Effect of partial substitution of superplasticizer by silanes in Portland cement pastes

Journal Pre-proof Effect of partial substitution of superplasticizer by silanes in Portland cement pastes Cézar Augusto Casagrande, Lidiane Fernanda Jochem, Lucas Onghero, Paulo Ricardo De Matos, Wellington Longuini Repette, Philippe Jean Paul Gleize PII:

S2352-7102(19)31662-6

DOI:

https://doi.org/10.1016/j.jobe.2020.101226

Reference:

JOBE 101226

To appear in:

Journal of Building Engineering

Received Date: 21 August 2019 Revised Date:

9 December 2019

Accepted Date: 27 January 2020

Please cite this article as: Cé.Augusto. Casagrande, L.F. Jochem, L. Onghero, P. Ricardo De Matos, W.L. Repette, P.J.P. Gleize, Effect of partial substitution of superplasticizer by silanes in Portland cement pastes, Journal of Building Engineering (2020), doi: https://doi.org/10.1016/j.jobe.2020.101226. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

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EFFECT OF PARTIAL SUBSTITUTION OF SUPERPLASTICIZER BY SILANES IN PORTLAND CEMENT PASTES

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CÉZAR AUGUSTO CASAGRANDE1,2 *; LIDIANE FERNANDA JOCHEM3; LUCAS ONGHERO2; PAULO RICARDO DE MATOS2; WELLINGTON LONGUINI REPETTE2; PHILIPPE JEAN PAUL GLEIZE2.

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1 Technologic Center of Campus Agreste, Federal University of Pernambuco (UFPE) - 55014-900, Caruaru, PE, Brazil.

7 8

2 Department of Civil Engineering (ECV). Federal University of Santa Catarina (UFSC) – 88040-970, Florianópolis. SC. Brazil.

9 10

3 Academic Department of Civil Construction (DACOC). Technological Federal University of Parana (UTFPR), Curitiba, PR, Brazil.

11

* Corresponding author: C. A. Casagrande; [email protected].

12 13

Email: [email protected]; [email protected]; [email protected]; [email protected]; [email protected]; [email protected]

14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

ABSTRACT This work evaluated the effect of partially replacing a superplasticizer by silanes on the fresh and hardened properties of Portland cement pastes. For this, pastes with water/cement ratio of 0.20 and 25 and 50% replacement of a polycarboxylate-ester-based superplasticizer by tetraethoxysilane (TEOS), 3-glycidoxypropyltrimethoxysilane (GPTMS) and aminoethylaminopropyltrimethoxysilane (AEAPTMS) were produced. The specific gravity, entrapped air content and mini-slump of the pastes were determined in the fresh state. The hydration kinetics was evaluated by isothermal calorimetry up to 120 hours. Finally, the compressive and flexural strength of the pastes, as well as the ratio between them (referred to as toughness) were determined at 7 and 28 days. The results showed that 25% replacement by either the silanes increased the mini-slump and decreased the air content of the pastes in comparison to the reference. In contrast, 50% incorporation of GPTMS and AEAPTMS increased the air content and decreased the mini slump of the pastes, while 50% TEOS did not significantly affected these properties. The isothermal calorimetry indicated that TEOS reduced the induction period of the pastes, while the other silanes increased it, delaying the acceleration period start in up to 53 hours in comparison to the reference. In addition, the increase in both silane content progressively increased the cumulative heat up to 120 hours, reaching values 11% higher than the reference. The incorporation of TEOS progressively increased the flexural strength, while did not significantly changed the compressive strength, both at 7 and 28day. The same trend was found for the compressive strength of the pastes containing GPTMS and AEAPTMS, while the flexural strength of these pastes decreased as the replacement level increased. Finally, 7 and 28-days toughness of the pastes increased as the content of TEOS increased, while no clear trend was found for GPTMS and AEAPTMS. KEYWORDS: silanes; TEOS; GPTMS; AEAPTMS; hydration. 1

INTRODUCTION

The production of cementitious materials is one of the most pollutant industrial activities currently development [1]. For the reduction of this impact, there are two main approaches, one is the study of the dosage of the cementitious composite in order to optimize the dosage of the constituents of the material in order to consume less cement or natural aggregates [2–9].

2 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71

Another approach is to manipulate separately the composition of each constituent of the cementitious material. In this second approach, particularly in the case of chemical admixtures (plasticizers, superplasticizers, air incorporators, etc.), the use of organosilanes or siloxanes has attracted the attention of researchers, mainly because silanes hydrolyze in contact with water. Thus, it present interesting reactivity for studies in cementitious materials. Normally, silane molecules are used in conjunction with traditional superplasticizer admixture in search of a more potent new additive from the reaction of silane molecules with the admixture in a chemically controlled environment, so that the result is an admixture with potentiated effects [10–15]. Furthermore, silanes are used as nanoparticle modifying additives for addition in cementitious materials [16–18]. Also silanes are widely applied as a protective medium for cementitious composites located in aggressive medium, such as chemical sealant. Once applied to the surface of the composite, the silane molecules react with each other, condensing and forming an inorganic film, thereby protecting the cementitious substrate [19–22]. Silanes are chemical reagents with silicon-based molecules as the main agent. For the most part, they have two types of reactivity in the same molecule, usually a polar and an apolar (functional group). These molecules are usually referred to by their hydrolyzable group of the alkoxy type (CxHyO-) and by a functional group, usually organic with strong electronegativity (Fig. 1). However, in addition to this configuration, there are silanes with four hydrolyzable groups, such as the silane most widely used in the most diverse processes, tetraethoxysilane (TEOS), besides tetramethoxysilane (TMOS), among others [23].

Fig. 1 – Idealized scheme of the silane molecule. Si is the silicon atom, O is the oxygen atom, “Y” represents a functional group and “x” represent a hydrolyzable group. Adapted from [23]. 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86

This versatility opens potentials for the application of silanes in cementitious matrixes. When in contact with water, the silanes hydrolyze part of their molecule, presenting OH-ions that have an attraction for the highly positive surface of the grains of cement, which are calcium rich, also interact with organic plasticizer additives. This affinity has already been proven in studies where silanes were added to calcium hydroxide solutions (characteristics of the pore solution in cement paste) [10,24]. Studies evaluating the use of silanes as additives for the modification of properties in the fresh and hardened state of cementitious composites presented a potential for the use of this type of additive in cement-based materials, as summarized in Table 1.

3 87 88

Table 1 Summary of recent studies on accelerated cementitious matrices

Authors Xu & Chung Cao & Chung

Ref. [25] [26]

Type of silane APTMS and GPTMS APTMS and GPTMS

Study in Paste and mortar Mortar

Main objective of the work Dispersion of silica fume Dispersion of fibers

Xu & Chung

[27]

APTMS and GPTMS

Paste

Dispersion of carbon fiber

Franceschini et al.

[10]

VTES, TMOS, TEOS, APTES and APMDES MTES, ETES, BTMS, HTES, OTES, DDTES, ODTMS, PTES and TEOS

Organo-C-S-H synthesis Organo C-S-H synthesis

Production of new organo-calcium-silicate layered from silane and Ca(OH)2

Minet et al.

[13,28]

Švegl et al.

[29]

APTES and AEAPTES

Paste and mortar

Evaluation of silanes in cementitious matrix

Fan et al.

[12]

APTMS

Paste

Production of new silane-sulfated admixture

Fiber treatment for improve mechanical properties Evaluation of new synthetized admixture in Plank, J & Witt, J [14] MAPS Paste cementitious matrix Collodeti et al. [18] APTMS and GPTMS Paste Improve nanosilica dispersion Evaluation of new synthetized admixture in Plank et al. [15] APTES Paste cementitious matrix APTES, AEAPTMS and Evaluation of the silanes in cementitious Kong et al. [31] Paste TEOS matrix APTMS, VTMS and Evaluation of the silanes in cementitious Feng et al. [17] Mortars GPTMS matrix Lu et al. [11] TMSPMA Paste and mortar Production of new silane-admixture He et al. [32] TMSPMA Paste Production of new silane-admixture Functionalization of steel microfiber for Casagrande et al. [33] TEOS Fiber treatment improve mechanical properties TEOS, AEAPTMS and Evaluation of partial substitution of This study Paste GPTMS admixture by silanes 89 Note: Aminopropyltrimethoxysilane (APTMS), 3-Glycidoxypropyltrimethoxysilane (GPTMS), 90 triethoxyvinylsilane (VTES), tetramethoxysilane (TMOS), 3-amino-propyltriethoxysilane (APTES), 91 tetraethoxysilane (TEOS), 3-aminopropylmethyldiethoxysilane (APMDES), methyltriethoxysilane (MTES), 92 ethyltri-ethoxysilane (ETES), n-butyltrimethoxysilane (BTMS), n-hexyl-triethoxysilane (HTES), n93 octyltriethoxysilane (OTES), n-dodecyltriethoxysilane (DDTES), n-octadecyltrimethoxysilane (ODTMS), 94 phenyltriethoxysilane (PTES), aminoethylaminopropyltriethoxylsilane (AEAPTES), 95 aminoethylaminopropyltrimethoxylsilane (AEAPTMS), vinyltrimethoxysilane (VTMO), N-maleic γ96 aminopropyltriethoxysilane (MAPS) and 3-(trimethoxysilyl)propylmethacrylate (TMSPMA).

Benzerzour et al.

97 98 99 100 101 102 103 104 105 106 107

[30]

VTMO

Fiber treatment

Pioneers in the silane in cement-based applications, the Chung’s research group in the early twenty’s started to show the potential of the silane, mainly, for particle dispersion and for hardened properties in pastes and mortar [25–27]. In this way, a new approach has been shown by obtaining new hybrid C-S-H by reaction of several types of silanes in Ca(OH)2 solution [10,13,28]. Švegl et al. [29] reported that aminosilanes (N-2aminoethyl-3- aminopropyltrimethoxysilane and aminopropyltrimethoxysilane) may improve the spread of cement paste and reduced the water demand. Flexural and compressive strength of cement mortars were markedly increased by the addition of aminosilanes after 28 days of curing. On the other hand, detailed workability mechanism of silanes in cementitious materials has been rarely reported.

4 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154

Collodeti et al. [18] used amino and epoxy silanes and found that there were strongly impact of the silanes in the induction period and hydration of the cement. This was justified by interaction of the silanes with the calcium ions in the solution. Kong et al. [31] executed an extensive experimental program, where pastes with addition of silanes presents several behaviors. Was concluded that AEAPTMS shows the strongest fluidizing capability in fresh cement paste. AEAPTMS and APTES strongly retard the cement hydration by extending the induction period and reducing the hydration degree. The addition of TEOS solely reduces the maximum exothermic peak without delaying the onset of the acceleration period, which implies a different retardation. AEAPTMS and APTES increase the strength of mortars after 7, 28 and 90 days curing. AEAPTMS and APTES show positive effect on the toughness of mortars after 7, 28 and 90 days curing indicated by increasing the ratio of flexural strength to compressive strength. Also, Feng et al. [17] showed, by isothermal calorimetry, that the silane addition, as reactant or treated nanoparticles, affect the kinetics of cement hydration, justifying that this may be caused by adsorption and reaction of silanes on the surface of cement hydration products. In addition, concluded that the addition of small amounts of silanes or silane derivatives in ordinary mortars shows general enhancement on the flexural strength and certain improvement on the compressive strength after 7-day and 28-day curing. Else, silane may be used as fiber surface treatment for improvement of fiber-matrix strength. Benzerzour et al. [30] used VTMO for surface treatment of synthetic fibers and obtained an improvement in the mechanical properties of the samples with silane treated fibers in comparison with reference fiber. In the same direction, Casagrande et al. [33] used TEOS for steel microfibers surface treatment. It was found that until 1% of silane in surface treatment may result in significant improvement of the mechanical properties of the fiber-matrix interface. The evaluations of the morphology and the composition of the fiber- matrix interface reveal that the improvement in performance generated by the silane treatment relates to physical and chemical changes. The treatment with TEOS increases the roughness of the fiber surface, increasing the total area of contact and, as a result, the pullout force. In fiber treated with a high content of TEOS, silane deposits are formed over the surface of the fiber. These deposits act as mechanical anchorages that generate inclined forces and increase the surface mobilized during the pullout test. Even with some studies have already been published, there is great doubt about the effects of silanes on cementitious matrixes, due to a large number of types of silanes in the market. In this regard, an investigation on the manipulation of the additive in order to enhance its performance may contribute to the lower consumption of plasticizing admixtures and/or improve the cement efficiency. This work aimed to better understand the interactions mechanisms and effects of silanes in cementitious composites. The effect of superplasticizer substitution by TEOS, GPTMS and AEAPTMS in cement pastes were evaluated. Pastes with water/cement ratio of 0.20 were produced with partial replacements of 25 and 50% of superplasticizer by silanes. The samples were evaluated in the fresh state by workability in terms of minislump spread, specific gravity and entrapped air content. Furthermore, a discussion on the hydration kinetics of the cement by isothermal conduction calorimetry was conducted. In the hardened state, the fracture surface and flexural and compression strength at 7 and 28 days were evaluated.

5 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181

2 EXPERIMENTAL PROGRAM 2.1 MATERIALS A high early strength Portland cement was used, commercially available in Brazil as CPV-ARI [34], equivalent to Type III by ASTM C150 [35]. This type of cement was chosen because it is high in clinker content (>95%), that may minimize the side effects by others additions. The particle size distribution (PSD) of the cement was obtained using a Microtrac s3500 equipment (laser diffraction method in dry suspension), and is shown in Fig. 2. The physical properties of the cement are presented in Table 3. The chemical and mineralogical compositions of the cement are presented in Table 3 (the latter calculated by the Bogue’s equations [36]). As superplasticizer, a commercial polycarboxylate-ester based (MPEG PCEs) highrange water-reducing admixture (TecFlow 8000, GCP) was used, with pH of 4 ±1, viscosity of 440 mPa·s-1, specific gravity of 1.10 g·cm-3 and solids content of 47.1%. Fig. 3 shows the FTIR spectrum of the superplasticizer obtained by a Jasco model 4200 FT-IR with a ATR model PRO450-S/470-H and ZnSe crystal, at 4000-550 cm-1. In the FTIR spectrum is possible to identify the following bands: hydroxyl band in ~3300 cm-1, aliphatic characteristic band in ~2990 cm-1, in ~2370 the band characteristic to carbon dioxide (associated with the gas in the environment), the carbonyl group at ~1700 cm-1, 15001200 cm-1 is identified the carboxyl and hydroxyl groups associated to main superplasticizer-cement interaction and >1200 cm-1 is the fingerprint region associated to complicated series of absorptions. The latter in mainly related to all manner of bending vibrations within the molecule such region, each different compound produces a different pattern in this part of the spectrum.

Fig. 2. PSD of the Portland cement used. 182 183 184 185 186 187

6 188 189 190

191 192

Table 2 Physical properties of the Portland cement used. Surface area Blaine (cm·g-1) 407.0 Expansion (mm) 0.2 Start 145.0 Setting time (min.) End 183.0 1 day 22.4 3 days 37.5 Compression strength (MPa) 7 days 42.8 28 days 51.1 -3 Specific gravity (g·cm ) 3.11 Table 3 Chemical and mineralogical composition of the Portland cement used. Chemical composition Mineralogical composition Oxides (%) Compound (%) SiO2 18.90 C3S 52.24 Al2O3 3.69 C2S 15.07 K2O 0.47 C3A 1.89 NaO2 0.40 C4AF 8.52 2.76 Fe2O3 CaO 62.94 MgO 4.22 SO3 3.09 Loss of ignition

3.20

193 100

50 40 30

CO₂

OH¯ C=OO¯

C-O/C=O

O-H

~990

C-H

~1190

~2990

60

~1700

70

~3380

Transmittance (%)

80

~2370

90

HC-O-OH

20 10 0 4000

> 1200 cm¯¹ is the fingerprint region

3500

3000

2500 2000 1500 Wavenumber (cm¯¹)

1000

500

Fig. 3. FTIR of the superplasticizer used 194 195 196 197 198 199

The silanes used in this work were TEOS (Tetraethoxylsilane) with four hydrolysable ethoxy groups, GPTMS (3-Glycidoxypropyltrimethoxysilane) with three hydrolysable methoxy groups and one ionizable epoxy group and AEAPTMS (Aminoethylaminopropyltrimethoxysilane) with three methoxy groups and one ionizable amine group. The physicochemical characteristics of the silanes are shown in Table 4.

7 200 201 202

Table 4 Properties of the silanes used in this work. TEOS

GPTMS

AEAPTMS

Tetraethoxysilane

3-Glycidoxypropyltrimethoxysilane

Aminoethylaminopropyltrimetho xysilane

Manufacturer

Aldrich (USA)

Gelest (EUA)

Gelest (EUA)

Purity Hydrolysable Group Functional group Chemical formula Molar mass Density

> 98.0 %

> 97.5 %

> 95.0 %

Ethoxy

Methoxy

Methoxy

-

Epoxy

Amino

C8H20O4Si

C9H20O5Si

C8H22N2O3Si

208.33 0.99

236.34 1.07

226.36 1.03

Name

Structural formula

203 204 205 206 207 208 209 210 211 212 213

2.2

METHODS

2.2.1 Mix proportioning A cement paste with composition of 1.0: 0.20: 1.00% by weight (cement: water: superplasticizer) was produced as the reference mixture. Then, the superplasticizer was replaced by the silanes in the levels of 25 and 50% (by weight). Table 5 presents the composition of the cement pastes produced. The name of the mixture is given by the silane used, followed by the replacement level. Table 5 Composition of the cement pastes produced. Mixture REF TEOS 25% TEOS 50% GPTMS 25% GPTMS 50% AEAPTMS 25% AEAPTMS 50%

214 215 216 217 218 219

Pastes composition by mass Cement Water Silane Superplasticizer 1 0.20 0.00% 1.00% 1 0.20 0.25% 0.75% 1 0.20 0.50% 0.50% 1 0.20 0.25% 0.75% 1 0.20 0.50% 0.50% 1 0.20 0.25% 0.75% 1 0.20 0.50% 0.50%

2.2.2 Sample preparation The cement pastes were produced in a mortar mixer with 5 liters and 285 rpm capacity, according to the following steps. Firstly, the cement was added to the mixer container; due the hydrophobic nature of the silantes, mainly TEOS, the silanes were mixed with the superplasticizer in a container and homogenized manually for 5 minutes

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forming an emulsion, then was added to the material in the mechanical mixer container; and the liquids (water and admixture or silane emulsion) were gradually added over 1 min mixing at low speed (140 rpm). Then, the sample was mixed for another 4 minutes at high speed (285 rpm). The fresh state and the isothermal calorimetry tests were conducted immediately after the sample preparation at the mixer. For the flexural and compressive strength tests, three prismatic specimens of 40 x 40 x 160 mm were cast for each mixture and test age (7 and 28 days). After 24 hours, the specimens were demolded and cured in a humid chamber (temperature of 23 °C and 95% relative humidity) until the respective test ages. 2.2.3 Test methods The flowability of the pastes was evaluated by the mini-slump flow test [37]. A conical mold with 19 x 38 x 57 mm (upper diameter x lower diameter x height) was filled with the sample, and subsequently lifted. Then, the average of two measurements of the sample spread was taken as the result of the test. Furthermore, the specific gravity and entrained air content were determined following the ASTM C173 [38], with a 476 cm³ container. In order to evaluate the effect of the silanes on the cement hydration kinetics, isothermal calorimetry tests were conducted on a TAM AIR calorimeter (TA Instruments). The heat release was measured up to 120 hours at 23 ˚C, in a sample of about 25g of paste. The end of the induction period was determined as the intersection of the horizontal baseline and the regression line of the accelerated period of the normalized heat flow curve, as suggested by Betioli et al. [39]. Flexural and compressive strength tests were performed according to NBR 19279 [40] and ASTM C349 [41], respectively, at the ages of 7 and 28 days. For each composition and age, three specimens were submitted to the bending test for flexural strength determination. Subsequently, the six specimen halves were submitted to the compression test. Average values were adopted. Finally, analysis of variance (ANOVA) was used to check the influence of the parameters “type of silane” and “replacement level” on the compressive and flexural strength of the pastes, for a reliability of 95%. 3 RESULTS AND DISCUSSION 3.1 FRESH STATE Fig. 4 presents the results of the mini-slump test. The 25% replacement of the superplasticizer by the silanes resulted in an increase of 32% in the mini-slump spread, on average. Although the increase in the silane content reduced the superplasticizer content, the silanes probably improved the lubrication and compactness of the paste, favoring the interaction of the silanes with cement surface, acting as a hydrophobic layer to water reaction and as steric repulsion [11]. In contrast, the increase in the replacement level up to 50% decreased the flowability of all pastes in comparison to the 25% silanes-containing mixtures. This may be explained by the considerable reduction in the superplasticizer content, in addition to the low dispersion of the silanes in aqueous solution (which are hydrophobic). The existence of non-dispersed and/or non-adsorbed molecules may result in cluster formations. During the shear, these clusters get across the particles trajectory, hindering the flow of the material and resulting in flowability reductions [42].

9 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281

Despite the flowability reduction of the 50% silanes mixtures compared to the 25% silanes mixtures, the incorporation of 50% of TEOS and GPTMS resulted in equivalent (GPTMS) or greater spread (TEOS) in comparison to the reference, while the 50% AEAPTMS decreased the sample spread in about 50%. This may be attributed to type of silane alcoxy groups in the AEAPTMS, which are methoxy groups and have a hydrolysis faster than ethoxy groups, in comparison with TEOS. This hydrolysis may favor the agglomeration/reaction with neighboring molecules of the AEAPTMS, resulting in less interaction with cement. Another fact is that silanes need to be hydrolyzed before reacting with the substrates [43]. This chemical reaction takes time [44], which apparently was not enough for the AEAPTMS, resulting in the low reactivity of these molecules. Nevertheless, the results obtained for 25% replacement of either the silanes or 50% of TEOS and GPTMS indicates that the replacement of superplasticizer by silane may be beneficial for the flowability of cement pastes, in line with that reported by Kong et al. [31] and Švegl et al. [29].

AEAPTMS

282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301

Fig. 4. Influence of the silanes on mini-slump of the pastes Fig. 5 presents the results of specific gravity and entrained air content of the pastes in fresh state. The specific gravity and air content of the reference sample were respectively 2.19 g·cm-3 and 3.96%. The 25% replacement of superplasticizer by either silane increased the specific gravity (and decreased the air content) of the pastes. On the other hand, the increase in the GPTMS and AEAPTMS content up to 50% reduced the specific gravity of the pastes, resulting in entrained air contents of 4.64% and 8.02%, respectively. This trend of increase in the air content with the increase in the silane content was also reported by Švegl et al. [29]. In turn, the increase in TEOS content up to 50% did not affect the specific gravity of the paste. The reduction in the specific gravity of the pastes with the increase in the GPTMS and AEAPTMS content may be due to the apolar nature of these silanes. The silane molecule is not water-soluble in the first moments, and does not exhibit immediate reactivity with the cementitious compounds after their addition, as occurs with superplasticizer admixtures. In addition, TEOS has four groups with hydrolysis potential, while AEAPTMS and GPTMS respectively have an amine group (NH2) and an epoxy group in it functional groups, which can lead to different reaction rates in comparison to the TEOS. This effect result in low reactivity kinetics, also verified by Collodetti et al. [18].

10 Both effects result in lower amounts of superplasticizer available to disperse the cement particles, resulting in a more consistent (i.e., less flowable) paste, as verified in the minislump tests. This hinders the exit of the air, thus resulting in lower specific gravity.

TEOS Specific gravity (g.cm -3 )

2,25 2.25

GPTMS

AEAPTMS AMPTS 10 8

2.20 2,20 Reference

2,15 2.15

6 4

2,10 2.10

Entrained air (%)

302 303 304 305

2

2.05 2,05

0 25 50 Superplasticizer replacement (%) Fig. 5. Physical properties in fresh state. Note: specific gravity (dotted lines) and entrained air (full lines). 0

306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331

3.2

HYDRATION KINETICS

The calorimetric curves of cement pastes are presented in Fig. 6: the heat flow curves obtained are presented in Fig. 6 (a), and the cumulative heat curves are presented in Fig. 6 (b). Calorimetry is a useful method to evaluate the kinetics of cement hydration. The initial peak is caused by the combination of the exothermal wetting of the particles, Aluminate phases (C3A) hydration and ettringite (Ca6Al2(SO4)3·(OH)12·26H2O) formation [45]. Then the induction period occurs. This is characterized by low cement hydration rate and low heat released, and can be seen in Fig. 6 (a) and semi-horizontal line in the first hours. After that, the acceleration period starts, mainly caused by alite dissolution and precipitation of Calcium Silicate Hydrate (C-S-H) and Portlandite (Ca(OH)2), ending in the main peak of heat release. Finally, there is a deceleration period, characterized by a low cement hydration rate and when the sulfate carrier is already consumed by ettringite formation, additionally monosulfoaluminate (3CaO·Al2O3·CaSO4·12H2O) precipitates. In Fig. 6 (a), three different behaviors were identified. For TEOS, acceleration effect was verified, and for GMTPS and AEAPTMS a retarder effect was verified. Also, for all the samples analyzed, as higher was the silane incorporation level, lower was the heat flow peak and higher was the heat released. For GPTMS and AEAPTMS, the impact of the silane replacement in the induction period was lower than series with 25%. When TEOS was incorporated, as higher its content, shorter was the acceleration period, and lower was the heat of hydration peak. The increase in the TEOS content progressively reduced the induction period length and anticipated the occurrence of the main heat release peak, from 25.8 hours in the reference to 11.0 hours in 50% TEOS. This means that less cement particles are hydrating in this period of the reaction, i.e., the reaction degree is lower. In contrast, the increase in the AEAPTMS and GPTMS

11 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353

significantly increased the induction period length and delayed the occurrence of the main peak of heat release. Furthermore, the GPTMS incorporation led to delays in the main peak release of about 30 hours in comparison to the paste with the same contents of AEAPTMS, reaching 83.4 hours for 25% GPTMS. This may be justified by the great reaction ant interaction of the hydrolyzed silane with molecules hydrolyzed in the vicinity that favored the condensation/precipitation of silanes and reduce it potential of reaction with cement surface, reflecting in the behavior observed. These behaviors are in agreement with studies of Kong et al. [31] and Collodetti et al. [18]. As can be noted, the silanes have strongly impacted the induction period, which are related to ettringite presence, portlandite (Ca(OH)2) precipitation and C-S-H formation [46,47]. In addition, the heat flow peak of the silane-containing samples are lower than the reference (4.95 mW·g-1 of cement for the reference vs. 3.02 – 4.09 for the silane-containing mixtures), confirming that the silanes incorporation affected the cement hydration kinetics. Both the silanes and the superplasticizer may interact with the ions present in the aqueous phase (e.g. Ca2+ and OH-), altering the kinetics of C-S-H and Portlandite precipitation. In the case of the TEOS, the accelerating effect may be due to the fact that the silane is rapidly hydrolyzed in the basic solution, forming silanols groups (SiOH) (Eq. 1). This SiOH groups have great surface energy and may favor the C-S-H formation/precipitation, due to higher concentrations of Si4+ and OH- in the solution. As a result, this increases the alite solubility by this unbalance in solution, resulting in accelerating effect.


 (  ) +   → ()
 (  ) +    354 355 356 357 358 359 360 361 362 363 364 365

Eq. 1

In turn, when AEAPTMS or GPTMS are added, the silanes interact either with the and OH- ions from the solution or with the cement and C-S-H surface. As a result, the degree of saturation with respect to C-S-H and Portlandite in aqueous phase of the Ca2+ in the solution may decrease as the AEAPTMS or GPTMS content increases. Since the concentration of Ca2+ ions in the solution is lower, the crystallization process of the Portlandite is delayed. This delay process is also reported by Kong et al. [31], Collodetti et al. [18], Švegl et al. [29] and Feng et al. [17]. This discussion about C-S-H and portlandite precipitation mechanism is also present when superplasticizer is added to cement-based materials, and the precipitation of C-S-H and Portlandite are the main factors (or the “trigger”) of the retarding/accelerating process [48].

Ca2+

12

AEAPTMS_25% AEAPTMS_50%

(a)

AEAPTMS_25% AEAPTMS_50%

(b) Fig. 6. Isothermal calorimetry results of the pastes. (a) Normalized heat flow ; (b) normalized cumulative heat. 366 367 368 369 370 371 372 373 374 375 376 377 378 379

In the case of the acceleration time of GPTMS and AEAPTMS, the trends verified were different from those found with TEOS: with 25% of silane, a longer period of acceleration was verified; however, when 50% was added, the acceleration period returned to the time similar to that of the reference paste. Nonetheless, in both cases the heat flow peaks were smaller than that of the reference, indicating that the silanecontaining samples had lower reaction rates during the acceleration period. Fig. 7 presents the heat released after 120 hours. It is possible to note that the all the pastes with silane had higher cumulative heat released after 120 hours in comparison to the reference paste. This indicates a higher degree of hydration of the cement up to 120h, suggesting a beneficial effect of the partial replacement of superplasticizer by silanes, contrarily to the findings reported by Collodetti et al. [18]. This means that for the time analyzed, the silane addition was beneficial for the degree of hydration.

13

AEAPTMS

Fig. 7. Influence of the silane replacement on cumulative heat after 120 hours. 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411

3.3

HARDENED STATE

The results of mechanical strength tests are shown in Fig. 8. In general, when TEOS partially replaced the superplasticizer, both flexural and compression strength increased. In contrast, when AEAPTMS or GPTMS were incorporated, equivalent or lower strengths were observed. The flexural strengths of the reference sample were 15.4 and 18.5 MPa at 7 and 28 days, respectively (Fig. 8 (a)). For samples with TEOS, the strength was enhanced progressively with the replacement of the superplasticizer by silane. For AEAPTMS and GPTMS samples, no improvements in mechanical properties were found. These results can be explained by the fresh state properties: the pastes with 50% of silane replacement presented losses in their workability and high contents of entrapped air. That is directly related to loss in the mechanical properties, as also verified by Švegl et al. [29]. The loss in mechanical properties as a function of entrapped air content in the cement-based matrix is extremely depended, and, commonly the reduction in the strength have exponential decreases as the air content increases [49,50]. If a ceramic material is loaded, the stress is concentered in the extremities of these cracks, increasing the probability of these cracks to propagates and growth, resulting in a low mechanical behavior as expected for a material free of defects [51]. Considering the situation of a cement-based matrix, where there is intrinsic porosity air distributed through the material, it is possible to predict that this material had a low flexural strength. This effect is highlighted when excessive entrapped air is present in the material, as verified in the samples with superplasticizer replacement by the silanes GPTMS and AEAPTMS. Thus, the losses in the mechanical properties may be attributed to entrapped air of the materials. This is more evident when correlated with the cumulative heat released (measured by the calorimetry), where the samples with silane reached higher values of heat released after 120 hours, indicating a higher degree of cement hydration, which should reflect in higher mechanical strength in comparison with the reference.

14

AEAPTMS (a)

AEAPTMS (b) Fig. 8. Hardened stated properties of the pastes at 7 and 28 days. (a) Flexural strength ; (b) Compression strength. 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427

ANOVA was conducted in order to check the statistical difference between the 28days flexural strength test results. The ANOVA results showed that the type of silane (F = 15.147; p-value = 1.387·10-4) and the interaction of the type and content of silane incorporation (F = 4.007; p-value = 0.0169) are influent in the flexural strength, with 95% reliability. On the other hand, only the content of silane substitution had no significant impact on the flexural strength at 28 days (F = 3.199; p-value = 0.064). Considering that, for the silanes tested in this work, incorporation of up to 50% of TEOS or 25% of GPTMS and AEAPTMS did not cause significant damages to the flexural strength of the pastes. For compressive strength results (Fig. 8 (b)), the reference sample showed values of 77.16 and 102.60 MPa at 7 and 28 days, respectively. Differently to the flexural strength, the addition of silane caused no changes in the compressive strength of the reference sample at 28 days. This may be justified by the entrapped air and the type of test executed. In the flexural strength test, the sample is extremely sensible to entrapped air and quality of the matrix, while in the compression, the propagation of crack is very different. When subjected to compression, the matrix is capable of transmitting stresses through the

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micro-cracks [52,53] and as expected for a ceramic composite, there is a high capacity to support compressive stress, reflected in high values strength (about 100 MPa in this study). Comparing the 28-day results of the compression tests with the 7-days results is possible to note that all the samples analyzed had a similar behavior, these results may be justified by the similar porosity of the samples with a lower degree of cement hydration, which limited the evolution of the mechanical properties. With the evolution of the hydration, the sample with less entrapped air had more potential to support load (in this case the reference sample), resulting in the highest compressive strength values at 28 days, as expected. One hypothesis regarding the worst mechanical performance of the samples at 7 days is the effect of the silanes on the hydration kinetics of the cement. The silanes, as well as the superplasticizers, can act to prevent the C-S-H precipitation in the early ages [17,18,48,53], resulting in a delay of hydration of the cement and subsequently retardation in the evolution of the strength of that material. To confirm the effects of the silane incorporation in the compression strength of the samples, ANOVA was conducted for checking which effect is significant in this property. As in flexural strength, the analysis was conducted only for 28 days of cement hydration. The ANOVA showed that the type of silane (F = 10.450; p-value = 9.73·10-4), the interaction of the type and content of silane incorporation (F = 3.055; p-value = 0.043) and the content of silane incorporation (F = 10.491; p-value = 9.54·10-4) had significant impact on the compressive strength at 28 days. This confirms the impact of the silane application in the cement composites, in the fresh and hardened state. The entrapped air, a hypothesis regarding the lower performance obtained in the samples with silane, can be verified on fractured surfaces shown in Fig. 9. These fracture surfaces were obtained in the samples at 7 days of hydration, however the configuration of the macro-porosity has no trend to change after the setting. It can be seen in Fig. 9 (a) that the reference, qualitatively, presents small entrapped air bubbles, similar to those observed in samples with 25% and 50% TEOS , respectively in Fig. 9 (b) and Fig. 9 (c). This characteristic increases the potential for a stronger material, justifying a better mechanical performance compared to the samples with AEAPTMS and GPTMS. The most interesting fractures are observed in Fig. 9 (d) and Fig. 9 (f), which evidence the effect of the silane in the mixture, mainly in the air entrapped. In samples with 50% replacement, as in Fig. 9 (e) and (g), large entrapped air bubbles can be seen in the fracture surface, justifying the lower mechanical performance and the specific gravity of these samples.

20 mm (a)

16

20 mm

20 mm (b)

(c)

20 mm

20 mm (d)

(e)

20 mm

20 mm

(f) (g) Fig. 9. Fracture surface of the pastes at 7 days of hydration. REF in (a); TEOS 25% in (b); TEOS 50% in (c); AEAPTMS 25% in (d); AEAPTMS 50% in (e); GPTMS 25% in (f); GPTMS 50% in (g) 464

17

AEAPTMS

(a)

AEAPTMS

(b) Fig. 10. Flexural to compression ratio, at 7 days in (a) and 28 days in (b). 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482

The measurement of the toughness of the ceramic materials is very difficult due to the elastic behaviors and the low capacity of the matrix to resist the crack propagation, resulting in a brittle material. Thus, the toughness was analyzed by the flexural to compression ratio. Fig. 10 (a) presents the flexural to compression strength ratio for 7 days, and for 28 days is shown in Fig. 10 (b). At 7 days, it was verified that the results obtained for the flexural to compression ratio do not present a trend as a function of the replacement of the superplasticizer by silanes. However, it was verified that when TEOS and AEAPTMS were used, there was an increase in this ratio, suggesting a potential improvement in the mechanical properties (toughness) of the material. On the other hand, by using GPTMS there was a decrease in such ratio, indicating a negative impact on the toughness when this type of silane was used. For 28 days, there was an increase in the flexural to the compression ratio of the pastes. For all samples analyzed, there were increases in the toughness comparing with the reference sample, thus promoting a beneficial behavior for the paste. These results may be associated with the more reliable results obtained in the flexural strength, which is more sensible to entrapped air as compared to the compressive strength. The reference

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series, as verified, presented less entrapped air in the fractured surface, therefore presenting less probability to the deviation of the cracking propagation, resulting in a similar performance in comparison to the series with silane incorporation, which reflected in the flexural to compression ratio.

4

CONCLUSIONS

This work evaluated the changes in the fresh and hardened properties of cement pastes with the partial replacement of superplasticizer by TEOS, GPTMS, and AEAPTMS silanes. Based on the results obtained, the following conclusions can be drawn:

•

•

•

•

5

When 25% of silanes were added, a trend of improving the workability in terms of spread diameter was verified. On the other hand, 50% incorporation resulted in no improvement in the spread of the pastes, and more silane may be not appropriate for general upgrading. The pastes presented increases in specific gravity until 25% of silane addition, showing a denser matrix in comparison with the reference sample. However, when de 50% was added, the flowability of the paste was impaired and the entrained air is increased, resulting in lower densities than that of the reference. The silane addition impacts strongly in the cement hydration kinetics. With TEOS, there was an acceleration of the hydration kinetics while with GPTMS and AEAPTMS there was a retarding effect in the hydration. Both effects were attributed to the interaction of the silanes with the Ca2+ and OH- ions of the solution, altering the chemical concentration that regulates the Ca(OH)2 and C-S-H formation/precipitation. In addition, the heat flow peak and cumulative heat released were altered, as well as the acceleration period. The properties in the hardened state of the cementitious pastes have been significantly affected by replacing the superplasticizer by silanes. When TEOS was used, there was a trend to increase the compressive and flexural strength at 7 days, whereas when GPTMS or AEAPTMS was used, a trend of reduction of these properties was observed regardless of the test age, mainly for 50% replacement. These effects may be attributed to the entrapped air verified in the fresh state and confirmed by images of the fracture surface. The feasibility of applying the silanes considering the improvement in the mechanical properties showed a potential use for TEOS together with the superplasticizer. In contrast, the silanes GPTMS and AEAPTMS, did not led to satisfactory fresh and hardened properties, when used in the contents investigated in the current work.

ACKNOWLEDGMENTS

The authors would like to acknowledge the LINDEN-UFSC and the Laboratory of Application of Nanotechnology in Civil Construction (LabNANOTEC-UFSC) for the support during the experimental program. We also would like to acknowledge the Brazilian governmental research agencies Coordination for the Improvement of Higher Education Personnel (CAPES), the National Council for Scientific and Technological Development

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(CNPq) and the Foundation for Research and Innovation of the State of Santa Catarina (FAPESC) for the financial support to laboratories involved in the research.

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Highlights •

25% of superplasticizer replacement by silanes enhanced the flowability of the pastes.

•

25 and 50% of superplasticizer replacement by silanes significantly changed the hydration kinetics.

•

The superplasticizer replacement by TEOS improved the mechanical properties of the matrix.

AUTHOR DECLARATION

We wish to confirm that there are no known conflicts of interest associated with this publication. We confirm that the manuscript has been read and approved by all named authors and that there are no other persons who satisfied the criteria for authorship but are not listed. We further confirm that the order of authors listed in the manuscript has been approved by all of us. We confirm that we have given due consideration to the protection of intellectual property associated with this work and that there are no impediments to publication, including the timing of publication, with respect to intellectual property. In so doing we confirm that we have followed the regulations of our institutions concerning intellectual property. We understand that the Corresponding Author is the sole contact for the Editorial process (including Editorial Manager and direct communications with the office). He is responsible for communicating with the other authors about progress, submissions of revisions and final approval of proofs. We confirm that we have provided a current, correct email address which is accessible by the Corresponding Author and which has been configured to accept email from [email protected].

Sincerely yours,

Cézar Augusto Casagrande Lidiane Fernanda Jochem Lucas Onghero Paulo de Matos Wellington Repette Philippe Gleize

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Manuscript title: Effect of partial substitution of superplasticizer by silanes in Portland

cement pastes

Author 1: Cézar Augusto Casagrande ☒

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Author 2: Lidiane Fernanda Jochem ☐

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Author 3: Lucas Onghero ☒

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Author 4: Paulo Ricardo de Matos ☐

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Author 5: Wellington Longuini Repette ☐

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Author 6: Philippe Jean Paul Gleize ☐

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