The investigation of factors affecting the water impermeability of inorganic sodium silicate-based concrete sealers

Construction and Building Materials xxx (2015) xxx–xxx

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The investigation of factors affecting the water impermeability of inorganic sodium silicate-based concrete sealers Lihong Jiang a, Xiao Xue a,b, Weidong Zhang a,⇑, Jingna Yang c, Hongqiang Zhang a, Yanwen Li a, Rongpu Zhang a, Ziying Zhang a, Lijin Xu b, Jian Qu a, Jianrong Song a, Jie Qin a a b c

Technical Center, China State Construction Engineering Co., Ltd., Beijing 101300, PR China Department of Chemistry, Renmin University of China, Beijing 100872, PR China Department of Polymer Engineering, Beijing University of Chemical Technology, Beijing 10029, PR China

h i g h l i g h t s  The waterproofing efficacy of a concrete sealer is determined by its surface tension.  The waterproofing efficacy of a concrete sealer is determined by its gelation time.  Some fluorocarbon surfactants greatly reduce the surface tension of a concrete sealer.  The gelation time of a concrete sealer increases as the content of Na2SiO3 increases.  The low surface tension mainly imparts good water impermeability to the concrete.

a r t i c l e

i n f o

Article history: Received 24 April 2015 Received in revised form 27 May 2015 Accepted 1 June 2015 Available online xxxx Keywords: Concrete sealer Sodium silicate Waterproofing properties Gelation time Surface tension

a b s t r a c t The composition and preparation process for an inorganic sodium silicate-based concrete sealer are introduced in the current paper. The factors affecting water impermeability of the concrete sealer are systematically explored. In addition to the concentration of sodium silicates and the viscosity of the concrete sealer, the surface tension and gelation time of the concrete sealer also affect the waterproofing efficacy of the concrete sealer. Some super-active fluorocarbon surfactants are very effective in reducing the surface tension of the concrete sealer to an ideally low value. The gelation time of the sodium silicate-based concrete sealer surprisingly increases as the concentration of the active ingredient increases but decreases as the concentration of the catalyst increases. Additionally, the gelation time decreases as the testing temperature increases. The good waterproofing properties of the developed sodium silicate-based concrete sealer result from its low surface tension and appropriate density, viscosity and gelation time. Ó 2015 Published by Elsevier Ltd.

1. Introduction The ingress of water and water-soluble aggressive agents (e.g., chloride ions, carbon dioxide, sulfur dioxide and sulfates) into concrete objects results in reinforcement corrosion and consequently the deterioration of the reinforced concrete structures [1–6], which has become a worldwide problem in the past few decades. Considerable resources are required to repair and rehabilitate the deteriorated structures, and the cost of repair is sometimes even higher than the original investment [7–10]. Water is mainly responsible for the degradation processes of concrete structures [2,6,10]. Therefore, in recent years, different

⇑ Corresponding author. Tel.: +86 1089498866; fax: +86 1089498030. E-mail address: [email protected] (W. Zhang).

surface protection treatments for concrete have been adopted to prevent the penetration of water into concrete structures, so that the durability of both new and existing concrete structures can be extended [7]. Of the commonly used protective surface treatments, sodium silicate-based concrete sealers (also referred to as concrete impregnations) has attracted extensive attention from both academia and industry due to the following advantages: they react with portlandite in the cement matrix to form inorganic calcium-silicate hydrates (CASAH gels) [11,12,10] and hence avoid aging problems; the durability of the surface treatments is increased [12], and the waterproofing effect is nearly permanent; they are waterborne and thus environmentally friendly. Even though sodium silicate-based concrete sealers are relatively frequently used in civil engineering [7], to date, there appear to be only a small number of studies that are related to their performance [7,12,10], little or no attention has been paid to the

http://dx.doi.org/10.1016/j.conbuildmat.2015.06.001 0950-0618/Ó 2015 Published by Elsevier Ltd.

Please cite this article in press as: Jiang L et al. The investigation of factors affecting the water impermeability of inorganic sodium silicate-based concrete sealers. Constr Build Mater (2015), http://dx.doi.org/10.1016/j.conbuildmat.2015.06.001

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L. Jiang et al. / Construction and Building Materials xxx (2015) xxx–xxx

manufacturing process of the sodium silicate-based concrete sealers, and the exact mechanisms by which the concrete sealers act to improve the performance of concretes are unclear [7,11,12,10]. Furthermore, even in these limited papers, the experimental data on the waterproofing properties of sodium silicate-based impregnations are quite different. While some authors conclude that the treatment of the concrete surface with sodium silicate-based pore blockers does not prevent water absorption and, consequently, this treatment cannot prevent the initiation of corrosion of steel reinforcement [13], others report that the waterproofing performance of sodium silicate-based pore blockers can be significantly improved by post-treatment with alkyl quaternary ammonium salt solution [14]. In addition, there are still several issues concerning this type of concrete surface protection that remain open. Among those issues, to our knowledge, is the fact that the factors affecting the water impermeability of sodium silicate-based concrete sealers have not yet been fully elucidated. Therefore, we conducted systematic investigations of the factors affecting the density, viscosity, surface tension and gelation time of sodium silicate-based concrete sealers. In this paper, the effects of various factors on these four key parameters are presented and discussed. In addition, test data on the waterproofing performance of the developed sodium silicate-based concrete sealers are also presented.

The preparation of the sodium silicate-based concrete sealers proceeded as follows: the sodium silicate was first added into the mixing setup, followed by the addition of deionized water, catalyst, sodium hydrate and surfactant. The mixtures were stirred at high speeds for 30 min, and the concrete sealers were obtained. 2.3. Density and pH measurements Following the procedures specified by the Chinese national standard GB/T 8077-2012 [15], which referenced and adopted ISO 4316-1977 [16] and ISO 1675-1985 [17], the density and pH values of the sodium silicate-based concrete sealers with different compositions were measured using a pycnometer and an acidometer, respectively. 2.4. Viscosity measurements The viscosity values of the sodium silicate-based concrete sealers with different compositions were measured using a 4-mm orifice ISO Flow Cup following the procedures specified by the Chinese national standard GB/T 1723-1993 [18], which is equivalent to ISO 2431-1993 [19] and ASTM D 5125-10-2014 [20]. 2.5. Surface tension measurements In accordance with the Chinese national standard GB/T 8077-2012/ISO 304-1985 [21], the surface tension values of the deionized water, the sodium silicate solution and the sodium silicate-based concrete sealers with different compositions were measured at 25 °C using a Du-Nuoy TensiometerKruss-K100 (Kruss GmbH, Germany). 2.6. Gelation time measurements

2. Experimental section 2.1. Selection of the materials To prepare the inorganic sodium silicate-based concrete sealers, commercially available materials were selected. Soluble sodium silicate, also known as waterglass, purchased from Beijing Sodium Silicate Factory, is an industrial grade sodium silicate solution (34.2 wt%, 8.2 wt% Na2O, 26 wt% SiO2 and SiO2/Na2O ratio of 3.1– 3.4 by mole). The catalyst solution for specific use in the preparation of sodium silicate-based concrete sealers was purchased from Lotech Corporation, Hangzhou, Zhejiang, China. The catalyst is claimed to be a complex compound with a molecular structure consisting of Ca2+  A2. A2 represents an active chemical group whose exact component has not yet been disclosed by any manufacturer due to commercial confidentiality. To study the effects of different surfactants on the surface tension of the sodium silicate-based concrete sealers, the representative surfactants were selected as follows: anionic surfactants trisodium citrate and sodium dodecyl benzene sulfonate (SDBS), cationic surfactant tetrabutylammonium chloride, nonionic surfactant alkylphenol ethoxylate (OP-10) and fluorocarbon surfactants, grades FC-03, FC-117 and FC-118, purchased from Shanghai Jian Hong Industrial Co., Ltd. The physical properties of these surfactants are summarized in Table 1. These representative surfactants were selected not only because they are commonly used surfactants but also because they are relatively cheap and can decrease the costs of the resultant concrete sealers. In addition, trisodium citrate also has notable retarding effect and can be used to adjust the gelation time of the sodium silicate-based concrete sealers. In addition, deionized water and reagent-grade sodium hydrate were used to adjust the viscosity and mole ratio of SiO2/Na2O. 2.2. Preparation of the sodium silicate-based concrete sealer The sodium silicate-based concrete sealers were composed of sodium silicate, deionized water, catalyst solution, sodium hydrate and surfactant. The ranges of the concentrations of the sodium silicate, the surfactants and the catalyst solution were 23.9–34 wt%, 0.003–0.1 wt% and 0.15–2.55 wt%, respectively.

Table 1 Physical properties of the surfactants selected in this work. Surfactants

Physical state

Density (g/cm3)

pH

Trisodium citrate SDBS Tetrabutylammonium chloride OP-10 FC-03 FC-117 FC-118

Solid Solid Solid Liquid Liquid Liquid Liquid

1.008 >0.18 1.05 1.04 0.80–0.99 0.80–0.99 0.80–0.99

Solution: 7.5–9.0 Solution: 7.0–10.5 – 5–7 6.0–8.0 6.0–8.0 6.0–8.0

The gelation time of a sodium silicate concrete sealer is defined as the time between pouring the concrete sealer into the container and the time at which the flow of the solution ceases to be discernible. The gelation time values of the sodium silicate-based concrete sealers with different compositions were measured at (20 ± 3) °C in accordance with China’s construction materials industrial standard JC/T 1018-2006 [22]. To this end, 0.5 g of Ca(OH)2 was placed in a 50-ml glass beaker, followed by the addition of 15 ml of deionized water. The mixture was stirred for 2 min using a glass rod, and then 20 ml of concrete sealer was slowly added into the mixture. The test started to be timed at this moment. The specimen was stirred for 5 min. Thereafter, the stirring was stopped and the mixture was allowed to stand for an additional 60 min. The glass beaker was slightly shaken, and the fluid level was observed. The initial gelation time was the moment when the gelation could be observed, and the final gelation time was the time at which the flow of the solution in the 45°-tilted beaker ceased to be discernible. The results reported are based on the mean of at least three parallel measurements. 2.7. Impermeability measurements Concrete specimens with the strength grade C30, whose composition is presented in Table 2, were casted using plastic formworks and used as substrates for water impermeability measurements of the sodium silicate-based concrete sealers. The cement, the fineness module of the medium sand and the maximum particle size of the crushed stone used in the concrete composition are type 1/42.5 R, 2.3–3.0, and 40 mm, respectively. Twelve truncated-cone-shaped and three cubic concrete specimens were manufactured according to standard procedures and then cured in standard conditions [(20 ± 2) °C, relative humidity P 95%] for 28 days. For each truncated-cone-shaped specimen, the height is 150 mm and the diameters of the upper surface and the bottom surface are 175 mm and 185 mm, respectively. The geometry of the cubic specimens is 100 mm  100 mm  100 mm. Six truncated cone-shaped specimens were used as controls, and the other six were used as testing substrates. Three cubic specimens were tested to determine the compressive strength of the concrete specimens. The average measured compressive strength was 42 MPa. After the curing period, six reference specimens were continuously cured for seven additional days in the same standard conditions. Demolding agents on the bottom surfaces of the testing specimens were removed using a stainless steel wire brush, and the specimens were then cleaned. Thereafter, the bottom surfaces of the substrates that were in contact with the formworks were dipped into the sodium silicate concrete sealer bath for 24 h under standard laboratory conditions [(23 ± 2) °C, RH (50 ± 5)%]. The immersion depth was more than 10 mm. The impregnated specimens were then cured in standard conditions for 6 days prior to testing. Following the Chinese national standard GB/T 50082-2009 [23], which is comparable with EN 123908-2000 [24], ASTM C6421997 [25], NT Build 369 [26] and DIN 1048 part 5-1991 [27], the water impermeability measurements were carried out on six testing specimens and six reference specimens using the concrete impermeability tester (HP-4.0, Shanghai Le Ao Test Instrument Co., Ltd, China) shown in Fig. 1. The measurements were performed under standard laboratory conditions.

Please cite this article in press as: Jiang L et al. The investigation of factors affecting the water impermeability of inorganic sodium silicate-based concrete sealers. Constr Build Mater (2015), http://dx.doi.org/10.1016/j.conbuildmat.2015.06.001

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L. Jiang et al. / Construction and Building Materials xxx (2015) xxx–xxx Table 2 The composition and mixture ratio of the concrete specimens used in the impermeability measurements of the sodium silicate concrete sealers. Water/cement ratio (w/c)

Compositions (kg/m3) Cement

Water

Medium sand

Crushed stone

0.60

317

190

654

1189

Mixture ratio cement:sand:stone:water

Sand ratio (%)

Slump

1:2.06:3.75:0.60

35.5

80 ± 10

portlandite in the cement matrix and not corrode concrete and rust rebars. Therefore, the addition of any additives into the concrete sealers that may decrease their pH values must be avoided. For instance, in the preparation of sodium silicate-based concrete sealers, surfactants are generally added to decrease their surface tension to improve their penetrating ability. Reasonable care should be taken in the selection of surfactants. Because some conventional surfactants (e.g., fatty alcohol polyoxyethylene ether disodium sulfosuccinate) are acidic, the addition of these surfactants may decrease the pH values of the concrete sealers. According to China’s construction materials industrial standard JC/T 1018-2006, the pH value of a qualified sodium silicate-based concrete sealer should be 13 ± 1. 3.2. Density

Fig. 1. Photo of the concrete impermeability tester used in this work.

The hydraulic pressure was adjusted to 1.2 MPa and kept there for 8 h. After that, the specimens were broken and the depth of penetration values were measured at 10 points. The results reported are based on the mean of six parallel measurements. Note that the reference specimens should be impermeable at a hydraulic pressure of 0.8 MPa for 8 h, or else all of the measurements should be repeated.

3. Results Theoretically, when the sodium silicate-based concrete sealers permeate into the concrete surface, the reaction between the active sodium silicate and the portlandite in the cement matrix occurs as follows [4,7,11,12,10,13]:

ð1Þ

3.1. pH values Generally speaking, sodium silicate-based concrete sealers are used directly on the surfaces of concrete and cement mortar. Their pH values should be in line with each other because only under this circumstance can sodium silicates react with the

3.3. Viscosity Theoretically, sodium silicate-based penetrating concrete sealers should have low enough viscosity to achieve satisfactory penetration [28] because a thin liquid is more quickly attracted than a viscous liquid substance [29]. The viscosity of a sodium

1.6 1.5 3

The calcium–silicate hydrates (CASAH gels) produced can block the pores in the concrete. As a consequence, the hardness of the concrete surface is enhanced and the impermeability and durability of the concrete structure are increased [7,11,12,10,13]. Therefore, the sodium silicates are generally viewed as pore blockers [4]. Naturally, the pore-blocking ability and thus the waterproofing ability of a sodium silicate-based concrete sealer are highly dependent on the quantities of the product (CASAH gels), which are mainly determined by the quantities of the active ingredient – sodium silicates – which have penetrated into the concrete structures (considering that the concentration of portlandite in the cement matrix can be viewed as high enough). The quantity of sodium silicate-based concrete sealer penetrating into concrete structures is affected by the concentration of sodium silicates in the sealer, the viscosity, the surface tension and the gelation time of the sealer. Therefore, all factors that affect these four parameters necessarily affect the impermeability of the sodium silicate-based concrete sealers and are thus investigated in this section.

Density (g/cm )

Na2 SiO3 þ yH2 O þ xCaðOHÞ2 ! xCaO  SiO2  yH2 O þ 2NaOH

Concrete sealers belonging to the same generic type were often found to have quite different waterproofing performance, generally resulting from the different concentrations of the active ingredient [28]. This is because the concentration of the active reactant not only determines the quantities of the product but also directly affects the gelation time of the sealers. The concentration of the active ingredient can be characterized either by the solid content or by the density of the sealer. The density of the sealer was used to characterize the concentration of sodium silicates in this work. Fig. 2 shows the variation of the density of sodium silicate-based concrete sealers as a function of the weight concentration of the sodium silicate. As expected, the density of sodium silicate-based concrete sealers increases as the weight concentration of the sodium silicate increases.

1.4 1.3 1.2 1.1

24

26

28

30

32

34

Concentration of Na2SiO3 (wt%) Fig. 2. Variation of the density of sodium silicate-based concrete sealers as a function of the weight concentration of the sodium silicate.

Please cite this article in press as: Jiang L et al. The investigation of factors affecting the water impermeability of inorganic sodium silicate-based concrete sealers. Constr Build Mater (2015), http://dx.doi.org/10.1016/j.conbuildmat.2015.06.001

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silicate-based concrete sealer is controlled by the content of sodium silicate, additives and water. Thus, matching performance of the ingredients is required for a reasonable viscosity. Generally speaking, the viscosity positively correlates with the concentration of sodium silicates. Therefore, practically, the viscosity of the concrete sealer cannot be too low. An appropriate viscosity value can not only guarantee good penetration but also sufficient sodium silicate and products. Fig. 3 presents the variation of the viscosity of sodium silicate-based concrete sealers as a function of the weight concentration of the sodium silicate. The concentration values of the sodium silicate were 23.9, 25.6, 27.3, 29, 30.7 and 32.4 wt%. Again, as anticipated, the viscosity of the sodium silicate-based concrete sealers also increases as the weight concentration of the sodium silicate increases. Although the viscosity of a sodium silicate-based concrete sealer is mainly determined by the concentration of the sodium silicate, it can also be increased by the addition of some thickening agents. However, the addition of thickening agents may affect the transparency of the concrete sealer, which is not in accordance with the requirements specified by China’s construction materials industrial standard JC/T 1018-2006. 3.4. Surface tension It is widely accepted that when a liquid comes into contact with a porous building material such as concrete, the liquid is sucked into the pores of the material by capillary forces, which are determined by the surface tension of the liquid, the contact angle between the liquid and the pore walls, and the radius of the pores [29]. The lower the surface tension, the better is the penetration. According to China’s construction materials industrial standard JC/T 1018-2006, the surface tension of a qualified sodium silicate-based concrete sealer should be less than or equal to 26 mN/m. The measured surface tension values of water and the sodium silicate solution with concentration of 34.2 wt% used in this work are 72.8 and 44.9 mN/m, respectively. Clearly, surfactants should be added to decrease the surface tension of the concrete sealers to a lower value. The effects of different types of commonly used surfactants on the surface tension of sodium silicate-based concrete sealers composed of a specific amount of deionized water (5 wt%) and catalyst (0.15 wt%) and a variable amount of the sodium silicate solution (94.75–94.947 wt%) and a surfactant (0.003–0.1 wt%) are presented in Fig. 4. As indicated in Fig. 4, the surface tension of sodium silicate-based concrete sealers initially increases and then

15 14

Viscosity (s)

13 12

decreases as the weight concentrations of the tetrabutylammonium fluoride, sodium dodecyl benzene sulfonate and alkylphenol ethoxylate increases (Fig. 4a–c); the surface tension of sodium silicate-based concrete sealers initially decreases as the concentration of sodium citrate increases, then abruptly increases as the concentration of sodium citrate increases and finally decreases again as the concentration of sodium citrate increases further (Fig. 4c); the surface tension of sodium silicate-based concrete sealers decreases as the concentrations of the fluorocarbon surfactants increase and then levels off above the critical micelle concentration (CMC) (Fig. 4d), showing the expected behavior. Additionally, the most effective surfactant is FC-03, which can reduce the surface tension of sodium silicate-based concrete sealers to 24.9 mN/m, which is lower than the minimum surface tension value of 26 mN/m specified by China’s construction materials industrial standard JC/T 1018-2006. The patterns of surface tension variation with surfactant concentration shown in Fig. 4a–c are markedly different from the classical behavior shown in Fig. 4d. The unexpected surface tension behavior of the sodium silicate/sodium citrate system is similar to that observed in surface tension curves of sodium dodecyl sulfate/poly dimethyldiallylammonium chloride (SDS/poly-dmdaac) systems [30]. The structure of colloidal particles in sodium silicate + 2x solution is {[(SiO2)mnSiO2 }2xNa+ [31]. Therefore, 2 2(n  x)H ] the behavior of sodium silicate is similar to that of a charged polymer. Therefore, the initial decrease in the surface tension at low surfactant concentrations is most likely ascribed to the formation of surface sodium silicate/surfactant complexes, as a result of the electrostatically driven cooperativity. The subsequent increase in surface tension at higher surfactant concentrations is attributed to the decrease in the amount of sodium citrate and sodium silicate at the interface because the formation of sodium silicate/surfactant complexes in solution becomes more favorable than that of surface complexes. In the final stage, the second decrease in surface tension results from the increase in the amount of sodium citrate and sodium silicate at the interface, as a consequence of strong sodium silicate/surfactant interaction at the interface and the adsorption of a sodium/surfactant complex at the interface [30]. The initial decrease in surface tension with increasing surfactant concentration is probably shifted to much lower concentrations for surface tension curves of sodium silicate/tetrabutylammonium fluoride, sodium silicate/sodium dodecyl benzene sulfonate and sodium silicate/alkylphenol ethoxylate systems. The initial decrease and the subsequent increase in surface tension for these three systems originate from the same source as mentioned above. Of the chemical bonds, the CAF bond has the strongest bonding energy and thus is much more stable than the CAH bond. In addition, the CAF bond cannot be easily polarized and consequently processes low polarity. Therefore, the interaction between the charged sodium silicates and fluorocarbon surfactants is weak, giving rise to the expected behavior. 3.5. Gelation time

11 10 9 8 7 24

26

28

30

32

34

Concentration of Na2SiO3 (wt%) Fig. 3. Variation of the viscosity of sodium silicate-based concrete sealers as a function of the weight concentration of the sodium silicate.

As indicated by its definition, the gelation time of a sodium silicate-based concrete sealer characterizes the speed of the reaction between sodium silicate and calcium hydroxide. An over-short gelation time means too fast a reaction. As a result, the capillary pore channels are blocked too early and the depth of penetration is reduced. An excessively long gelation time means not only that the construction time is prolonged but also that the concentration of sodium silicates and thus the viscosity are too high. Consequently, the depth of penetration is also decreased. The gelation time of sodium silicate-based concrete sealers is affected by the concentrations of the sodium silicate and the catalyst. It is also determined by the testing temperature. Fig. 5 shows

Please cite this article in press as: Jiang L et al. The investigation of factors affecting the water impermeability of inorganic sodium silicate-based concrete sealers. Constr Build Mater (2015), http://dx.doi.org/10.1016/j.conbuildmat.2015.06.001

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L. Jiang et al. / Construction and Building Materials xxx (2015) xxx–xxx

(a)

(b)

36

Surface tension (mN/m)

Surface tension (mN/m)

SDBS sodium citrate

34

35 34 33 32 31

33 32 31 30 29 28

30 0.00

0.02

0.04

0.06

0.08

0.10

0.00

Concentration of a cationic surfactant (wt%)

0.04

0.06

0.08

0.10

(d)

(c)

31 Surface tension (mN/m)

29.5 Surface tension (mN/m)

0.02

Concentration of anionic surfactants (wt%)

29.0 28.5 28.0 27.5

FC-03 FC-117 FC-118

30 29 28 27 26 25

0.00

0.02

0.04

0.06

0.08

0.10

Concentration of a nonionic surfactant(wt%)

0.00

0.02

0.04

0.06

0.08

0.10

Concentration of fluorocarbon surfactancts (wt%)

Fig. 4. Variation of the surface tension of sodium silicate-based concrete sealers as a function of the weight concentration of a cationic surfactant (a), two anionic surfactants (b), a nonionic surfactant (c) and three fluorocarbon surfactants (d).

the weight concentration of the sodium silicate increases, with the involvement of CO2, the gel network structure is formed as follows [31]:

500

Gelation time (min)

450 400

Na2 O þ nSiO2 þ 2nH2 O þ CO2 ! 2Na2 CO3 þ nSiðOHÞ4

ð2Þ

350 300 250

ð3Þ

200 150 100 24

26

28

30

32

34

36

Concentration of Na2SiO3 (wt%) Fig. 5. Variation of the gelation time of sodium silicate-based concrete sealers as a function of the weight concentration of the sodium silicate.

the variation of the gelation time of sodium silicate-based concrete sealers as a function of the weight concentration of the sodium silicate. Surprisingly, the gelation time of sodium silicate-based concrete sealers increases as the weight concentration of the sodium silicate increases. It is common knowledge that the speed of chemical reactions generally increases as the concentrations of the reactants increase. This unexpected result most likely results from the formation of a gel network structure (ASiAOASiA), which interferes with the ability of the molecules of sodium silicate to make adequate contact with the molecules of calcium hydroxide. As

It should be stressed that this interesting phenomenon can also be observed in cement–waterglass grouting materials, whose gelation time was also found to increase as the concentration of sodium silicates increases [32]. Fig. 6 shows the variation of the gelation time of sodium silicate-based concrete sealers as a function of the weight concentration of the catalyst. The concentration of the sodium silicate is 31.4 wt%. As expected, the gelation time of sodium silicate-based concrete sealers initially decreases as the concentration of the catalyst increases and then levels off above a threshold concentration. It is widely accepted that the speed of a chemical reaction cannot be infinitely accelerated by a catalyst. The temperature dependence of the gelation time of sodium silicate-based concrete sealers is presented in Fig. 7. The concentration of the sodium silicate is 31.4 wt%. As indicated in Fig. 7, the gelation time of sodium silicate-based concrete sealers decreases as the testing temperature increases, which is also in accordance with expectation. Clearly, if a concrete sealer is used in summer, when the surface temperature of concrete roof is easy

Please cite this article in press as: Jiang L et al. The investigation of factors affecting the water impermeability of inorganic sodium silicate-based concrete sealers. Constr Build Mater (2015), http://dx.doi.org/10.1016/j.conbuildmat.2015.06.001

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L. Jiang et al. / Construction and Building Materials xxx (2015) xxx–xxx Table 3 The measured pH, density, viscosity, surface tension, gelation time and water impermeability and the requirements specified by China’s construction materials industrial standard JC/T 1018-2006.

200 180

Gelation time (min)

160 140 120 100 80 60

Testing parameters

Measured values

Specifications

pH Density (g/cm3) Viscosity (s) Surface tension (mN/m) Gelation time (min) Depth of impermeability (mm)

13 1.46 11.6 24.9 160 20

13 ± 1 P1.1 11 ± 1 626 6180 ± 30 630

40 20 0

0.0

0.5

1.0

1.5

2.0

2.5

Concentration of catalyst (wt%) Fig. 6. Variation of the gelation time of sodium silicate-based concrete sealers as a function of the weight concentration of the catalyst.

4. Discussion

300 270 Gelation time (min)

As shown in Table 3, the measured pH, density, viscosity, surface tension, gelation time and water impermeability of the developed sodium silicate-based concrete sealer completely meet the requirements specified in China’s construction materials industrial standard JC/T 1018-2006, indicating that the developed sodium silicate-based concrete sealer is a quite qualified concrete sealer.

240 210 180 150 120 90 60 30 10

20

30

40

50

60

70

o

Testing temperature ( C) Fig. 7. Temperature dependence of the gelation time of sodium silicate-based concrete sealers.

to reach 70–80 °C [33,34], some retarders should be added into the sealer to retard the gelation process for a deeper penetration of the concrete sealer and thus a better waterproofing performance. 3.6. Water impermeability Of the performance parameters of a sodium silicate-based concrete sealer, water impermeability is the most significant one because it directly determines the quality and applicability of the sodium silicate-based concrete sealer. The optimum composition of the sodium silicate-based concrete sealers was determined by orthogonal testing to be as follows: sodium silicate solution (91.76 wt%), deionized water (6 wt%), catalyst solution (0.2 wt%), sodium hydrate (2 wt%) and surfactant (0.04 wt%). The measured average depth of water penetration values for reference specimens and specimens treated with the developed sodium silicate concrete sealer were 52 and 20 mm, respectively. The water impermeability of impregnated samples is notably increased compared with the reference specimens. In addition, the pH, density, viscosity, surface tension and gelation time were also tested following the related standards and the results obtained are summarized in Table 3. For the convenience of comparison, the above mentioned corresponding requirements specified in the China’s construction materials industrial standard JC/T 1018-2006 are listed in Table 3 again.

The experimental results presented above clearly show that a well-designed sodium silicate-based concrete sealer is very effective in reducing the permeability to water of concrete structures. Considerable engineering practice and field tests have also demonstrated that the commercially available sodium silicate-based concrete sealers are very useful for improving the water impermeability of concrete structures and extending their durability. However, as mentioned above, it has been argued that the treatment of the concrete surface with sodium silicate-based pore blockers cannot prevent water absorption because hydrophilic silicate gels are formed after chemical reaction with the concrete with the involvement of CO2 [13]. Clearly, the investigations on the water impermeability of sodium silicate-based concrete sealers have not yielded a universal conclusion. These controversial arguments will be discussed in greater detail below. As noted by Litvan [28], a generic type of a concrete sealer in itself cannot ensure good performance. Concrete sealers belonging to the same generic type can have quite different water proofing ability due to the different concentration of the active ingredient. Furthermore, penetrating concrete sealers must have low enough viscosity to achieve satisfactory penetration. In addition to these two factors, as illustrated in this work, for good water impermeability of a concrete sealer, it is necessary to achieve a deep penetration of sufficient active ingredients into the concrete structures; this is also affected by the surface tension and gelation time of the concrete sealer. A lower surface tension generally leads to a higher wetting ability, and an appropriate gelation time can usually ensure that enough active ingredients penetrate into the concrete structures. As shown in Table 1 of reference 13, the sodium silicate concrete sealer was simply composed of 30–60% sodium silicate and 1–5% catalyst, and there was no surfactant. As shown above, the measured surface tension of the sodium silicate solution with concentration of 34.2 wt% was 44.9 mN/m. Therefore, it is reasonable to speculate that the surface tension of the concrete sealer was high without the addition of some super active surfactants. In addition, as shown in Fig. 6, when the concentration of the catalyst was 1 wt%, the gelation time of the concrete sealer was approximately 94.5 min. Evidently, the concentration of the catalyst of the concrete sealer developed in reference 13 was too high and thus the gelation time was too short and the active ingredients of the concrete sealer cannot sufficiently penetrated into the concrete substrates. Therefore, the waterproofing efficacy of the surface treatment with sodium silicates was not good.

Please cite this article in press as: Jiang L et al. The investigation of factors affecting the water impermeability of inorganic sodium silicate-based concrete sealers. Constr Build Mater (2015), http://dx.doi.org/10.1016/j.conbuildmat.2015.06.001

L. Jiang et al. / Construction and Building Materials xxx (2015) xxx–xxx

The effects of the surface tension and the density of a sodium silicate-based concrete sealer on its waterproofing performance may be clearly described using Young–Laplace equation. It is common knowledge that concrete is inherently a porous material [1,2,13,28], and its surface inevitably contains micro-pores, micro-voids and micro-cracks [1,2,13]. These structural defects provide easy paths for liquid penetration into the interior of the concrete [2,13]. The distributions of these micro-defects are generally not homogenous and their shapes are not regular, yet, they can be simply viewed as a sufficiently narrow tube of circular cross-section (radius R). When the sodium silicate-based concrete sealer comes into contact with concrete, the capillary pressure (DP) may be expressed using Young–Laplace equation as follows:

DP ¼

2c cosh R

ð4Þ

where c is the liquid–solid interfacial free energy, which is numerically equal to the interfacial tension, and h is the contact angle [35]. For a given concrete substrate, at the specific liquid–solid interfacial free energy, the capillary pressure is proportional to the cosh. The cosh was found to increase linearly as the surface tension of the liquid decreases [36]. Therefore, the lower the surface tension of the sodium silicate-based concrete sealer, the bigger the capillary pressure and consequently the stronger the capability of the liquid penetrating into the solid. For a liquid of density q, the penetration height (h), also known as the Jurin height [37,38], of the liquid may be written as:

h¼

2c cosh pgR

ð5Þ

where g is the gravitational acceleration. Clearly, the lower the density of a sodium silicate-based concrete sealer is, the deeper the sealer penetrates into the concrete. Because the viscosity of the sodium silicate-based concrete sealer positively correlates to the concentration of the sodium silicate and thus the density of the sealer, the penetration capability and penetration height of the concrete sealer decrease as the viscosity of the sealer increases. However, as mentioned above, to ensure both good penetration and sufficient sodium silicate and products, the density and viscosity of the sodium silicate-based concrete sealer should be adjusted to reach appropriate values. Evidently, the effects of different technical parameters of a sodium silicate-based concrete sealer on its waterproofing performance should be systematically considered. To obtain an ideal waterproofing effect, a sodium silicate-based concrete sealer should have low enough surface tension and appropriate pH value, density, viscosity and gelation time. Note that the technical requirements specified by the China’s construction materials industrial standard JC/T 1018-2006 referenced the corresponding technical parameters of a commercially available sodium silicate-based concrete sealer-Deep Penetration Sealer (DPS), Evercrete Corporation USA, whose density, pH value, viscosity, surface tension, gelation time and water penetration value are 1.07 g/cm3, 11 ± 1, 11 ± 1s, 36 mN/m, 400 h and 35 mm, respectively. Compared with the technical parameters listed in Table 3, the sodium silicate-based concrete sealer developed in this work has higher density and pH value, equivalent viscosity, shorter gelation time and much lower surface tension and depth of water impermeability. Clearly, the sodium silicate-based concrete sealer developed in this work has better waterproofing performance.

5. Conclusions The factors affecting the water impermeability of inorganic sodium silicate-based concrete sealers were systematically

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investigated for the first time in this work. The results presented in this paper offer the following conclusive evidence:  In addition to the known factors, such as the concentration of the active ingredient and the viscosity of a sodium silicate-based concrete sealer, the water impermeability of the concrete sealer is also determined by the surface tension and gelation time of the concrete sealer.  The most commonly used conventional surfactants cannot effectively reduce the surface tension of the developed sodium silicate-based concrete sealer to an ideally low value. Some super-active fluorocarbon surfactants can reduce the surface tension of the concrete sealer to 24.9 mN/m.  The gelation time of a sodium silicate-based concrete sealer unexpectedly increases as the concentration of sodium silicates increases, while it decreases as the concentration of the catalyst increases. It also decreases as testing temperature increases.  The low surface tension and appropriate density, viscosity and gelation time of the developed sodium silicate-based concrete sealer endow the impregnated concrete with good water impermeability.

Acknowledgments Prof. L. Jiang and Dr. X. Xue contributed equally to this work. This work was performed under the ‘‘Water-Borne Cool Coatings for Building Energy Efficiency’’ project with funding from the Technical Center of China State Construction Engineering Co., Ltd. (Grant No. 00.000.072). References [1] Swamy RN, Tanikawa S. An external surface coating to protect concrete and steel from aggressive environments. Mater Struct 1993;26:465–78. [2] Barbucci A, Delucchi M, Cerisola G. Organic coatings for concrete protection: liquid water and water vapour permeabilities. Prog Org Coat 1997;30:293–7. [3] Diamanti MV, Brenna A, Bolzoni F, Berra M, Pastore T, Ormellese M. Effect of polymer modified cementitious coatings on water and chloride permeability in concrete. Constr Build Mater 2013;49:720–8. [4] Brenna A, Bolzoni F, Beretta S, Ormellese M. Long-term chloride-induced corrosion monitoring of reinforced concrete coated with commercial polymermodified mortar and polymeric coatings. Constr Build Mater 2013;48:734–44. [5] Seneviratne AMG, Sergi G, Page CL. Performance characteristics of surface coatings applied to concrete for control of reinforcement corrosion. Constr Build Mater 2000;14:55–9. [6] Delucchi M, Barbucci A, Cerisola G. Study of the physico-chemical properties of organic coatings for concrete degradation control. Constr Build Mater 1997;11:365–71. [7] Baltazar L, Santana J, Lopes B, Rodrigues MP, Correia JR. Surface skin protection of concrete with silicate-based impregnations: influence of the substrate roughness and moisture. Constr Build Mater 2014;70:191–200. [8] Medeiros M, Helene P. Efficacy of surface hydrophobic agents in reducing water and chloride ion penetration in concrete. Mater Struct 2008;41:59–71. [9] Ibrahim M, Al-Gahtani AS, Maslehuddin M, Almusallam AA. Effectiveness of concrete surface treatment materials in reducing chloride-induced reinforcement corrosion. Constr Build Mater 1997;11:443–51. [10] Franzoni E, Pigino B, Pistolesi C. Ethyl silicate for surface protection of concrete: performance in comparison with other inorganic surface treatments. Cement Concrete Comp 2013;44:69–76. [11] Thompson JL, Silsbee MR, Gill PM, Scheetz BE. Characterization of silicate sealers on concrete. Cement Concrete Res 1997;27:1561–7. [12] Pigino B, Leemann A, Franzoni E, Lura P. Ethyl silicate for surface treatment of concrete – part II: characteristics and performance. Cement Concrete Comp. 2012;34:313–21. [13] Dai J, Akira Y, Wittmann FH, Yokota H, Zhang P. Water repellent surface impregnation for extension of service life of reinforced concrete structures in marine environments: the role of cracks. Cement Concrete Comp. 2010;32:101–9. [14] Kagi DA, Ren KB. Reduction of water absorption in silicate treated concrete by post-treatment with cationic surfactants. Build Environ 1995;30:237–43. [15] The Chinese national standard GB/T 8077-2012. The methods for testing uniformity of concrete admixture. [16] ISO 4316-1977. Surface active agents; Determination of pH of aqueous solutions; Potentiometric method.

Please cite this article in press as: Jiang L et al. The investigation of factors affecting the water impermeability of inorganic sodium silicate-based concrete sealers. Constr Build Mater (2015), http://dx.doi.org/10.1016/j.conbuildmat.2015.06.001

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Please cite this article in press as: Jiang L et al. The investigation of factors affecting the water impermeability of inorganic sodium silicate-based concrete sealers. Constr Build Mater (2015), http://dx.doi.org/10.1016/j.conbuildmat.2015.06.001