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Effects of the pozzolanic reactivity of nanoSiO2 on cement-based materials
Cement & Concrete Composites 55 (2015) 250–258
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Effects of the pozzolanic reactivity of nanoSiO2 on cement-based materials Pengkun Hou a,b, Jueshi Qian c,⇑, Xin Cheng a,b, Surendra P. Shah d a
School of Materials Science and Engineering, University of Jinan, Jinan, Shandong 250022, China Shandong Provincial Key Laboratory of Preparation and Measurement of Building Materials, Jinan, Shandong 250022, China c College of Materials Science and Engineering, Chongqing University, Chongqing 400045, China d Department of Civil and Environmental Engineering, Northwestern University, Evanston, IL 60208, USA b
a r t i c l e
i n f o
Article history: Received 7 August 2013 Received in revised form 5 May 2014 Accepted 21 September 2014 Available online 6 October 2014 Keywords: NanoSiO2 Pozzolanic reactivity Surface-treatment
a b s t r a c t The aim of this work is to understand the characteristics of the pozzolanic reactivity of nanoSiO2 from studies of its pozzolanic reaction kinetics, morphology and structure of the hydrates and the influences of these features on the properties of cement-based materials, so as to explore a more targeted way of using nanoSiO2 in cement or concrete. It revealed that the pozzolanic reaction of nanoSiO2 is of the first-order and the apparent reaction rate constant of nanoSiO2-4 nm is about one order of magnitude bigger than that of silica fume, but the specific reaction rate constant is about one half to that of silica fume. A compacter gel structure and poorer crystallinity of the hydrates of nanoSiO2 to those of silica fume are found, as well. The rate of hydration of cement at very early ages is enhanced by nanoSiO2, but the rate slows down with aging due to the compact gel structure. To make the use of the high pozzolanic reactivity and ultrafine particle size of nanoSiO2, as well as its resulting compact gel structure, colloidal nanoSiO2 was applied onto the hardened cement mortar by brushing technique and a less permeable surface was resulted, which shows the potential of using nanoSiO2 as a surface treatment material for cement-based materials. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction The application of nanotechnology in cement and concrete has attracted much attention in recent years. It is being gradually accepted that by adding a portion of nanoparticle, even at a very small dosage, the properties of cement-based material can be enhanced to a great extent in respects of workability, strength gain and durability [1–3]. And novel functional properties, such as photocatalicity property introduced by nanoTiO2, piezoelectricity by nanotube, can be obtained, as well [4–6]. Among all the nanopartilces, nanoSiO2 has been most widely investigated. NanoSiO2 is often produced by the sol–gel method by the hydrolysis process of trimethylethoxysilane or tetraethoxysilane, and its particle size is often smaller than 100 nm. Due to its pozzolanic reactivity and ultrafine particle size, it was reported that nanoSiO2 improved the compressive strength of cement-based materials significantly, and made the microstructure denser as well [7–10]. Attempts have been made to use nanoSiO2 to prepare HPC or low-carbon cementitious materials [11,12]. ⇑ Corresponding author. Tel.: +86 23 65126109. E-mail address: [email protected] (J. Qian). http://dx.doi.org/10.1016/j.cemconcomp.2014.09.014 0958-9465/Ó 2014 Elsevier Ltd. All rights reserved.
Although it has been well-documented that nanoSiO2 introduced a lot of benefits to cement-based materials, some other researches demonstrated that the property-enhancing characteristics of nanoSiO2 was correlated to the curing age: the rate of strength gain of nanoSiO2-modified system slowed down at later ages, which became smaller than that of the blank sample three months later [13–17]. There are many studies showing a smaller compressive strength of nanoSiO2-modified cement-based materials, and almost all the phenomena were attributed the poor dispersion of nanoSiO2 in the blends [18]. More recently, Nazari claimed that the decreased crystalline Ca(OH)2 content required for C–S–H gel formation could be the reason [19]. Considering nanoSiO2 has a greater reactivity than the conventional pozzolans, its hydration characteristics and the gel properties may be accounted for property-gain of cement-based materials, which is short of investigation thus far [20]. In this paper, study on the pozzolanic reactivity of nanoSiO2 and its influences on the properties of cement-based materials will be conducted. After that, advantages of utilizing the pozzolanic reactivity and particle size feature of nanoSiO2 in surface treatment of hardened cement mortar will be shown. We hope that a clearer recognition of the pozzolanic reactivity of nanoSiO2 and its
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influences on cement-based materials can be made and its advantages can be better explored. 2. Experimental 2.1. Materials A Type I Portland Cement with a 28-day compressive strength of 45.1 MPa was used in this study. The bulk compositions of the cement are shown in Table 1. Commercially available colloidal nanosilica (CNS), other than nanosilica powder, was used to achieve a homogeneous dispersion in cement paste. CNSes with average particle sizes of 4 nm, 10 nm and 20 nm produced by the sol–gel technique and stabilized by sodium were used in this study [14]. Eq. (1) can be used to summarize the synthesis process of nanosilica, i.e. the hydrolysis of the precursor of TEOS into Si5O4(OH)12 and finally SiO2 in acidic or alkaline environment. In this work, nanosilica used was synthesized in alkaline environment as reported by the provider. The purity of the CNSes were higher than 99%. Viscosity measured by rheometer showed that all water and CNSes showed comparable viscosity of about 10 Pa S. Considering the content of the sodium introduced into cement-based material is greatly less than the inherent sodium content in cement (alkali limit in cement is about 0.6%), its influence on cement hydration was ignored. A silica fume (SF) with the particle size ranged from 100 nm to 2000 nm was used for a comparison study (chemical compositions of SF are shown in Table 1). Particle size distributions of CNSes and SF are depicted in Fig. 1, from which it shows that all CNSes exhibit a single dispersion peak, illustrating a well dispersion of the materials. While for SF, a double-peak distribution curve is seen. Morphology images of CNS and SF (as shown in Fig. 2) reveal that both pozzolans are round in shape for individual particles, however, some clusters were seen. Considering the specific surface area of the pozzolan plays an important role in its reactivity, BET technique was used to obtain this value. However, as BET test can only be conducted on solid powder, it is impossible to obtain such value on colloidal nanoSiO2 sample. In this work, specific surface area of the SF was measured by BET technique and the specific surface areas is shown in Fig. 1, in which the specific surface area value of CNSes provided by the supplier are also presented. Considering the characteristic particle size of CNS as shown in the particle size distribution curve equals to the value provided by the supplier well, specific surface area of CNS as provided by the supplier was used during the following discussions. XRD patterns (as shown in Fig. 3) indicate the poor-crystallinity of both pozzolans. A similar full width at half maximum indicates that the two materials have a comparable crystallinity.
nSiðOC2 H5 Þ4 þ 2nH2 O ! nSiO2 þ 4nC2 H5 OH
ð1Þ
Table 1 Physiochemical compositions of cement and silica fume. Materials
Type I cement
Silica fume
SiO2 Al2O3 Fe2O3 SO3 CaO MgO LOI Total Density, g/cm3 Fineness as surface area, m2/kg
20.2 4.7 3.3 3.3 62.9 2.7 1.1 98.2 3.1 380
90.1 0.6 2.0 – 0.5 5.1 1.0 99.3 2.2 21,000
251
2.2. Sample preparation To explore the pozzolanic reactivity of the pozzolans, 20 g of chemical grade Ca(OH)2 (CH) were mixed with 5 g of CNS/SF at a water-to-binder ratio (w/b) of 2.0 to simulate a cement-CNS/SF system. It was assumed that 20 g of CH can be generated by 100 g of cement [21]. After mixing, samples were sealed in plastic vials and the CH content at different ages were determined by the TGA technique. During sampling, core parts of the sample were selected and a quick sampling process was conducted to ensure a less carbonization and a good reproductivity of the test. Unless otherwise stated, blends were mixed at the highest speed of the mixer for five minutes to achieve a homogeneous dispersion of the particles. Unless otherwise stated, cement pastes mixed with and without 5% CNS/SF at a w/b ratio of 0.4 were prepared and investigated throughout this study. Samples were demolded 1 day after casting and cured in saturated lime solution at about 23 °C until testing. Plain cement mortar of a w/c ratio of 0.6 and a sand/cement ratio of 3.0 was used to evaluate the surface-sealing effect of CNS by measuring the water absorption ratio of the mortars before and after CNS treatment applied by using brushing technique. 2.3. Test methods 2.3.1. CH content Thermogravimetric analysis (TGA, METTLER, TGA/sDTA 851) was carried out to measure the CH content of the CH-CNS/SF blends. Samples were heated in nitrogen atmosphere from 50 °C to 950 °C at a heating rate of 15 °C/min. The weight loss between 440 °C and 510 °C was considered to be the decomposition of CH crystal into lime [22,23]. Before measuring, powder samples were vacuum oven-dried at 105 °C for four hours. CH content was calculated based on the ignited sample. 2.3.2. Rate of hydration The hydration temperature was measured by a semi-adiabatic calorimeter to assess the effect of CNS on the hydration heat of cement pastes. Samples were prepared at a constant w/b ratio of 0.4, with 100 g of blends (including cement, mixing water and CNS) at a temperature of about 27 °C. Cement was replaced by various amount of CNS by mass. Mixes were cast in £5 cm 10 cm plastic cylinders within 3 min after initial cement and water contact. The sample was then covered, placed in the calorimeter, and the temperature of the sample was recorded every 3 min for 20 h. 2.3.3. Morphology Hitachi S-4800 FE-SEM was used to analyze the morphology of the crushed cement paste after the hydration process of the sample was hindered by soaking in acetone. The vacuum oven-dried sample was coated with 20 nm of gold to make it conductive before observation. Image analysis of graphs obtained from backscattered electron microscopy technique (BSE, Hitachi S-3400) was used to evaluate the hydration degree of cement paste. Before testing, thin sample section of approximately 5 mm was cut out of the specimen cast in 2 cm 2 cm 8 cm mold and mounted on a metal sample holder for polishing. The well-polished sample was soaked in acetone for 1 day before been vacuum oven-dried at 50 °C for 1 day. Sample was also made conductive with gold coating. 2.3.4. IR spectra The IR spectroscopic experiments were conducted using a spectrometer (Thermo Nicolet, Nexus 870). The vacuum oven-dried (50 °C for 4 h) samples were mixed with KBr at a sample-to-KBr mass ratio of 1/100 and compressed to give self-supporting thin
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From the supplier
From BET test
2
Specific surface area/m /g
1000
100
10
1 CNS-4nm
CNS-10nm
CNS-20nm
SF
Fig. 1. Particle size distribution and specific surface areas of nanoSiO2 and silica fume.
Fig. 2. Morphology images of CNS and SF.
pastes or mortars were ceased by soaking in acetone for 1 day. Then samples were vacuum oven-dried at 50 °C for three days before testing.
Fig. 3. XRD patterns of CNS and SF.
layer. Samples were scanned 64 times with a resolution of 4.0 cm1. 2.3.5. XRD In this work, XRD tests were conducted on a Rigaku DMAX. The acceleration voltage and acceleration current were 40 kv and 40 mA, respectively. A step size and dwelling time of 0.05 and 2 s were used during the tests. 2.3.6. Pore size distribution Mercury intrusion calorimetry (MIP, Micromeritics, AutoPore 9500 IV) was used to study the pore size distribution of hardened cement pastes and mortars. The hydration process of cement
2.3.7. Water sorptivity Plain cement mortar was cast in molds of dimensions 4 cm 4 cm 16 cm and demolded one day after casting. Samples were cured at 20 °C and 95% RH for 27 days before been sliced into small pieces of dimensions of about 4 cm 4 cm 1 cm with a water-cooled saw and carefully cleaned. Then samples were dried at 60 °C for 24 h to avoid any decomposition of the products before surface treatment by brushing CNS on the surface of the slices for 3 times at a time interval of 20 min, and then the CNS-coated samples were sealed with clear plastic packing tape and cured at 50 °C for 7 days. After that, tape on sample surface was removed and the samples were dried at 60 °C for 48 h before soaking in water. Sample weight under a surface-saturated dry condition was measured on a scale with a resolution of 0.01 g and the water absorption ratio at different soaking times were calculated and results of three replicate samples were averaged and taken as the representative value. 3. Results and discussions 3.1. Pozzolanic reactivity of nanoSiO2
3.1.1. Hydration kinetics To assess the pozzolanic reactivities of the pozzolans, their rates of consuming CH were determined by the TGA technique. The CHconsuming rate of silica fume was also examined to make a
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comparative study. As the reproductivity of the CH test was good, only one measurement was conducted on each sample. It can be seen in Fig. 4(a) that the CH content in the pozzolan-CH blends decreases with age and reaches a constant value that can be regarded as the end point of the pozzolanic reaction. The difference between the final CH content in the blends could be due to the difference of the hydration degree of the pozzolans, which could be attributed to the inherent reactivity and the dispersion state of the nano, the properties of the hydrates as discussed in the following sections, etc. It is also shown in the inserted graph that the finer the pozzolan, the sharper decrease of the CH content at early age, indicating a higher rate of the pozzolanic reaction. In order to quantitatively evaluate the reactivities of the pozzolans, the hydration kinetics parameters of these reactions were determined from the time-dependent curves of CH content in the blends by first assuming the chemical reaction order of the pozzolanic reaction and then verifying the assumption through the analysis of the experimental data. It has been reported that the pozzolanic reaction of pozzolans, such as fly ash and silica fume, was the first-order chemical reaction, i.e. the reaction rate is controlled by the concentration of the reactant [24]. The rate law for a reaction that is first-order with respect to a reactant A is dct(A)/ct(A) = kdt, where k stands for the reaction rate coefficient. The integrated first-order rate law is ln {ct(A)} = kt + {c0(A)}, and a plot of ln {ct(A)} vs. time t gives a straight line with a slope of k and an intercept of ln {c0(A)}. For the liquid–solid reaction in the pozzolan-CH system, the variation of the volume during reaction can be regarded as negligible, and thus the variation of the concentration of the reactant, dct(A), is proportional to the variation of the percentage of reactant that temporarily do not take part in the reaction, but will eventually take part in the reaction, at time t. Assuming the hydration of nanoSiO2 is of the first-order, the linear regression of the natural logarithmic scatter graph of the relevant percentage of unconsumed CH that would eventually involves in the pozzolanic reaction to the total amount of CH taking part in the reaction at time t is shown in Fig. 4(b). The slopes of the regression lines, i.e. the reaction rate constants and the regression coefficients, are listed in Table 2, from which the assumption of the first-order reaction mode is verified. The reaction rate constants shown in Table 2 indicate that the rate constant of CNS-4 nm is about 7–9 times greater than other pozzolans and the rate constants of CNS-10 nm and CNS-20 nm is comparable to that of silica fume. Considering the specific surface area greatly affects the reaction rate, in Table 2, the normalized reaction rate constants (normalized by specific surface area) of all the pozzolans are also shown.
(a)
3.1.2. Morphology and mineralogy The morphology image of the pozzolanic hydration products of nanoSiO2 is shown in Fig. 5, and it is compared with that of silica fume. It shows that the hydrates of nanoSiO2 are more compact and featureless, while the microstructure of the hydrates of SF is more loose and porous. The compact structure of the hydrates of nanoSiO2 contributes to a densified microstructure and improved durability of cement-based materials [8]. XRD patterns of the pozzolanic hydration products of CNS and silica fume are depicted in Fig. 6. The comparable peak intensity of CH at 2 theta angle of ca. 28.5° implies a similar amount of CH consumed by the two pozzolans. By comparing the intensity and width of the diffuse peaks at 2 theta angle range of ca. 29– 30°, it can be deduced that the C–S–H gel formed in CNS–CH system has a lower crystallinity. It has been well-documented that the crystallinity of the hydration products has a great influence on the mechanical property of cement-based materials and a suitable ratio of the crystals to the noncrystals is desired to yield a higher mechanical property [27]. Given this, an optimal dosage of nanoSiO2 can achieve a proper crystal-to-noncrystal ratio in nanoSiO2-added cement so as to acquire a higher compressive strength, and this could be ascribed to the fact that a high dosage of nanoSiO2 is detrimental to the compressive strength gain of cement-based materials as many researchers reported.
(b) CNS-4nm CNS-10nm CNS-20nm SF
100
0
110
90
80
70
60
80
50
40
30
60
CNS-4nm CNS-10nm CNS-20nm SF
100
ln[Percentage(CHunreacted)
120
CH unconsumed/%
It should be buried in mind that the specific surface area of the pozzolan dispersed in cement pastes is just an estimated value that based on the assumption that pozzolans are well dispersed, which, however, is not always the case. Thus the normalized reaction rate constants can only act as an indicator showing the real reactivity of the pozzolans. It shows that CNS-4 nm has a relatively higher reactivity than those of CNS-10 nm and CNS-20 nm and the latter two have comparable normalized reaction rate constant. It is interesting to note that silica fume has the highest normalized reaction rate constant, which is about 2.4 times of that of CNS-4 nm, and this could be attributed to the differences in the forming process of the pozzolans. For silica fume, a sharp cooling of the melt of the SiO2-containing byproduct of the production of ferrosilicon and silicon metals may introduce a higher reactivity, and this could also be one of the reasons why silica fume has a greater hydration acceleration effect than nanoSiO2 as many researchers reported [25,26]. It can thus be deduced from this study that both chemical composition and particle size are crucial factors that govern the properties of high-Si supplementary materials.
20 0.1
1
10
100
40
2 4 6 8 10 12
20 0
500
1000
1500
Time/hrs
2000
2500
0
200
400
600
Time/hrs
Fig. 4. CH consuming capacity (a) and the linear regression of pozzolanic reaction functions (b) of CNS/SF.
800
1000
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Table 2 Pozzolanic reaction rate constants of nanoSiO2 and silica fume. Pozzolans 3
Apparent reaction rate constant/k (10 ) Regression coefficient/R2 Normalized reaction rate constant (g/m2, 103)
CNS-4 nm
CNS-10 nm
CNS-20 nm
Silica fume
73.0 0.98 0.162
7.6 0.96 0.038
8.0 0.99 0.059
6.3 0.99 0.315
Fig. 5. Morphology of pozzolanic hydration products of CNS-4 nm and silica fume (CNS/SF:CH = 5:20; w/b = 2.0, 4 months old).
3.2. Influences of nanoSiO2 on properties of cement The influences of nanoSiO2 on the properties of cement were determined by assessments of the hydration rate at ages and the hydration extents of cement at both early and later ages. 3.2.1. Rate of hydration at early age The hydration acceleration effect of nanoSiO2 on cement, i.e. the seeding effect, was demonstrated by the hydration temperature evolution of cement pastes with addition of nanoSiO2 of various dosages and sizes, as shown in Fig. 8. It shows that a quicker and higher temperature peak formation occurs when nanoSiO2 is added, and the increase of the dosage or the fineness of nanoSiO2 results in a greater hydration acceleration effect. A similar shape of the temperature–time curve of paste with 5% nanoSiO2 to that of paste with 1% implies that a greater dosage of CNS-10 nm than 1% contributes little to the hydration acceleration of cement.
Fig. 6. XRD patterns of hydration products of silica fume and CNS (CNS/ SF:CH = 5:20; w/b = 2.0, 4 months old).
3.1.3. Silicate anion structure The infrared spectroscopy technique was used to evaluate the evolution of the silicate anion structure of the pozzolanic hydration products of nanoSiO2. The characteristic absorbance peaks at ca. 1100 cm1 and 980 cm1 represent the amorphous silicate of the pozzolans and their hydration products of middle groups of silicate tetrahedra, respectively [28]. It can be seen in Fig. 7 that at 7 days the peak intensity ratio of the amorphous silicate to the middle group silicate tetrahedra changes with the average particle size of the pozzolans, and the finer the pozzolan, the lesser the value, which indicates a higher reaction rate of the pozzolan. At 4 months old, the IR curve configurations of nanoSiO2 and silica fume are almost the same, indicating a similar silicate anion structure of the two pozzolans at later ages, while the polymerization degree of them are both higher than that of the plain cement paste [16,29,30].
3.2.2. Degree of hydration of cement at later ages Backscattered electron microscopy (BSE) technique under the automatic number/composition contrast mold was used to measure the degree of cement hydration. By measuring the area of unhydrates (bright spot in BSE images) to that of the whole image area, the hydration degree of cement was traced in Ref. [31]. Considering that areas taken by pores in BSE images would bring errors to the calculation, especially at early ages, pore volume/area of the pastes as detected by MIP was excluded of the calculation. It should also be noted that BES images reflect the average atomatic number contrast of the constituents and the accuracy of the calculation would depend on the selection of the gray value threshold and a weak contrast in early age sample due to a smaller water inclusion into the hydrates would bring higher errors, thus only later age hydration degree was measured through this technique. Typical BSE images of hardened cement pastes without and with 5% nanoSiO2-4 nm at 8 months are shown in Fig. 9 and the hydration degree of cement, which is the average value of five BSE images, is also depicted in Fig. 9. To accurately evaluate the hydration degree, the smallest magnification as 100 was used to obtain the biggest image that testing equipment could provide.
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Fig. 7. IR spectra of the pozzolanic hydration products of nanoSiO2 and silica fume (CNS/SF:CH = 5:20; w/b = 2.0).
Fig. 8. Influences of nanoSiO2 on the hydration temperature of cement pastes.
It shows that hydration degrees of nanoSiO2-modified cement paste is smaller than the control paste at 8 months, indicating a hindrance effect of nanoSiO2 on cement hydration at later ages. At 8 months old, the pore structures of the CNS-add and control sample are comparable and thus the hindrance effect is very evident at this time. Considering the hydration of cement at later age is controlled by the ion diffusion ability through hydrates [32], a compact gel structure of the pozzolanic hydration products of nanoSiO2 would result in the block of the diffusion and thus decreases the hydration degree of cement and finally the slow down of the strength gain [33–34]. 3.2.3. Pore size distribution The pore size distribution of pastes before and after an addition of 5% nanoSiO2-4 nm at 3.5 and 9 months were studied by using MIP technique and the results are shown in Fig. 10. It demonstrates that the pore structure of cement paste is densified by nanoSiO2 at 3.5 months and this complies well with other studies [35]. It also shows that the pore size distribution curve of nanoSiO2-modified paste is comparable to that of control paste at 9 months old, and the pore volume of nanoSiO2-modified paste is even greater than that of the control paste in pore size range smaller than 10 nm. This result is consistent with results reported by Thomas and Jennings, who claimed a more porous structure of nanoSiO2-added C3S paste of 3 months old [33]. And this could be due to the compact gel structure of the hydration products of CNS, which blocks
diffusion of unhydrates and results in a more porous bulk structure. It is also known that pores smaller than 10 nm are also present in C–S–H gel and additional C–S–H gel introduced by pozzolanic reaction of nanoSiO2 may result in more gel pores. However, from Fig. 3 we can see that the pozzolanic reaction of CNS-4 nm ceases after about 100 h hydration, thus the hydration blocking effect would be the main cause. 4. Application of the pozzolanic reactivity of CNS on surface treatment of hardened cement-based materials The influences of fine pozzolans on the properties of cementbased materials have been intensively investigated and three effects, i.e. the pozzolanic reactivity, the filling and seeding effects, were often referred to when interpreting their effects. A detailed investigation of the pozzolanic reactivity of nanoSiO2 reveals a quicker hydration rate, a greater apparent reactivity, and a more amorphous and compact microstructure of hydrates than those of silica fume. Although these features greatly improves the mechanical properties of cementitious materials at early ages, they could also be detrimental to the property gain at later ages due to the block of ion diffusion through these compact gel. From the work mentioned above, it can be seen that an ultrafine particle size, a high pozzolanic reactivity and a compact hydrate structure of nanoSiO2 are the primary features that should be considered when it is applied in cement-based materials. Considering
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0CNS-8months
5CNS-8months
110
95.8%
Degree of hydration
100
85.9%
90 80 70 60 50 40 30 20 0 CNS
5 CNS
Fig. 9. Influences of CNS-4 nm on the hydration degree of cement at 8 months (note: the pore volume of cement pastes without and with 5% CNS at 9 months of 23.6% and 24.4% were used in the calculation).
18
18 16 0% CNS 3.5months 5% CNS 3.5months
2
14
Cumulative pore area (m /g)
2
Cumulative pore area (m /g)
16
12 10 8 6 4 2 0 100
10
Pore size/nm
14
0% CNS 9months 5% CNS 9months
12 10 8 6 4 2 0 100
10
Pore size/nm
Fig. 10. Influence of nanoSiO2-4 nm on the pore size distribution of cement pastes at 3.5 and 9 months (from MIP tests).
a denser surface structure of cement-based material would be favorable for a more durable concrete, the influences of nanoSiO2 on the surface-treatment of hardened cement mortar was investigated. Fig. 11 depicts the effects of CNS-10 nm on the water sorptivity of hardened cement mortar after treatment procedure described in Section 2.3.7. It shows that after CNS is applied on the surface of hardened mortar, the water absorption ratios of mortar at 1.5 h and 22 h are reduced by 26% and 14.2%, respectively. IR spectroscopy technique was used to verify the presence of the hydration products of CNS on the surface of CNS-treated mortar. Powder (including unhydrated nanoSiO2 and newly formed C–S–H gel) on the surface of hardened cement mortar that
was treated with CNS through brushing technique and cured at 50 °C/95% RH for 7 days was carefully scraped and collected without disturbing the hardened cement mortar. It shows in Fig. 12 that the middle group silicate tetrahedral, Q2, although very small, which could be due to the small percentage of the hydrates in the sample collected or a small hydration degree of CNS, appears when compared with the pure CNS, indicating the occurrence of the pozzolanic reaction of nanoSiO2 on the surface of hardened cement mortar, and this theory has also been reported by Cardenas [36] to show the capability of using nanoparticle of sealing the surface of cement-based materials. Surface-sealing effect of CNS is also demonstrated by the pore size distribution analysis of cement
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100
Control CNS(brushing)
Percentage/%
80
60
40
20
0 100 Fig. 11. Water sorptivity of mortar after surface treatment with nanoSiO2 (w/ c = 0.6).
10
1
0.1
Pore size/nm Fig. 13. Pore size distribution of CNS-treated mortar (samples after water sorptivity measurement in Fig. 10 were used).
effective in utilizing the characteristics of nanoSiO2 in the surface-treatment [38]. 5. Conclusions In this work, investigations of the characteristics of pozzolanic reactivity of nanoSiO2 and its influences on the properties of cement at both early and later ages were made and possibility for its application on the surface treatment of cement-based material based on the recognition of the pozzolanic reactivity was shown. The following conclusions can be made:
Fig. 12. IR spectrum of the hydration products on the surface of CNS-treated mortar (28 days old, sample were CNS treated for 3 times and then cured at 50 °C/95% RH for 7 days) [38].
mortars that are used in water sorptivity test in Fig. 11. It is shown in Fig. 13 that the percentage of coarse pores in the surface of hardened cement mortar decreased by brushing CNS. Recently, we detaily studied the pore-filling effect of CNS on hardened cement pastes and it was found effective in sealing pores larger than 50 nm [37]. Results in Figs. 11–13 show the potential of using nanoSiO2 on surface treatment of cement-based material by exploring its pozzolanic reactivity, through which the surface becomes denser and less permeable. Since the surface part of cement-based material is exposed to a different environment than the underlying bulk concrete, more work is needed, including exploring ways of improving the hydration reaction degree of CNS and of increasing the penetration of nanoSiO2 into cement/concrete. Recently, we studied the effectiveness of using the precursor of nanoSiO2, tetraethoxysilane, TEOS, of treating hardened cement paste/mortar/ concrete, and by taking the advantage of the easy penetration of TEOS into pores and the in-situ hydrolysis of TEOS into nanoSiO2, obvious pozzolanic reaction was found, which proved to be more
(1) The pozzolanic reaction of micro/nanoSiO2 follows the firstorder chemical reaction mold and its reaction rate constant is related to the particle size and its production technique. (2) The pozzolanic reaction products of nanoSiO2 are more compact than those of silica fume. (3) Although the hydration process of cement at early age is significantly increased by nanoSiO2, its later age hydration rate is slowed down due to the compact structure of the pozzolanic hydration products of nanoSiO2. (4) NanoSiO2 can make the hardened cement mortar less water-absorbable by exploring its high pozzolanic reactivity or filler effect on the surface of mortar, which is worth of further investigation.
Acknowledgements The work is supported by Program for Scientific Research Innovation Team in Colleges and Universities of Shandong Province and Natural Science Foundation of China (Grant No. 51302105), which is greatly acknowledged. References [1] Collepardi M, Collepardi S, Skarp U, et al. Optimization of silica fume, fly ash and amorphous nano-silica in superplasticized high-performance concrete. ACI Mater J 2004;221:495–506. [2] Kawashima S, Kim JH, Corr DJ, et al. Study of the mechanisms underlying the fresh-state response of cementitious materials modified with nanoclays. Constr Build Mater 2012;36:749–57.
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[3] Gaitero JJ, Campillo I, Guerrero A. Reduction of the calcium leaching rate of cement paste by addition of silica nanoparticles. Cem Concr Res 2008;38(8– 9):1112–8. [4] Liu ZG, Yang RL, Wei JC, Zhao BH, et al. Piezoresistive properties of cement mortar with carbon nanotube. Adv Mater Res 2011;284–286(7):310–3. [5] Konsta-Gdoutos MS, Metaxa ZS, Shah SP. Highly dispersed carbon nanotube reinforced cement based materials. Cem Concr Res 2010;40(7):1052–9. [6] Folli A, Pade C, Hansen T, et al. TiO2 photocatalysis in cementitious systems: insights into self-cleaning and depollution chemistry. Cem Concr Res 2012;42(3):539–48. [7] Singh LP, Karade SR, Bhattacharyya SK. Beneficial role of nanosilica in cement based materials – a review. Constr Build Mater 2013;47:1069–77. [8] Ji T. Preliminary study on the water permeability and microstructure of concrete incorporating nano-SiO2. Cem Concr Res 2005;35:1943–7. [9] Ye Q, Zhang Z, Kong D. Influence of nano-SiO2 addition on properties of hardened cement paste as compared with silica fume. Constr Build Mater 2007;21(3):539–45. [10] Jo B, Kim C, Lim J. Characteristics of cement mortar with nano-SiO2 particles. Constr Build Mater 2007;21(6):1351–5. [11] Zhang M, Islam J. Use of nano-silica to reduce setting time and increase early strength of concretes with high volumes of fly ash or slag. Constr Build Mater 2012;29:573–80. [12] Zhang M, Islam J, Peethamparan S. Use of nano-silica to increase early strength and reduce setting time of concretes with high volumes of slag. Cem Concr Compos 2012;34:650–62. [13] Gundogdu D, Pekmezci BY, Atahan HN. Influence of nanosilica on the mechanical properties of mortars containing fly ash. In: Brameshuber E, editor. Proceedings of the internal RILEM conference on materials science, vol. 3; 2010. p. 345–54. [14] Sobolev K, Flores I, Hermosillo R, Torres-Martinez LM. Nanomaterials and nanotechnology for high-performance cement composites. Nanotechnology of concrete: recent developments and future perspectives; 2010. p. 93–120. [15] Nazari A, Riahi S. The effects of SiO2 nanoparticles on physical and mechanical properties of high strength compacting concrete. Compos Part B Eng 2010;42(3):570–8. [16] Hou P, Kawashima S, Kong D, et al. Modification effects of colloidal nanoSiO2 on cement hydration and its gel property. Compos Part B Eng 2013;45(1):440–8. [17] Hou P, Kawashima S, Wang K, Corr D, et al. Effects of colloidal nanosilica on rheological and mechanical properties of fly Ash-cement mortar. Cem Concr Compos 2013;35(1):12–22. [18] Berra M, Carassiti F, Mangialardi T, et al. Effects of nanosilica addition on workability and compressive strength of Portland cement pastes. Constr Build Mater 2012;35:666–75. [19] Nazari A, Riahi S. The role of SiO2 nanoparticles and ground granulated blast furnace slag admixtures on physical, thermal and mechanical properties of self compacting concrete. Mater Sci Eng A 2011;528(4–5):2149–57.
[20] Hou P, Wang K, Qian J, et al. Effects of colloidal nanoSiO2 on fly ash hydration. Cem Concr Compos 2012;34(10):1095–103. [21] Lam L, Yong YL, Poon CS. Degree of hydration and gel/space ratio of highvolume fly ash/cement systems. Cem Concr Res 2010;34:1541–7. [22] Ramachandran RS. Differential thermal method of estimating calcium hydroxide in calcium silicate and cement pastes. Cem Concr Res 1979;9(6):677–84. [23] Franke L, Sisomphon K. A new chemical method for analyzing free calcium hydroxide content in cementing material. Cem Concr Res 2004;34(7):1161–5. [24] Song Y, Qian J, Wang Z. Pozzolanic reactivity of coal ashes. J Chin Ceram Soc 2006;34(8):962–5. [25] Jo BW, Kim CH, Tae G, et al. Characteristics of cement mortar with nano-SiO2 particles. Constr Build Mater 2007;21(6):1351–5. [26] Thomas JJ, Jennings HM, Chen JJ. Influence of nucleation seeding on the hydration mechanisms of tricalcium silicate and cement. J Phys Chem 2009;113(11):4327–34. [27] Feldman RF, Beaudoin JJ. Microstructure and strength of hydrated cement. Cem Concr Res 1976;6:389–400. [28] Lilkov V, Dimitrova E, Petrov OE. Hydration process of cement containing fly ash and silica fume: the first 24 hours. Cem Concr Res 1997;27(4):577–88. [29] Huang C, Feldman RF. Hydration reactions in Portland cement-silica fume blends. Cem Concr Res 1985;15(4):585–92. [30] Kim JJ, Rahman MK, Al-Majed AA, et al. Nanosilica effects on composition and silicate polymerization in hardened cement paste cured under high temperature and pressure. Cem Concr Compos 2013;43:78–85. [31] Scrivener K, Nonat A. Hydration of cementitious materials, present and future. Cem Concr Res 2011;41:651–65. [32] Bullard J, Jennings HM, Livingston RA, et al. Mechanisms of cement hydration. Cem Concr Res 2011;41(12):1208–23. [33] Thomas J, Jennings H. The nanostructure of Low-CO2 concrete for a sustainable infrastructure. Report on year 1 activities and recommendations for future work. A scientific collaboration between Lafarge Center for Research (LCR) and Northwestern University (NWU); 2010. [34] Verbeck GJ, Helmuth R. Structures and physical properties of cement paste. In: Proceedings of the fifth intonational symposium on the chemistry of cement, Part III, Properties of cement paste and concrete, Tokyo, vol. III; 1968. p. 1–32. [35] Choolaei M, Morad Rashidi A, Ardjmand M. The effect of nanosilica on the physical properties of oil well cement. Mater Sci Eng A 2012;538(15):288–94. [36] Cardenas HE. Nanomaterials in concrete: advances in protection, repair, and upgrade. DEStech Publications; 2012. [37] Hou P, Cheng X, Qian J, et al. Characteristics of Surface-treatment of NanoSiO2 on the transport properties of hardened cement pastes with different waterto-cement ratios. Cem Concr Compos 2015;55:26–33. [38] Hou P, Cheng X, Qian J, et al. Effects and mechanisms of surface treatment of hardened cement-based materials with colloidal nanoSiO2 and its precursor. Constr Build Mater 2014;53:66–73.
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Cement & Concrete Composites journal homepage: www.elsevier.com/locate/cemconcomp
Effects of the pozzolanic reactivity of nanoSiO2 on cement-based materials Pengkun Hou a,b, Jueshi Qian c,⇑, Xin Cheng a,b, Surendra P. Shah d a
School of Materials Science and Engineering, University of Jinan, Jinan, Shandong 250022, China Shandong Provincial Key Laboratory of Preparation and Measurement of Building Materials, Jinan, Shandong 250022, China c College of Materials Science and Engineering, Chongqing University, Chongqing 400045, China d Department of Civil and Environmental Engineering, Northwestern University, Evanston, IL 60208, USA b
a r t i c l e
i n f o
Article history: Received 7 August 2013 Received in revised form 5 May 2014 Accepted 21 September 2014 Available online 6 October 2014 Keywords: NanoSiO2 Pozzolanic reactivity Surface-treatment
a b s t r a c t The aim of this work is to understand the characteristics of the pozzolanic reactivity of nanoSiO2 from studies of its pozzolanic reaction kinetics, morphology and structure of the hydrates and the influences of these features on the properties of cement-based materials, so as to explore a more targeted way of using nanoSiO2 in cement or concrete. It revealed that the pozzolanic reaction of nanoSiO2 is of the first-order and the apparent reaction rate constant of nanoSiO2-4 nm is about one order of magnitude bigger than that of silica fume, but the specific reaction rate constant is about one half to that of silica fume. A compacter gel structure and poorer crystallinity of the hydrates of nanoSiO2 to those of silica fume are found, as well. The rate of hydration of cement at very early ages is enhanced by nanoSiO2, but the rate slows down with aging due to the compact gel structure. To make the use of the high pozzolanic reactivity and ultrafine particle size of nanoSiO2, as well as its resulting compact gel structure, colloidal nanoSiO2 was applied onto the hardened cement mortar by brushing technique and a less permeable surface was resulted, which shows the potential of using nanoSiO2 as a surface treatment material for cement-based materials. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction The application of nanotechnology in cement and concrete has attracted much attention in recent years. It is being gradually accepted that by adding a portion of nanoparticle, even at a very small dosage, the properties of cement-based material can be enhanced to a great extent in respects of workability, strength gain and durability [1–3]. And novel functional properties, such as photocatalicity property introduced by nanoTiO2, piezoelectricity by nanotube, can be obtained, as well [4–6]. Among all the nanopartilces, nanoSiO2 has been most widely investigated. NanoSiO2 is often produced by the sol–gel method by the hydrolysis process of trimethylethoxysilane or tetraethoxysilane, and its particle size is often smaller than 100 nm. Due to its pozzolanic reactivity and ultrafine particle size, it was reported that nanoSiO2 improved the compressive strength of cement-based materials significantly, and made the microstructure denser as well [7–10]. Attempts have been made to use nanoSiO2 to prepare HPC or low-carbon cementitious materials [11,12]. ⇑ Corresponding author. Tel.: +86 23 65126109. E-mail address: [email protected] (J. Qian). http://dx.doi.org/10.1016/j.cemconcomp.2014.09.014 0958-9465/Ó 2014 Elsevier Ltd. All rights reserved.
Although it has been well-documented that nanoSiO2 introduced a lot of benefits to cement-based materials, some other researches demonstrated that the property-enhancing characteristics of nanoSiO2 was correlated to the curing age: the rate of strength gain of nanoSiO2-modified system slowed down at later ages, which became smaller than that of the blank sample three months later [13–17]. There are many studies showing a smaller compressive strength of nanoSiO2-modified cement-based materials, and almost all the phenomena were attributed the poor dispersion of nanoSiO2 in the blends [18]. More recently, Nazari claimed that the decreased crystalline Ca(OH)2 content required for C–S–H gel formation could be the reason [19]. Considering nanoSiO2 has a greater reactivity than the conventional pozzolans, its hydration characteristics and the gel properties may be accounted for property-gain of cement-based materials, which is short of investigation thus far [20]. In this paper, study on the pozzolanic reactivity of nanoSiO2 and its influences on the properties of cement-based materials will be conducted. After that, advantages of utilizing the pozzolanic reactivity and particle size feature of nanoSiO2 in surface treatment of hardened cement mortar will be shown. We hope that a clearer recognition of the pozzolanic reactivity of nanoSiO2 and its
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influences on cement-based materials can be made and its advantages can be better explored. 2. Experimental 2.1. Materials A Type I Portland Cement with a 28-day compressive strength of 45.1 MPa was used in this study. The bulk compositions of the cement are shown in Table 1. Commercially available colloidal nanosilica (CNS), other than nanosilica powder, was used to achieve a homogeneous dispersion in cement paste. CNSes with average particle sizes of 4 nm, 10 nm and 20 nm produced by the sol–gel technique and stabilized by sodium were used in this study [14]. Eq. (1) can be used to summarize the synthesis process of nanosilica, i.e. the hydrolysis of the precursor of TEOS into Si5O4(OH)12 and finally SiO2 in acidic or alkaline environment. In this work, nanosilica used was synthesized in alkaline environment as reported by the provider. The purity of the CNSes were higher than 99%. Viscosity measured by rheometer showed that all water and CNSes showed comparable viscosity of about 10 Pa S. Considering the content of the sodium introduced into cement-based material is greatly less than the inherent sodium content in cement (alkali limit in cement is about 0.6%), its influence on cement hydration was ignored. A silica fume (SF) with the particle size ranged from 100 nm to 2000 nm was used for a comparison study (chemical compositions of SF are shown in Table 1). Particle size distributions of CNSes and SF are depicted in Fig. 1, from which it shows that all CNSes exhibit a single dispersion peak, illustrating a well dispersion of the materials. While for SF, a double-peak distribution curve is seen. Morphology images of CNS and SF (as shown in Fig. 2) reveal that both pozzolans are round in shape for individual particles, however, some clusters were seen. Considering the specific surface area of the pozzolan plays an important role in its reactivity, BET technique was used to obtain this value. However, as BET test can only be conducted on solid powder, it is impossible to obtain such value on colloidal nanoSiO2 sample. In this work, specific surface area of the SF was measured by BET technique and the specific surface areas is shown in Fig. 1, in which the specific surface area value of CNSes provided by the supplier are also presented. Considering the characteristic particle size of CNS as shown in the particle size distribution curve equals to the value provided by the supplier well, specific surface area of CNS as provided by the supplier was used during the following discussions. XRD patterns (as shown in Fig. 3) indicate the poor-crystallinity of both pozzolans. A similar full width at half maximum indicates that the two materials have a comparable crystallinity.
nSiðOC2 H5 Þ4 þ 2nH2 O ! nSiO2 þ 4nC2 H5 OH
ð1Þ
Table 1 Physiochemical compositions of cement and silica fume. Materials
Type I cement
Silica fume
SiO2 Al2O3 Fe2O3 SO3 CaO MgO LOI Total Density, g/cm3 Fineness as surface area, m2/kg
20.2 4.7 3.3 3.3 62.9 2.7 1.1 98.2 3.1 380
90.1 0.6 2.0 – 0.5 5.1 1.0 99.3 2.2 21,000
251
2.2. Sample preparation To explore the pozzolanic reactivity of the pozzolans, 20 g of chemical grade Ca(OH)2 (CH) were mixed with 5 g of CNS/SF at a water-to-binder ratio (w/b) of 2.0 to simulate a cement-CNS/SF system. It was assumed that 20 g of CH can be generated by 100 g of cement [21]. After mixing, samples were sealed in plastic vials and the CH content at different ages were determined by the TGA technique. During sampling, core parts of the sample were selected and a quick sampling process was conducted to ensure a less carbonization and a good reproductivity of the test. Unless otherwise stated, blends were mixed at the highest speed of the mixer for five minutes to achieve a homogeneous dispersion of the particles. Unless otherwise stated, cement pastes mixed with and without 5% CNS/SF at a w/b ratio of 0.4 were prepared and investigated throughout this study. Samples were demolded 1 day after casting and cured in saturated lime solution at about 23 °C until testing. Plain cement mortar of a w/c ratio of 0.6 and a sand/cement ratio of 3.0 was used to evaluate the surface-sealing effect of CNS by measuring the water absorption ratio of the mortars before and after CNS treatment applied by using brushing technique. 2.3. Test methods 2.3.1. CH content Thermogravimetric analysis (TGA, METTLER, TGA/sDTA 851) was carried out to measure the CH content of the CH-CNS/SF blends. Samples were heated in nitrogen atmosphere from 50 °C to 950 °C at a heating rate of 15 °C/min. The weight loss between 440 °C and 510 °C was considered to be the decomposition of CH crystal into lime [22,23]. Before measuring, powder samples were vacuum oven-dried at 105 °C for four hours. CH content was calculated based on the ignited sample. 2.3.2. Rate of hydration The hydration temperature was measured by a semi-adiabatic calorimeter to assess the effect of CNS on the hydration heat of cement pastes. Samples were prepared at a constant w/b ratio of 0.4, with 100 g of blends (including cement, mixing water and CNS) at a temperature of about 27 °C. Cement was replaced by various amount of CNS by mass. Mixes were cast in £5 cm 10 cm plastic cylinders within 3 min after initial cement and water contact. The sample was then covered, placed in the calorimeter, and the temperature of the sample was recorded every 3 min for 20 h. 2.3.3. Morphology Hitachi S-4800 FE-SEM was used to analyze the morphology of the crushed cement paste after the hydration process of the sample was hindered by soaking in acetone. The vacuum oven-dried sample was coated with 20 nm of gold to make it conductive before observation. Image analysis of graphs obtained from backscattered electron microscopy technique (BSE, Hitachi S-3400) was used to evaluate the hydration degree of cement paste. Before testing, thin sample section of approximately 5 mm was cut out of the specimen cast in 2 cm 2 cm 8 cm mold and mounted on a metal sample holder for polishing. The well-polished sample was soaked in acetone for 1 day before been vacuum oven-dried at 50 °C for 1 day. Sample was also made conductive with gold coating. 2.3.4. IR spectra The IR spectroscopic experiments were conducted using a spectrometer (Thermo Nicolet, Nexus 870). The vacuum oven-dried (50 °C for 4 h) samples were mixed with KBr at a sample-to-KBr mass ratio of 1/100 and compressed to give self-supporting thin
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From the supplier
From BET test
2
Specific surface area/m /g
1000
100
10
1 CNS-4nm
CNS-10nm
CNS-20nm
SF
Fig. 1. Particle size distribution and specific surface areas of nanoSiO2 and silica fume.
Fig. 2. Morphology images of CNS and SF.
pastes or mortars were ceased by soaking in acetone for 1 day. Then samples were vacuum oven-dried at 50 °C for three days before testing.
Fig. 3. XRD patterns of CNS and SF.
layer. Samples were scanned 64 times with a resolution of 4.0 cm1. 2.3.5. XRD In this work, XRD tests were conducted on a Rigaku DMAX. The acceleration voltage and acceleration current were 40 kv and 40 mA, respectively. A step size and dwelling time of 0.05 and 2 s were used during the tests. 2.3.6. Pore size distribution Mercury intrusion calorimetry (MIP, Micromeritics, AutoPore 9500 IV) was used to study the pore size distribution of hardened cement pastes and mortars. The hydration process of cement
2.3.7. Water sorptivity Plain cement mortar was cast in molds of dimensions 4 cm 4 cm 16 cm and demolded one day after casting. Samples were cured at 20 °C and 95% RH for 27 days before been sliced into small pieces of dimensions of about 4 cm 4 cm 1 cm with a water-cooled saw and carefully cleaned. Then samples were dried at 60 °C for 24 h to avoid any decomposition of the products before surface treatment by brushing CNS on the surface of the slices for 3 times at a time interval of 20 min, and then the CNS-coated samples were sealed with clear plastic packing tape and cured at 50 °C for 7 days. After that, tape on sample surface was removed and the samples were dried at 60 °C for 48 h before soaking in water. Sample weight under a surface-saturated dry condition was measured on a scale with a resolution of 0.01 g and the water absorption ratio at different soaking times were calculated and results of three replicate samples were averaged and taken as the representative value. 3. Results and discussions 3.1. Pozzolanic reactivity of nanoSiO2
3.1.1. Hydration kinetics To assess the pozzolanic reactivities of the pozzolans, their rates of consuming CH were determined by the TGA technique. The CHconsuming rate of silica fume was also examined to make a
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comparative study. As the reproductivity of the CH test was good, only one measurement was conducted on each sample. It can be seen in Fig. 4(a) that the CH content in the pozzolan-CH blends decreases with age and reaches a constant value that can be regarded as the end point of the pozzolanic reaction. The difference between the final CH content in the blends could be due to the difference of the hydration degree of the pozzolans, which could be attributed to the inherent reactivity and the dispersion state of the nano, the properties of the hydrates as discussed in the following sections, etc. It is also shown in the inserted graph that the finer the pozzolan, the sharper decrease of the CH content at early age, indicating a higher rate of the pozzolanic reaction. In order to quantitatively evaluate the reactivities of the pozzolans, the hydration kinetics parameters of these reactions were determined from the time-dependent curves of CH content in the blends by first assuming the chemical reaction order of the pozzolanic reaction and then verifying the assumption through the analysis of the experimental data. It has been reported that the pozzolanic reaction of pozzolans, such as fly ash and silica fume, was the first-order chemical reaction, i.e. the reaction rate is controlled by the concentration of the reactant [24]. The rate law for a reaction that is first-order with respect to a reactant A is dct(A)/ct(A) = kdt, where k stands for the reaction rate coefficient. The integrated first-order rate law is ln {ct(A)} = kt + {c0(A)}, and a plot of ln {ct(A)} vs. time t gives a straight line with a slope of k and an intercept of ln {c0(A)}. For the liquid–solid reaction in the pozzolan-CH system, the variation of the volume during reaction can be regarded as negligible, and thus the variation of the concentration of the reactant, dct(A), is proportional to the variation of the percentage of reactant that temporarily do not take part in the reaction, but will eventually take part in the reaction, at time t. Assuming the hydration of nanoSiO2 is of the first-order, the linear regression of the natural logarithmic scatter graph of the relevant percentage of unconsumed CH that would eventually involves in the pozzolanic reaction to the total amount of CH taking part in the reaction at time t is shown in Fig. 4(b). The slopes of the regression lines, i.e. the reaction rate constants and the regression coefficients, are listed in Table 2, from which the assumption of the first-order reaction mode is verified. The reaction rate constants shown in Table 2 indicate that the rate constant of CNS-4 nm is about 7–9 times greater than other pozzolans and the rate constants of CNS-10 nm and CNS-20 nm is comparable to that of silica fume. Considering the specific surface area greatly affects the reaction rate, in Table 2, the normalized reaction rate constants (normalized by specific surface area) of all the pozzolans are also shown.
(a)
3.1.2. Morphology and mineralogy The morphology image of the pozzolanic hydration products of nanoSiO2 is shown in Fig. 5, and it is compared with that of silica fume. It shows that the hydrates of nanoSiO2 are more compact and featureless, while the microstructure of the hydrates of SF is more loose and porous. The compact structure of the hydrates of nanoSiO2 contributes to a densified microstructure and improved durability of cement-based materials [8]. XRD patterns of the pozzolanic hydration products of CNS and silica fume are depicted in Fig. 6. The comparable peak intensity of CH at 2 theta angle of ca. 28.5° implies a similar amount of CH consumed by the two pozzolans. By comparing the intensity and width of the diffuse peaks at 2 theta angle range of ca. 29– 30°, it can be deduced that the C–S–H gel formed in CNS–CH system has a lower crystallinity. It has been well-documented that the crystallinity of the hydration products has a great influence on the mechanical property of cement-based materials and a suitable ratio of the crystals to the noncrystals is desired to yield a higher mechanical property [27]. Given this, an optimal dosage of nanoSiO2 can achieve a proper crystal-to-noncrystal ratio in nanoSiO2-added cement so as to acquire a higher compressive strength, and this could be ascribed to the fact that a high dosage of nanoSiO2 is detrimental to the compressive strength gain of cement-based materials as many researchers reported.
(b) CNS-4nm CNS-10nm CNS-20nm SF
100
0
110
90
80
70
60
80
50
40
30
60
CNS-4nm CNS-10nm CNS-20nm SF
100
ln[Percentage(CHunreacted)
120
CH unconsumed/%
It should be buried in mind that the specific surface area of the pozzolan dispersed in cement pastes is just an estimated value that based on the assumption that pozzolans are well dispersed, which, however, is not always the case. Thus the normalized reaction rate constants can only act as an indicator showing the real reactivity of the pozzolans. It shows that CNS-4 nm has a relatively higher reactivity than those of CNS-10 nm and CNS-20 nm and the latter two have comparable normalized reaction rate constant. It is interesting to note that silica fume has the highest normalized reaction rate constant, which is about 2.4 times of that of CNS-4 nm, and this could be attributed to the differences in the forming process of the pozzolans. For silica fume, a sharp cooling of the melt of the SiO2-containing byproduct of the production of ferrosilicon and silicon metals may introduce a higher reactivity, and this could also be one of the reasons why silica fume has a greater hydration acceleration effect than nanoSiO2 as many researchers reported [25,26]. It can thus be deduced from this study that both chemical composition and particle size are crucial factors that govern the properties of high-Si supplementary materials.
20 0.1
1
10
100
40
2 4 6 8 10 12
20 0
500
1000
1500
Time/hrs
2000
2500
0
200
400
600
Time/hrs
Fig. 4. CH consuming capacity (a) and the linear regression of pozzolanic reaction functions (b) of CNS/SF.
800
1000
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Table 2 Pozzolanic reaction rate constants of nanoSiO2 and silica fume. Pozzolans 3
Apparent reaction rate constant/k (10 ) Regression coefficient/R2 Normalized reaction rate constant (g/m2, 103)
CNS-4 nm
CNS-10 nm
CNS-20 nm
Silica fume
73.0 0.98 0.162
7.6 0.96 0.038
8.0 0.99 0.059
6.3 0.99 0.315
Fig. 5. Morphology of pozzolanic hydration products of CNS-4 nm and silica fume (CNS/SF:CH = 5:20; w/b = 2.0, 4 months old).
3.2. Influences of nanoSiO2 on properties of cement The influences of nanoSiO2 on the properties of cement were determined by assessments of the hydration rate at ages and the hydration extents of cement at both early and later ages. 3.2.1. Rate of hydration at early age The hydration acceleration effect of nanoSiO2 on cement, i.e. the seeding effect, was demonstrated by the hydration temperature evolution of cement pastes with addition of nanoSiO2 of various dosages and sizes, as shown in Fig. 8. It shows that a quicker and higher temperature peak formation occurs when nanoSiO2 is added, and the increase of the dosage or the fineness of nanoSiO2 results in a greater hydration acceleration effect. A similar shape of the temperature–time curve of paste with 5% nanoSiO2 to that of paste with 1% implies that a greater dosage of CNS-10 nm than 1% contributes little to the hydration acceleration of cement.
Fig. 6. XRD patterns of hydration products of silica fume and CNS (CNS/ SF:CH = 5:20; w/b = 2.0, 4 months old).
3.1.3. Silicate anion structure The infrared spectroscopy technique was used to evaluate the evolution of the silicate anion structure of the pozzolanic hydration products of nanoSiO2. The characteristic absorbance peaks at ca. 1100 cm1 and 980 cm1 represent the amorphous silicate of the pozzolans and their hydration products of middle groups of silicate tetrahedra, respectively [28]. It can be seen in Fig. 7 that at 7 days the peak intensity ratio of the amorphous silicate to the middle group silicate tetrahedra changes with the average particle size of the pozzolans, and the finer the pozzolan, the lesser the value, which indicates a higher reaction rate of the pozzolan. At 4 months old, the IR curve configurations of nanoSiO2 and silica fume are almost the same, indicating a similar silicate anion structure of the two pozzolans at later ages, while the polymerization degree of them are both higher than that of the plain cement paste [16,29,30].
3.2.2. Degree of hydration of cement at later ages Backscattered electron microscopy (BSE) technique under the automatic number/composition contrast mold was used to measure the degree of cement hydration. By measuring the area of unhydrates (bright spot in BSE images) to that of the whole image area, the hydration degree of cement was traced in Ref. [31]. Considering that areas taken by pores in BSE images would bring errors to the calculation, especially at early ages, pore volume/area of the pastes as detected by MIP was excluded of the calculation. It should also be noted that BES images reflect the average atomatic number contrast of the constituents and the accuracy of the calculation would depend on the selection of the gray value threshold and a weak contrast in early age sample due to a smaller water inclusion into the hydrates would bring higher errors, thus only later age hydration degree was measured through this technique. Typical BSE images of hardened cement pastes without and with 5% nanoSiO2-4 nm at 8 months are shown in Fig. 9 and the hydration degree of cement, which is the average value of five BSE images, is also depicted in Fig. 9. To accurately evaluate the hydration degree, the smallest magnification as 100 was used to obtain the biggest image that testing equipment could provide.
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255
Fig. 7. IR spectra of the pozzolanic hydration products of nanoSiO2 and silica fume (CNS/SF:CH = 5:20; w/b = 2.0).
Fig. 8. Influences of nanoSiO2 on the hydration temperature of cement pastes.
It shows that hydration degrees of nanoSiO2-modified cement paste is smaller than the control paste at 8 months, indicating a hindrance effect of nanoSiO2 on cement hydration at later ages. At 8 months old, the pore structures of the CNS-add and control sample are comparable and thus the hindrance effect is very evident at this time. Considering the hydration of cement at later age is controlled by the ion diffusion ability through hydrates [32], a compact gel structure of the pozzolanic hydration products of nanoSiO2 would result in the block of the diffusion and thus decreases the hydration degree of cement and finally the slow down of the strength gain [33–34]. 3.2.3. Pore size distribution The pore size distribution of pastes before and after an addition of 5% nanoSiO2-4 nm at 3.5 and 9 months were studied by using MIP technique and the results are shown in Fig. 10. It demonstrates that the pore structure of cement paste is densified by nanoSiO2 at 3.5 months and this complies well with other studies [35]. It also shows that the pore size distribution curve of nanoSiO2-modified paste is comparable to that of control paste at 9 months old, and the pore volume of nanoSiO2-modified paste is even greater than that of the control paste in pore size range smaller than 10 nm. This result is consistent with results reported by Thomas and Jennings, who claimed a more porous structure of nanoSiO2-added C3S paste of 3 months old [33]. And this could be due to the compact gel structure of the hydration products of CNS, which blocks
diffusion of unhydrates and results in a more porous bulk structure. It is also known that pores smaller than 10 nm are also present in C–S–H gel and additional C–S–H gel introduced by pozzolanic reaction of nanoSiO2 may result in more gel pores. However, from Fig. 3 we can see that the pozzolanic reaction of CNS-4 nm ceases after about 100 h hydration, thus the hydration blocking effect would be the main cause. 4. Application of the pozzolanic reactivity of CNS on surface treatment of hardened cement-based materials The influences of fine pozzolans on the properties of cementbased materials have been intensively investigated and three effects, i.e. the pozzolanic reactivity, the filling and seeding effects, were often referred to when interpreting their effects. A detailed investigation of the pozzolanic reactivity of nanoSiO2 reveals a quicker hydration rate, a greater apparent reactivity, and a more amorphous and compact microstructure of hydrates than those of silica fume. Although these features greatly improves the mechanical properties of cementitious materials at early ages, they could also be detrimental to the property gain at later ages due to the block of ion diffusion through these compact gel. From the work mentioned above, it can be seen that an ultrafine particle size, a high pozzolanic reactivity and a compact hydrate structure of nanoSiO2 are the primary features that should be considered when it is applied in cement-based materials. Considering
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0CNS-8months
5CNS-8months
110
95.8%
Degree of hydration
100
85.9%
90 80 70 60 50 40 30 20 0 CNS
5 CNS
Fig. 9. Influences of CNS-4 nm on the hydration degree of cement at 8 months (note: the pore volume of cement pastes without and with 5% CNS at 9 months of 23.6% and 24.4% were used in the calculation).
18
18 16 0% CNS 3.5months 5% CNS 3.5months
2
14
Cumulative pore area (m /g)
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Cumulative pore area (m /g)
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12 10 8 6 4 2 0 100
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Pore size/nm
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0% CNS 9months 5% CNS 9months
12 10 8 6 4 2 0 100
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Fig. 10. Influence of nanoSiO2-4 nm on the pore size distribution of cement pastes at 3.5 and 9 months (from MIP tests).
a denser surface structure of cement-based material would be favorable for a more durable concrete, the influences of nanoSiO2 on the surface-treatment of hardened cement mortar was investigated. Fig. 11 depicts the effects of CNS-10 nm on the water sorptivity of hardened cement mortar after treatment procedure described in Section 2.3.7. It shows that after CNS is applied on the surface of hardened mortar, the water absorption ratios of mortar at 1.5 h and 22 h are reduced by 26% and 14.2%, respectively. IR spectroscopy technique was used to verify the presence of the hydration products of CNS on the surface of CNS-treated mortar. Powder (including unhydrated nanoSiO2 and newly formed C–S–H gel) on the surface of hardened cement mortar that
was treated with CNS through brushing technique and cured at 50 °C/95% RH for 7 days was carefully scraped and collected without disturbing the hardened cement mortar. It shows in Fig. 12 that the middle group silicate tetrahedral, Q2, although very small, which could be due to the small percentage of the hydrates in the sample collected or a small hydration degree of CNS, appears when compared with the pure CNS, indicating the occurrence of the pozzolanic reaction of nanoSiO2 on the surface of hardened cement mortar, and this theory has also been reported by Cardenas [36] to show the capability of using nanoparticle of sealing the surface of cement-based materials. Surface-sealing effect of CNS is also demonstrated by the pore size distribution analysis of cement
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100
Control CNS(brushing)
Percentage/%
80
60
40
20
0 100 Fig. 11. Water sorptivity of mortar after surface treatment with nanoSiO2 (w/ c = 0.6).
10
1
0.1
Pore size/nm Fig. 13. Pore size distribution of CNS-treated mortar (samples after water sorptivity measurement in Fig. 10 were used).
effective in utilizing the characteristics of nanoSiO2 in the surface-treatment [38]. 5. Conclusions In this work, investigations of the characteristics of pozzolanic reactivity of nanoSiO2 and its influences on the properties of cement at both early and later ages were made and possibility for its application on the surface treatment of cement-based material based on the recognition of the pozzolanic reactivity was shown. The following conclusions can be made:
Fig. 12. IR spectrum of the hydration products on the surface of CNS-treated mortar (28 days old, sample were CNS treated for 3 times and then cured at 50 °C/95% RH for 7 days) [38].
mortars that are used in water sorptivity test in Fig. 11. It is shown in Fig. 13 that the percentage of coarse pores in the surface of hardened cement mortar decreased by brushing CNS. Recently, we detaily studied the pore-filling effect of CNS on hardened cement pastes and it was found effective in sealing pores larger than 50 nm [37]. Results in Figs. 11–13 show the potential of using nanoSiO2 on surface treatment of cement-based material by exploring its pozzolanic reactivity, through which the surface becomes denser and less permeable. Since the surface part of cement-based material is exposed to a different environment than the underlying bulk concrete, more work is needed, including exploring ways of improving the hydration reaction degree of CNS and of increasing the penetration of nanoSiO2 into cement/concrete. Recently, we studied the effectiveness of using the precursor of nanoSiO2, tetraethoxysilane, TEOS, of treating hardened cement paste/mortar/ concrete, and by taking the advantage of the easy penetration of TEOS into pores and the in-situ hydrolysis of TEOS into nanoSiO2, obvious pozzolanic reaction was found, which proved to be more
(1) The pozzolanic reaction of micro/nanoSiO2 follows the firstorder chemical reaction mold and its reaction rate constant is related to the particle size and its production technique. (2) The pozzolanic reaction products of nanoSiO2 are more compact than those of silica fume. (3) Although the hydration process of cement at early age is significantly increased by nanoSiO2, its later age hydration rate is slowed down due to the compact structure of the pozzolanic hydration products of nanoSiO2. (4) NanoSiO2 can make the hardened cement mortar less water-absorbable by exploring its high pozzolanic reactivity or filler effect on the surface of mortar, which is worth of further investigation.
Acknowledgements The work is supported by Program for Scientific Research Innovation Team in Colleges and Universities of Shandong Province and Natural Science Foundation of China (Grant No. 51302105), which is greatly acknowledged. References [1] Collepardi M, Collepardi S, Skarp U, et al. Optimization of silica fume, fly ash and amorphous nano-silica in superplasticized high-performance concrete. ACI Mater J 2004;221:495–506. [2] Kawashima S, Kim JH, Corr DJ, et al. Study of the mechanisms underlying the fresh-state response of cementitious materials modified with nanoclays. Constr Build Mater 2012;36:749–57.
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