarding the structure of C-S-H under experimental conditions. Despite the differences between C-SH in hydrated Portland cement and synthesized C-S-H, the latter provided micro/nanomechanical analysis with no interference from other phases of hydrated Portland cement, permitting more precise analysis of the effect of the Ca/Si molar ratio. The study was also designed to verify that scanning electron microscopy (SEM) morphological analysis of the indented areas is an important tool for interpreting the results obtained.

2. EFFECT OF THE CAO/SIO2 RATIO ON MECHANICAL PROPERTIES In Portland cement pastes, it is well-known that there are at least two different types of C-S-H, which are formed by different mechanisms and have different densities and distinct structures. These two types of C-S-H, called inner product (Ip) and outer product (Op)12 or high- (HD) and low-density (LD),13 contribute in different ways to the properties of cement-based materials. However, using nanoindentation measurements, it was determined that the elastic properties of the two C-S-H do not depend on mix proportions,2 which only affect the volume ratio of the two types of C-S-H. The nanoindentation technique has proven to be an effective tool in the evaluation of micro/nanomechanical properties (hardness and elastic modulus) of C-S-H in cement paste.14−22 The results obtained by several research groups are summarized in Table 1. In general terms, analysis of these results indicate a bimodal distribution of the elastic modulus and hardness consistent with the existence of two types of C-S-H, high (HD) and low density (LD). On the basis of a statistical indentation technique, the existence of a statistically significant third hydrated mechanical phase became evident, in addition to the previously known low-density (LD) and high-density (HD) CS-H phases.22 The micro/nanomechanical properties of this third phase were shown to follow similar packing density scaling relations to LD and HD C-S-H, while being significantly greater. This third phase is therefore termed the ultra-highdensity (UHD) phase.22 Trtic et al.23 questioned the use of 17220



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Figure 1. Micro/nanomechanical properties (elastic modulus and hardness) of C-S-H with a Ca/Si molar ratio of 0.7 (A) and 2.1 (B).

a NaOH solution (10 mol/L). After the suspension was maintained at 60 °C for 35 days under gentle stirring in a CO2free atmosphere, the precipitates were vacuum-filtered and washed with acetone and CO2-free deionized water to eliminate residual sodium/nitrate ions. Next, the precipitates were dried at 60 °C in a vacuum oven for 14 days. The small irregular particle formed was vacuum-impregnated in 2 × 2 cm cylinders with a low-viscosity resin and cut with a diamond saw. The surface was ground with silicon carbide papers (100, 220, 320, 400, 600, and 800 grades) and polished diamond pastes (four stages of decreasing fineness: 6, 3, 1, 0.25 μm) to obtain a very flat, smooth surface finish. After each grinding/polishing stage, the samples were placed in an ultrasonic bath to remove the dust and diamond particles left on the surface or in the porous structure. Hardness (H) and elastic modulus (E) of the synthesized CS-H with different Ca/Si (CaO/SiO2) molar ratios were measured by nanoindentation or instrumented indentation test at micronanoscale using a Nanoindenter XP (MTS System) with a Berkovich indenter. The micro/nanomechanical properties were derived from analyses of load−displacement data using the Oliver and Pharr method.30,31 The unpolished surface of the cylindrical specimens was carefully cut with a diamond saw into 5−6 mm thick sections. Twelve indentations were performed on each sample in three different regions (for each region, four individual indentations were performed in a 2 × 2 matrices, with 20 μm spacing) by

atomic packing density of the structure of tobermorite, jennite, and C-S-H. Thus, a high CaO/SiO2 ratio also presents a low H2O/SiO2 ratio and a greater packing density, leading us to believe that the C-S-H would present increased strength. To our knowledge, all the published papers concerning the measurement of elastic properties by nanoindentation of C-S-H were performed on cement pastes14−22 and unfortunately, in the majority of cases, the Ca/Si molar ratios of C-S-H, which are an indirect assessment of the degree of silicate polymerization, were not measured. Thus, the micro/nanomechanical properties of hydrated Portland cement are frequently subdivided according to the values obtained in the indentation test and are classified as calcium hydroxide, high-density (HD C-S-H), low-density C-SH (LD C-S-H), and nonhydrated clinker. However, the intermixing of these phases, particularly between HD C-S-H and LD C-S-H, also complicates the relation between its chemical composition and mechanical properties.

3. MATERIALS AND METHODS For the production of C-S-H, the direct precipitation method was followed.29 A 1 mol/L calcium nitrate solution was added gradually to a 0.22 mol/L solution of sodium silicate predissolved in CO2-free deionized water to achieve Ca/Si molar ratios of 0.7 and 2.1. The total water:solid ratio was 20. The pH of the mix was maintained between 13.1 and 13.3 using 17221



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applying five loading cycles of 2, 4, 8, 16, and 32 mN in each indented location (type 1 matrix). An additional test was performed using higher loads (type 2 matrix), to cover a greater region, principally to make marks that would be visible under SEM. In this case, a 2 × 3 point matrix was used up to a load of 512 mN (1, 2, 4, 8, 16, 32, 64, 128, 256, 512 mN). Loading was applied linearly for 10 s, the maximal load was then maintained for 5 s, and unloading occurred over a further 10 s. The images of the indentation matrix, residual plastic impressions, and fracture process were collected on a SEM. Another part of the precipitate was ground in an agate mortar and sieved (# 75 μm) for XRD, FT-IR, and FRX analyses. XRD was performed to detect shifts in the C-S-H 002 basal reflections. A Philips X’pert instrument was used that operates with Cu Kα radiation (λ = 1.5418 Å) with an output of 40 kV and 30 mA. Scanning was performed from 2° to 10° (2θ) in 0.02° steps, with a collection time of 5 s per step. The sample for the infrared spectroscopy assay was prepared by pressing the material into a pellet shape, using a proportion of 95% potassium bromide (KBr) and 5% analysis material. The pressing load for the 13 mm diameter pellet was 9 tons. The equipment used was a Shimadzu Privilege model. The analysis was performed by transmittance, at a speed of 0.2 cm/s and a resolution of 4 cm−1, with a range of 400 to 4000 cm−1. The final Ca/Si molar ratios were obtained by X-ray fluorescence (XRF).

4. RESULTS The micro/nanomechanical properties of the synthesized C-SH were analyzed, considering as variables the indented area, the load used, and the Ca/Si molar ratio. In Figure 1, the values of elastic modulus and hardness for Ca/Si molar ratios of 0.7 and 2.1 are presented. XRF was used to measure the final Ca/Si ratios of 0.6 and 1.6 for mixtures of 0.7 and 2.1, respectively. This reduction in the Ca/Si molar ratio commonly occurs in the formation of C-S-H, because its structure presents a maximum Ca/Si ratio of 1.5 to 1.75, considering the absence of Ca(OH)2.6,23 Statistical analysis (ANOVA) showed a nonsignificant effect between the areas and a nonsignificant effect for the applied load used (Figure 1). A significant effect of the Ca/Si ratio was verified, for which a mean elastic modulus (load 2 mN) of 26.6 GPa (±1.8) and 19.3 GPa (±1.2) was determined for Ca/Si ratios of 0.7 and 2.1, respectively (Figure 2). The difference in hardness was more pronounced: 1.08 GPa (±0.08) and 0.19 GPa (±0.05) for Ca/Si ratios of 0.7 and 2.1, respectively. Comparisons using a load of 2 mN are the consensus for assessing the micro/nanomechanical properties of cement pastes;2,15,32 however, because in this work several cycles of loading were performed and the load used was not a significant variable, for quality comparisons in some figures and tables, the average value of all loads is assumed. Figure 3, showing micrographs of an indented area for the two Ca/Si ratios (type 1 grids), demonstrated that interference occurs in the results obtained due to the roughness of the material surface. One way to minimize this effect is to perform loading cycles involving greater loads.32 This procedure also permits the evaluation of larger areas. For this reason, several cycles of loading for each indentation were adopted. Figures 4 and 5 present the indentations (load up to 512 mN) in the two grids for Ca/Si molar ratios of 0.7 and 2.1 respectively. The presence of microcracks on the surface can be observed in Figure 4, but the values obtained showed no change as a function of these cracks, except for indentation III,

Figure 2. Effect of Ca/Si molar ratio on the mean elastic modulus (a) and mean of hardness (b).

which struck near the fissure area. Table 2 shows the values of the indentations shown in Figures 4 and 5. It is important to consider that in the indentation tests performed on cement pastes, as reported in the literature,7,8,33 C-S-H synthesis shows a lower roughness. Figure 6 and Table 3 show that minimal variation occurred in the elastic modulus and hardness for the mean values in load cycles from 2 mN up to 32 mN and up to 512 mN, and for the mean values of all the loads of the indentation grids, both in the smaller area and load (type 1 grids) and in the larger area and load (type 2 grids), indicating the formation of a homogeneous material.20 The increase in the elastic modulus according to the increase in load (Ca/Si = 2.1) is attributed to contributions of the lower compacted layers on loading, resulting in increased density (Figure 5b,c), and the decrease (Ca/Si = 0.7) is attributed to the microcracks that occurred in the indentation (Figure 4b, 4c). The elastic modulus for the Ca/Si ratio of C-SH of 0.7 and 2.1 were 27 and 20 GPa, respectively, presenting an approximate difference of 25%. The values of hardness were 1.10 and 0.32 GPa (mean of the two grids) for the Ca/Si ratio of C-S-H of 0.7 and 2.1, respectively, presenting a more significant difference. FT-IR confirmed the theory1,28 that a reduction in the Ca/Si ratio increases the polymerization of silica tetrahedra, as shown in Figure 7, where the presence of type Q1 and Q2 connections occurred for 0.7 C-S-H and a reduction in the spectrum in Q2 17222



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Figure 3. Micrographs of the indented grids (1−32 mN) for Ca/Si molar ratios of 0.7 (a) and 2.1 (b).

for 2.1 C-S-H was observed, indicating a lower degree of polymerization. The results of XRD (Figure 8) showed that the reduction in the Ca/Si ratio caused an increase in the interplanar distance of C-S-H, because it interferes in the quantity of water molecules in its structure.34,35 Considering the models proposed by Bonnacorsi et al.36 for tobermorite 14 Å, formed with a Ca/Si ratio of approximately 0.7, a theoretical composition Ca5Si6O16(OH)2·7H20 is presented, whereas for a Ca/Si ratio of 1, the structure presents load balancing with at least three molecules of water, maintaining the approximate composition Ca5Si5O14(OH)2·4H2O and interplanar distance 11.2 Å.36 In this work, the formation of C-S-H with interplanar distances of 12 Å and 15 Å was observed for a Ca/Si ration of 2.1, thus indicating greater heterogeneity compared with the Ca/Si ration of 0.7. The results presented here cannot be directly transferred to C-S-H formed from the hydration of Portland cement, but as mentioned previously, similarities exists between the nanostructure of C-S-H in Portland cement and synthesized C-S-H; these similarities are evident between their micro/nanomechanical properties, determined by the same method and apparatus described herein. Studies characterizing the nanostructure of the cement paste resulted in mean values of indentation modulus 17.2 ± 2.9 GPa and 16.8 ± 2.3 GPa (in two samples of Portland cement paste with w/c = 0.5),33 indicating the formation of low-density C-S-H, as mentioned in the literature.15,18 Results obtained from TEM micrographs for the two types of C-S-H (Ca/Si = 2.1 in Figure 9 and Ca/Si = 0.7 in Figure 10) reveal the characteristics of C-S-H and the similarities of its disordered particulate arrangement to that of C-S-H from Portland cement.

Figure 4. Micrographs of the indented grids (1−512 mN) for a Ca/Si molar ratio of 0.7 (a), with zoom in the indentation I (b), high value, and indentation VI (c), low value (at 32 mN).

5. DISCUSSION As outlined in the Introduction, where it is desirable to increase the elastic modulus and hardness, C-S-H should be produced with a low Ca/Si molar ratio.1 However, for this to occur, the packing factor or intrinsic porosity of the C-S-H gel must be maintained constant. The interplanar distance of C-S-H can also affect its porosity and mechanical properties. While evaluating the indentation modulus by computational methods, Shahsavari et al.26 estimated an elastic modulus of 14, 11, and 9 Å (based on the Merlino model) for tobermorite at 55, 90, and 103 GPa, respectively. Figures 4b and 5b clarify that when assessing porosity, at least qualitatively, the use of thin materials assists in data interpretation, as suggested by Trtic et al.23 We expected that the elastic modulus of C-S-H with a Ca/Si ratio of 2.1 would be smaller than that of C-S-H with a Ca/Si molar ratio of 0.7. This difference was probably affected by the greater 17223



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Figure 6. Mean micro/nanomechanical properties (1−512 mN grid, Ca/Si ratio of 0.7 and 2.1).

because the first module presents hardness greater than that of the second module, it is clear that in this case, the particle packing of C-S-H or density is the predominant factor for these mechanical characteristics. Analysis of the results obtained corroborated this finding (Table 1).21 The results of work by Thomas et al.27 also contributed to this theory, because the relations between the composition and density of tobermorite, jennite, and nanoscale CaO−SiO2−H2O show that the greater the H2O/SiO2 molar ratio, the greater the atomic packing density of the structure of tobermorite, jennite, and C-S-H. Thus, a high CaO/SiO2 ratio also presents a low H2O/SiO2 ratio and a greater packing density, leading us to believe that the C-S-H would present increased resistance. However,

Figure 5. Micrographs of the indented grids (1−512 mN) for a Ca/Si molar ratio of 2.1 (a), with zoom in the indentation I (b), second lowest value, and indentation V (c), lowest value (at 32 mN).

micro- and nanoporosity evident in the macrograph of the former (Figure 5b), with a Ca/Si ratio of 2.1. In the case of C-S-H produced by cement hydration, the literature seems to confirm that high-density C-S-H has a mean ratio of Ca/Si greater than that of low-density C-S-H, but

Table 2. Elastic Modulus (E) and Hardness (H) Values for 32 mN and 512 mN of the Indentations Shown in Figures 4 and 5 Ca/Si = 0.7 (Figure 4) 32 mN

Ca/Si = 2.1 (Figure 5) 512 mN

32 mN

512 mN

indented mark

E (GPa)

H (GPa)

E (GPa)

H (GPa)

E (GPa)

H (GPa)

E (GPa)

H (GPa)

I II III IV V VI

29.6 27.5 − − 25.7 24.7

1.77 0.98 − − 1.07 1.09

25.0 23.4 − − 21.4 26.7

0.90 0.98 − − 0.89 0.91

17.4 18.2 24.3 23.7 16.2 −

0.35 0.38 0.72 0.51 0.38 −

16.8 18.1 22.4 25.1 12.8 −

0.44 0.48 0.51 0.56 0.28 −

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Table 3. Mechanical Properties of C-S-H (E, elastic modulus; H, hardness) ratio Ca/Si = 0.7 matrix-type 1 load

E(GPa)

H(GPa)

2 mN 32 mN 512 mN mean

26.6 ± 1.8 27.0 ± 2.2 − 26.8 ± 3.2

1.08 ± 0.8 1.15 ± 0.3 − 1.09 ± 0.3

ratio Ca/Si = 2.1 matrix-type 2

E(GPa) 27.0 26.9 24.1 26.6

± ± ± ±

1.8 1.8 1.8 2.9

matrix-type 1

H(GPa)

E(GPa)

H(GPa)

± ± ± ±

19.3 ± 1.2 20.3 ± 3.8 − 19.5 ± 4.5

0.19 ± 0.05 0.36 ± 0.11 − 0.27 ± 0.12

1.09 1.23 0.90 1.10

0.10 0.10 0.10 0.24

matrix-type 2 E(GPa) 15.8 20.0 19.0 18.4

± ± ± ±

0.6 1.6 1.6 4.3

H(GPa) 0.20 0.47 0.45 0.37

± ± ± ±

0.08 0.09 0.09 0.16

Figure 7. FT-IR spectra of 0.7 C-S-H and 2.1 C-S-H.

Figure 9. C-S-H Ca/Si = 2.1 TEM micrographs.

ratio could have influenced the form of packing, changing its properties, indicating its potential to control and modify the performance of cement-based materials. In terms of performance, almost all the progress in the mechanical properties of cement-based materials achieved in recent decades has been accomplished by the reduction of capillary porosity and the optimization of granular stacking, approaching an asymptote, which suggests that future improvement of such features may come, in part, from changes in and dominance of the nanostructural characteristics of C-S-H, through control of the Ca/Si ratio and, more specifically, the arrangement and packing of the nanocrystallites that compose it.

Figure 8. XRD spectra of 0.7 C-S-H and 2.1 C-S-H.

variations can occur due to the characteristics of C-S-H, as shown in Table 1.19 Because of the characteristics of the material, mechanical analysis of the nanostructure is difficult. Despite the knowledge that an increase in the Ca/Si ratio contributes to axial growth, reduction of the interplanar distance, and increased density, leading to increased C-S-H strength, experimental data indicate that the form of particle packing is one of the most important factors in material strength. Considering the limitations of comparing synthesized C-S-H with that formed in Portland cement, this study showed that alterations in the Ca/Si ratio affect the mechanical properties of C-S-H. This behavior could be influenced by the micro- and nanoporosity of C-S-H. However, the Ca/Si

5. CONCLUSIONS This study showed that the instrumented nanoindentation technique combined with observation of the indented areas by 17225



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Although the study was performed using synthesized C-S-H, which limits the direct application of the results to C-S-H in Portland cement, and has additional limitations based on the variables adopted in the research, effects were observed on mechanical properties in relation to alterations in the Ca/Si ratio and the micro- and nanoporosity of C-S-H particles. Greater control of the nanostructure or particle packing of C-SH could lead to future improvements and more efficient cement-based products.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +55 48 3444 3753; fax: +55 48 3444 3748; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the Conselho Nacional de ́ Desenvolvimento Cientifico e Tecnológico (CNPq) and the Fundaçaõ de Amparo a Pesquisa do Estado de Santa Catarina (FAPESC) for providing the financial support for this research, the NanoMechanical Laboratory (LABNANO-UFPR) for performing the nanoindentation tests, and the Laboratório Central de Microscopia Eletrônica (LCME-UFSC) for the SEM/TEM micrographs.

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REFERENCES

(1) Manzano, H.; Dolado, J. S.; Ayuela, A. Acta Mater. 2009, 57, 1666−1675. (2) Constantinides, G.; Ulm, F.-J. Cem. Concr. Res. 2004, 34, 67−80. (3) Scrivener, K. L.; Kirkpatrick, R. J. Cem. Concr. Res. 2008, 38, 128−136. (4) Minet J. Synthèse caractérization de silicates de calcium hydratés hybrides. Thesis de Doutorado, Université Paris-sud 11, França, 2003. (5) Taylor, H. F. W. Adv. Cem. Based Mater. 1993, 1, 38−47. (6) Allen, A. J.; Thomas, J. J.; Jennings, H. M. Nat. Mater. 2007, 6, 311−316. (7) Richardson, I. G. Cem. Concr. Res. 1999, 29, 1131−1147. (8) Zhang, X.; Chang, W.; Zhang, T.; Cong, K. O. J. Am. Ceram. Soc. 2000, 83, 600−604. (9) Pellenq, R. J. M.; Van Damme, H. MRS Bull. 2004, 5, 319−323. (10) Taylor, H. F. W. Chemistry of Cement; Academic Press: New York, 1964. (11) Zhang, X.; Chang, W.; Zhang, T.; Ong, C. K. J. Am. Ceram. Soc. 2000, 83, 2600−2603. (12) Richardson, I. G.; Groves, G. W. J. Mater. Sci. 1993, 28, 265− 277. (13) Tennis, P.; Jennings, D.; Hamlin, M. Cem. Concr. Res. 2000, 30, 855−863. (14) Constantinides, G.; Ulm, F.-J.; Van Vliet, K. Mater. Struct. 2003, 36, 191−197. (15) Constantinides, G.; Ulm, F.-J. J. Mech. Phys. Solids 2007, 55, 64− 90. (16) Acker, P. Micromechanical analysis of creep and shrinkage mechanisms. In Creep, Shrinkage and Durability Mechanics of Concrete and other Quasi-brittle Materials; Ulm, F.-J.; Bazant, Z. P.; Wittmann, F. H., Eds.; Elsevier: Oxford, UK, Cambridge, MA; 2001. (17) Mondal, P.; Shah, S. P.; Marks, D. L. Cem. Concr. Res. 2007, 37, 1440−1444. (18) Mondal, P.; Shah, S. P.; Marks, D. L. ACI Mater. J. 2008, 105− 106. (19) Hugues, J. J.; Trtic, P. Mater. Charact. 2004, 53, 223−231. (20) Zhu, W.; Hughes, J. J.; Bicanic, N.; Pearce, C. J. Mater. Charact. 2007, 58, 1189−1198.

Figure 10. C-S-H Ca/Si = 0.7 TEM micrographs.

scanning electron microscopy is an excellent tool for evaluating the elastic modulus and hardness of C-S-H. More specifically, the following conclusions can be made: • With this technique, the mechanical properties of C-S-H were successfully evaluated using only 16 indentation points, with several cycles of loading for each indentation. • The influence of indentation load intensity (1 to 32 mN and 1 to 512 mN) was not a significant factor, with some cases presenting an increase in the elastic modulus due to contributions from lower layers due to compaction at a greater depth of penetration, while in other cases decreases occurred caused by microcracks. • The experimental results corroborate those based on the force field atomistic methods used by Manzano, Dolado, and Ayuela,1 verifying that the elastic modulus and hardness increase when the Ca/Si molar ratio of C-S-H decreased, achieving elastic modulus values of 27 and 20 GPa for Ca/Si ratios of 0.7 and 2.1, respectively. Our results also determined that micro- and nanoporosity significantly influence the outcome. • The results of the research are limited to synthesized CS-H but highlight the potential of the Ca/Si ratio to modify the mechanical properties, while permitting investigation of C-S-H without the presence of other phases of hydrated Portland cement. 17226



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