Available online at www.sciencedirect.com ScienceDirect Procedia Engineering 172 (2017) 776 – 783 Modern Building Materials, Structures and Techniques, MBMST 2016 Microstructural analysis of self-compacting concrete modified with the addition of nanoparticles Paweł Niewiadomskia,*, Damian Stefaniuka, Jerzy Hołaa a Faculty of Civil Engineering, Wroclaw University of Technology, Wybrzeze Wyspianskiego 27, 50-370 Wroclaw, Poland Abstract Microstructural analysis of self-compacting concrete modified with the addition of nanoparticles was carried out. Investigations included SCC concretes modified with different amounts of SiO2, TiO2 and Al2O3 nanoparticle additives and one reference concrete made without nanoparticles. Porosity, maximal pore dimensions, microstructure 3-D models, hardness and elastic modulus were determined. Based on the conducted studies, it can be concluded that the addition of nanoparticles improves the microstructure of self-compacting concrete. It is confirmed both by the results of porosity of the tested samples and by the results of hardness and elastic modulus of the cement matrix of investigated concretes. © Published by Elsevier Ltd. Ltd. This is an open access article under the CC BY-NC-ND license © 2017 2016The TheAuthors. Authors. Published by Elsevier (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility ofthe organizing committee of MBMST 2016. Peer-review under responsibility of the organizing committee of MBMST 2016 Keywords: Self-compacting concrete; Modification; Nanoparticles; Microstructure; Microtomography; Nanoindentation. 1. Introduction Development of the construction industry is mainly possible due to both the production of new materials and the improvement of existing ones. In the latter case, particularly helpful is nanotechnology which has a significant impact on current directions of research on the most popular construction material - concrete. In many laboratories around the world there are intensive studies on the modification of concrete with the addition of nano-sized materials, which due to their unique properties are believed to improve the characteristics of the composite achieved with their addition. The results of these studies are very promising [1-3]. * Corresponding author. Tel.: +48 508 859 290. E-mail address: [email protected] 1877-7058 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of MBMST 2016 doi:10.1016/j.proeng.2017.02.122 Paweł Niewiadomski et al. / Procedia Engineering 172 (2017) 776 – 783 From a review of literature it can be concluded that in the modification of cement pastes, mortars and concretes with additives of very fine grain which do not exceed the size of 100 nm, the nanoparticles SiO 2, Al2O3, CuO, TiO2, ZnO2, Fe2O3 and Cr2O3 [4-14] are often used. In the area of interest of researchers there are also other nanoadditives, such as nanoclay [15] or carbon nanotubes [16,17]. The paper [18] confirms the interest of the aforementioned nanomaterials in the context of their usage as an additive to concrete. To date, there have been attempts to modify the composition of concrete with different amounts of nanoparticles that range from 0.2% to 12% of the weight quantity of cement. However, in most studies it is indicated that the optimum amount of nanoparticles is between 0.5% to 4 ÷ 5% of the weight of cement [4-14]. The way of adding nanoparticles is an important issue when modifying cement-based composites with nano-sized materials. This is because of their strong tendency to form conglomerates which are undesirable in hardened concrete. Literature provides information that up until now nanoparticles were added to a concrete mix mainly as a suspension in water combined with a superplasticizer [19], a suspension in acetone [20] or as a dry nano-powder [8]. Most of the studies available in literature present the results of research of rheological characteristics of concrete mixes and also selected physical and mechanical characteristics of hardened concrete. Results of these studies show that in most cases the addition of nanoparticles has an adverse impact on the rheological properties of concrete mixes [11,21,22]. The biggest problem occurs with a decrease of the workability of mixes containing nanoparticles, which due to their large specific surface area are characterized by high water demand [22]. It should also be noted that the addition of nanoparticles (SiO2) reduces the initial setting time of a concrete mix [22]. In contrast, there is an improvement of the physical properties of a hardened concrete modified with the addition of nanomaterials. These properties are measured by such parameters as porosity [8,10], absorption [23-25], corrosion resistance [24,25] or shrinkage resistance [23]. There is also an improvement in most of the basic mechanical properties of hardened concrete, i.e. compressive strength [7,26], bending strength [23,26], abrasion resistance, [6] hardness [27,28] and also the elastic modulus [27,28]. The mechanism describing the reason of the increase in the value of the mechanical properties of cement composites due to the usage of nanoparticles is presented, among others, in studies [24,29]. These studies indicate that a high chemical reactivity of nanoparticles, and the pozzolan effect of some of them has a beneficial influence on the formation of phase C-S-H, which is responsible for, among others, the strength of concrete. At the same time, nanoparticles reduce the growth of Ca(OH)2 crystals, which adversely affect the mechanical properties of hardened concrete. In addition, nano-additives make a concrete structure more homogeneous. It is also worth mentioning that the addition of nanoparticles in the form of nanoclays can reduce the value of the lateral pressure of a concrete mix on a formwork, which means that it improves the so called "green strength" [15]. An important aspect when trying to assess the impact of nanoparticles on the properties of hardened concrete seems to be knowledge about the changes occurring in its microstructure with the participation of an ultra-fine additive. The basic properties of concrete, such as compressive strength or porosity, are frequently associated with the macroscopic properties of concrete. However, it is the understanding of the phenomena occurring at a micro or even nano scale that enables the characteristics of concrete at a macro scale to be consciously created. There are currently research techniques that allow the microstructure of concrete to be analysed in detail. This group of techniques includes, among others, x-ray computer microtomography and nanoindentation [30,31]. Computer x-ray microtomography is a non-invasive test method, which involves the reconstruction of a threedimensional image of an assessed object based on two-dimensional projections achieved during the scanning of a sample with x-rays (Fig. 1). This method allows both qualitative three-dimensional visualization of the internal structure of a tested material to be created, and quantitative data regarding e.g. porosity or the specific surface area of the tested material to be obtained [30-32]. In turn, indentation is a test method that is used for measuring the mechanical properties of a tested material such as, among other things: hardness and the elastic modulus. It is worth noting that the hardness tests are carried out for different levels of observation - for macro, micro and nano scales [33-35]. As a result, in addition to knowledge of the mechanical parameters of the tested material, nanoindentation also enables a detailed analysis of its microstructure to be carried out by the identification of its components in terms of both quantity and quality [36]. It is possible to distinguish the various phases which form the microstructure of concrete, because each of them is characterized with a different hardness value. Thus, e.g. in studies [33,37], to individual phases of concrete (micro pores, C-S-H, sand, cement clinker) their approximate hardness values were assigned. 777 778 Paweł Niewiadomski et al. / Procedia Engineering 172 (2017) 776 – 783 Most previous studies on the influence of nanoparticles on the properties of cement composites were carried out on cement pastes and mortars but rarely on concretes. This is probably mainly due to the significant cost of purchasing commercially available nanomaterials. There are therefore only a few studies [7,10,23-25] on the impact of the addition of nanoparticles on the rheology of a concrete mix, as well as on the physical and mechanical characteristics of hardened self-compacting concrete. It is worth mentioning that the use of self-compacting concrete in the construction industry is increasing due to the ease of executing constructions with complex shapes, the ability to perform a seal filling of formwork of a construction with a high degree of reinforcement, and also the lack of the need for mechanical compaction of a concrete mix [38]. For these reasons, it seems to be an important issue to investigate the impact of modification of self-compacting concrete with nano-sized additives. The purpose of this work was to conduct a microstructural analysis of self-compacting concrete modified with the addition of nanoparticles. The paper presents the results of own studies of several series of self-compacting concrete modified with the addition of nanoparticles. The studies included porosity, which was measured with the use of a computer microtomograph, hardness and also the elastic modulus measured using the nanoindentation technique. An assessment of the obtained results, showing the influence of the addition of selected nanoparticles on the widely understood microstructure of self-compacting concrete, was attempted. 2. Preparation of samples Mixes of self-compacting concrete used in the tests were made of the following components: tap water, Portland cement CEM I 52.5R, superplasticizer Sky Glenium 600 with a density of 1.06 g/cm 3 in an amount of 4% of the weight of cement, granite aggregate with an average density of 2.67 g/cm 3 and fractions of 10-5, 5-2, 2-1, 1,2-0,5, 0,6-0,1 mm, and also a fraction with a grain size of <0.1 mm acting as a fine filler. The used additives were three types of nano-powder: SiO2 with a particle size of 10-20 nm (99.5% trace metal basis) in an amount of 0.5%, 2.0% and 4.0% of the weight of cement; TiO2 with a particle size < 25 nm (99.7% trace metal basis) in an amount of 0.5%, 2.0% and 4.0% of the weight of cement and Al 2O3 with a particle size of <50 nm in an amount of 0.5%, 1.0%, 2.0% and 3.0% of the weight of cement. The W/C ratio for all the designed concrete mixes was equal to 0.42. From the above ingredients, 11 self-compacting concrete mixes were prepared, out of which one mix was prepared without the addition of nanoparticles as a comparison. A summary of the compositions of the designed mixes calculated for 1m3 is shown in Table 1. All the mixes are designated with the symbols S1 to S11. Analogous symbols are determined for the samples made from these mixes. Table 1. Summary of compositions of the designed self-compacting concrete mixes per 1m3. Number Water (kg) Cement (kg) Aggregate (kg) Nano-additive (kg) SP (kg) S1 Type of nano-additive and its amount (%) - 193.2 460.0 1640 0.0 18.4 S2 SiO2 – 0.5% 193.2 457.7 1640 2.3 18.4 S3 SiO2 – 2.0% 193.2 450.8 1640 9.2 18.4 S4 SiO2 – 4.0% 193.2 441.6 1640 18.4 18.4 S5 TiO2 – 0.5% 193.2 457.7 1640 2.3 18.4 S6 TiO2 – 2.0% 193.2 450.8 1640 9.2 18.4 S7 TiO2 – 4.0% 193.2 441.6 1640 18.4 18.4 S8 Al2O3 – 0.5% 193.2 457.7 1640 2.3 18.4 S9 Al2O3 – 1.0% 193.2 455.4 1640 4.6 18.4 S10 Al2O3 – 2.0% 193.2 450.8 1640 9.2 18.4 S11 Al2O3 – 3.0% 193.2 446.2 1640 13.8 18.4 The rheological properties of the self-compacting concrete mixes were tested with an Abrams cone. The measured parameters were: the maximum diameter of a concrete mix slump and also subsidence time T 500, during which the mix coming out of the cone reaches a slump size with a diameter equal to 500 mm. The results of research Paweł Niewiadomski et al. / Procedia Engineering 172 (2017) 776 – 783 of the rheological characteristics (maximum diameter of the slump and subsidence time T500) of the assessed concretes is presented and commented on in paper [39]. A series of beams with dimensions of 40x40x160 mm were made from each concrete mix whose symbol and composition is shown in Table 1. Samples were cured in a climatic chamber at a temperature of 20°C (± 1°C) and relative humidity of 95% (± 5%). After a period of one year, three types of samples were cut from previously prepared beams: cuboid samples with dimensions of 10x10x20 mm and 32.5x32.5x60 mm and also cylindrical samples with a diameter of 25 mm and a height equal to 20 mm. In order to avoid the influence of the edge effect on the results, the samples were cut from the middle of the beams. The cuboid samples were intended for tests using a computer microtomograph, while the cylindrical samples for tests using nanoindentation. In order to properly prepare the surface for the testing of hardness and the elastic modulus, the cylindrical samples were immersed in an epoxy resin in a vacuum machine with a pressure inside the vacuum chamber equal to 0.07 bar. Their surfaces were then polished, firstly with abrasive paper with a grade size of 320 and then with diamond suspension with a grade size of 9, 3 and 1 μm, until a smooth surface was achieved. 3. Description of the carried out tests A computer microtomograph Skyscan 1172 (Fig. 1) equipped with a camera with a resolution of 11 Mp was used to perform the three-dimensional reconstruction of the tested samples and also to conduct the porosity tests. In order to obtain the maximum possible image of the microstructure of the tested concretes, two kinds of samples of different sizes were prepared, which therefore enabled images at different scales to be obtained. Thus, in the samples with dimensions of 10 mm x 10 mm x 20 mm, a unit rotation step amounting to 0.15° was used. The sample was scanned six times in one setup, the exposure time of each setting was equal to 2170 ms, and the whole process of scanning one sample lasted 92 hours. As a result, a series of images with a resolution of 2.5 μm per 1 pixel was obtained. In turn, in the samples with dimensions of 32.5x32.5x60 mm, a unit step of rotation equal to 0.2° was used. The sample was scanned eight times in one setup, the exposure time of each setting was equal to 660 ms, and the whole process of scanning one sample lasted 29 hours. As a result, a series of images with a resolution of 27 μm per 1 pixel was obtained. In contrast, the nanoindenter TTX-NHT (Fig. 2) with a Berkovich indenter was used in the hardness and elastic modulus tests. The concrete sample tests of all the series were carried out in two cycles of loading: with a constant increase of force equal to 40 mN/min, and without loading. The maximal force that was obtained during the tests was equal to 20 mN. Due to the large heterogeneity of the tested material, each concrete sample was subjected to a minimum of 150 measurements of hardness in the area of the cement matrix and on a regular grid with spacing of 20 μm. The grid was applied in at least three different locations on the sample surface. For the purpose of measurements, a value of Poisson's ratio equal to 0.3 was assumed for the tested concretes. Fig. 1. View of the computer microtomograph during the test. Fig. 2. View of the nanoindenter during the test. 4. Test results and their analysis Figure 3 shows the results of the porosity tests of the four concrete series (S1, S3, S7 and S11). The tests were carried out using a computer microtomograph. Due to their long duration, a reference series and three series with the 779 780 Paweł Niewiadomski et al. / Procedia Engineering 172 (2017) 776 – 783 addition of each type of nanoparticles were evaluated. The results for both of the sample sizes were summarized so that Figures 3 a), c), e) and f) show samples reconstructed to images with a resolution of 1 pixel = 2.5μm, and Figures 3 b), d), f) and g) show samples reconstructed to images with a resolution of 1 pixel = 27μm.Visualization of the tested samples of a self-compacting concrete are shown in Figures 4 and 5 as an example for series S1 and S7. Fig. 3. The percentage participation of pores in the self-compacting concretes of series S1, S3, S7 and S11, with pore size from <0.015 mm to> 2.0 mm. From the above diagrams it can be seen that concretes modified with the addition of the nanoparticles SiO 2, TiO2, and Al2O3 had a lower content of pores with a size of up to 0.015 mm (for smaller samples) when compared to the 781 Paweł Niewiadomski et al. / Procedia Engineering 172 (2017) 776 – 783 reference concrete. This may mean that the addition of nano-powders improves the microstructure of the cement matrix which makes it more compact. This is also confirmed in a summary of the porosity of the tested samples which is presented in Table 2. In the same table it can also be noted that the total porosity of series S3 was much higher than in the reference series. This may be due to the fact that the S3 concrete mix did not reach a sufficient diameter of slump required for self-compacting concretes [39], which in turn could result in improper compaction of the concrete in the form. Fig. 4. A 3D model of the pores of the S1 sample series with resolution equal to: (a) 2.5um (VOI = 10x10x20 [mm]) and (b) 27um (VOI = 32.5x32.5x60 [mm]). Fig. 5. A 3D model of the pores of the S7 sample series with resolution equal to: (a) 2.5um (VOI = 10x10x20 [mm]) and (b) 27um (VOI = 32.5x32.5x60 [mm]). Table 2. The porosity and maximal pore diameter for the tested series of concretes. Series Resolution of 2,5 μm Resolution of 27 μm Porosity [%] Maximal pore diameter [mm] Porosity [%] Maximal pore diameter [mm] S1 0.541 1.250 0.493 2.248 S3 1.680 1.918 0.824 3.114 S7 0.500 1.137 0.380 2.464 S11 0.487 1.065 0.834 2.789 The results of the hardness H and elastic modulus E testing of every series of self-compacting concrete modified with various amounts of nano-powder containing SiO2, TiO2, and Al2O3 are shown respectively in Figures 6a and 6b. The authors decided to present the results from the interval of hardness values between 0.23 and 1.85 GPa because these results relate to the hardness obtained for the C-S-H phase of the cement matrix. Values lower than 0.23 GPa (micropores filled with epoxy resin) and higher than 1.85 GPa (not hydrated products of cement hydration) were not the purpose of the analysis [37]. Fig. 6. The results of nanoindentation of the tested series of concrete (a) hardness H (b) the elastic modulus E. 782 Paweł Niewiadomski et al. / Procedia Engineering 172 (2017) 776 – 783 The obtained results allow the conclusion that both hardness and the elastic modulus of the cement matrix of concretes, modified with the addition of nanoparticles, significantly improved compared to the matrix of reference concrete which did not contain a nano-additive in its composition. However, it should be noted that it is difficult to state that with an increase of an amount of nano-powder in the composition of self-compacting concrete, the mechanical properties of the cement matrix such as hardness and the elastic modulus also improve. What is more, the type of nanoparticles used in the composition of concrete does not significantly affect the obtained results, however the recorded increase in the value of the above parameters is strongly noticeable. Reasons for the increase of values of the mechanical properties of the tested concretes can be found in the high chemical reactivity of the used nanoparticles, which favourably affects the formation of the C-S-H phase which corresponds with, among other things, the mechanical properties of concrete. The obtained results may also be associated with the phenomenon of the growth in the length of the C-S-H phase chain in concrete with nanoparticles. This translates to an increase in the amount of the C-S-H phase with a high stiffness in relation to the C-S-H phase with a low stiffness in a cement matrix [40], which in turn may cause the cement matrix to be harder. Other results concerning strength of investigated self-compacting concrete are presented in paper [39]. 5. Conclusions Based on the conducted studies, it can be concluded that the addition of nanoparticles improves the microstructure of self-compacting concrete. Analysis of the porosity results proved that concretes modified with the addition of nanoparticles are characterized by lower porosity in relation to pore size of up to 0.015mm in comparison to concrete which does not contain nanoparticles in its composition. The positive influence of the addition of nano-powders on the microstructure of the tested concretes is also confirmed by the results of the total porosity of the samples. 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