Construction and Building Materials 251 (2020) 118945 Contents lists available at ScienceDirect Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat Mechanical properties cement based composites modified with nano-Fe3O4/SiO2 Elzbieta Horszczaruk a,⇑, Małgorzata Aleksandrzak b, Krzysztof Cendrowski b, Roman Je˛drzejewski c, Jolanta Baranowska d, Ewa Mijowska b a West Pomeranian University of Technology Szczecin, Faculty of Civil Engineering and Architecture, Al. Piastow 50, 70-311 Szczecin, Poland Nanomaterials Physicochemistry Department, Faculty of Technology and Chemical Engineering, West Pomeranian University of Technology, Szczecin, Al. Piastow 45, 70-311 Szczecin, Poland c Łukasiewicz Research Network – PORT Polish Center for Technology Development, ul. Stabłowicka 147, 54-066 Wrocław, Poland d Faculty of Mechanical Engineering and Mechatronics, West Pomeranian University of Technology Szczecin, Al. Piastow 19, 70-310 Szczecin, Poland b h i g h l i g h t s  FeO4/SiO2 nanoparticles were incorporated to cement composites.  Fe3O4/SiO2 increases the amount of CH/C-S-H phase in the cement matrix.  Inclusion of Fe3O4/SiO2 improved the Young Modulus of cement composites. a r t i c l e i n f o Article history: Received 25 February 2019 Received in revised form 11 March 2020 Accepted 29 March 2020 Keywords: Cement based composites Magnetite-silica nanostructures Mechanical properties Nanoindentation a b s t r a c t The nanomaterials are being increasingly used for modification of the cement composites properties. The aim of the research was determination of optimum content of the core-shell type Fe3O4/SiO2 nanoparticles in the cement paste for obtaining the tight structure of the high-strength paste. For this purpose, such investigation techniques as atomic force microscope (AFM) and nanoindentation were employed. The significant effect of the silica shell on the increase of the porous phase in the tested composite has been found. Also, an increase of amount of high-stiff C-S-H phase was observed, which is a result of using of the nanoadmixture. The AFM method turned out to be effective for determination of the limit amounts of the nanoadmixture for the further investigation. The nanoindentation has brought information about the changes in the quantity and quality (stiffness) of the particular C-S-H phases, caused by modification of the tested pastes with various amounts of nano-Fe3O4/SiO2. The use of 3% of nano-Fe3O4/SiO2 admixture in the cement paste together with a superplasticizer for reduction of the excessive porosity appeared to be an optimum solution in the case of the tested composites, as it has been demonstrated in the further durability tests. Ó 2020 Elsevier Ltd. All rights reserved. 1. Introduction Construction industry is a significant area, in which the quickly developing nanotechnology keeps up with the growing requirements of the consumers. The most modern application dedicated mainly to the building materials are developed on the construction market. Nanotechnology brings to this sector the extraordinary, ⇑ Corresponding author. E-mail addresses: elzbieta.horszczaruk@zut.edu.pl (E. Horszczaruk), malgorzata. wojtaniszak@zut.edu.pl (M. Aleksandrzak), krzysztof.cendrowski@zut.edu.pl (K. Cendrowski), roman.jedrzejewski@zut.edu.pl (R. Je˛drzejewski), jolanta. baranowska@zut.edu.pl (J. Baranowska), emijowska@zut.edu.pl (E. Mijowska). https://doi.org/10.1016/j.conbuildmat.2020.118945 0950-0618/Ó 2020 Elsevier Ltd. All rights reserved. improved properties, like mechanical strength, resistance to bacteria, self-cleaning ability, energy saving and thermal insulation, just to name a few. This leads to the revolution in the modern sustainable construction: safe and inexpensive buildings, railroads, airfield pavements and roads [1–4]. Titanium dioxide, zinc oxide and carbon nanotubes are the most common materials commercialized in the construction sector [2,5–10]. Like the other fields, the construction industry desperately needs new solutions for diminishing the pollution and wastes formed during the production processes. In this regard, the nanotechnological objects are some panacea, thanks to which the construction industry is more environmental friendly. 2 E. Horszczaruk et al. / Construction and Building Materials 251 (2020) 118945 Concrete is one of the main construction materials. The properties, in particular the mechanical strength, of such a composite material depend on the properties of its constituents, it means the cement paste and the aggregate, and on the strength of the bonds between them [11,12]. In the ordinary concrete, the least deformable is the aggregate, in which the stresses are concentrated and then transferred into the aggregate-paste contact zone. The strength of the cement paste, however, contributes more to the concrete strength than the strength of the bonds between the paste and the aggregate [13]. The cement paste is a subject of a number of modifications with various types of nanoparticles. The modern testing techniques, allowing for determination of its structure in the nanoscale are increasingly used for cement paste investigation. In regard to concrete, the so-called grid indentation method or statistical nanoindentation was used [14–18]. The results of these researches led to the formulation of sophisticated models of the Portland cement hydration products. The main binding phase is hydrated calcium silicate C-S-H of heterogeneous structure. The C-S-H phase consists of 4–5 nm elementary spheres, appearing in the form of a colloid with different packing densities. Three main types (phases) of C-S-H are distinguished. They are characterized by various densities: LD C-S-H, HD C-S-H and the phase of calcium hydroxide CH/C-S-H nanocomposite [19,20]. The strong differentiation of the hardness and Young’s modulus was observed for the particular phases when the cement composites were tested by nanoindentation method [21–23]. These results together with statistical methods of analysis make possible to model the composite nanostructure. The research [24] has demonstrated that the coefficient w/c influences the relative volume fraction of the particular hydration products in the hydrated cement paste. Increasing of w/c causes growth of the low-density C-S-H phase contributon and diminishing of the volume of HD-C-S-H. The volume of CH/ C-S-H phase does not change significantly. The iron oxides nanoparticles were investigated in regard to the improvement of the mechanical properties and durability of the cement composites [25–29]. There are, however, only few researches referring to the evaluation of the influence of nanomagnetite on the structure and strength of the cement composites [30– 32]. Our investigation [33] has demonstrated that the use of 3% admixture of nano-Fe3O4/SiO2 (in relation to the binder mass) in the cement mortars can improve their compressive strength and tighten the composite structure. Searching for nanoadmixture, which makes possible to permanently embed nano-Fe3O4/SiO2 into the cement matrix, the authors decided to use the SiO2 shell. As it has been demonstrated in the research [34], such structures are chemically stable at the high temperature. The results of investigation of the modification of the cement composites with the magnetite-silica nanostructures (nanoFe3O4/SiO2) of the core–shell type are presented in the paper. The aim of tests of the mechanical properties was determination of the changes in the cement matrix structure caused by the nanostructures. The use of the AFM technique made possible to determine the optimum content of the nanoadmixture for the further testing of the cement composites mechanical properties. 2. Experimental 2.1. Materials Portland cement CEM I 42.5N according to PN-EN 197-1 was used for preparing the cement pastes. The tests were conducted on the cement pastes with w/c = 0.5, containing the admixture of the Fe3O4/SiO2 nanostructures: 1, 2, 3, 4 and 5% of the cement mass. The chemical composition of the used cement is presented in the Table 1. The core-shell nano-Fe3O4/SiO2 were prepared according to the Stöber method using commercially available nanomagnetite (MERCK, Darmstadt, Germany) as a template. The silica shell on the surface of the nanomagnetite is synthesised by sol–gel process - hydrolysis of TEOS in the presence of ammonium [34]. 2 g of the nanomagnetite were dispersed in ethanol (1500.0 mL - EtOH) with addition of ammonia solution (2.5 mL – NH3
H2O). After obtaining iron oxide dispersion, 1.5 mL of tetraethyl orthosilicate (TEOS) was added. Further, obtained mixture was stirred for 18 h, at room temperature in the sealed container. Obtained core–shell structures were separated with a magnet, washed with the ethanol and then dry in air a temperature 60 °C. Fig. 1a and b shows the morphology (TEM images) of the nanoFe3O4/SiO2. The nano-Fe3O4/SiO2 structures present cube-shaped particles. Electron microscopy analysis of commercially available magnetite shows that, the nanoparticles were cubic in shape, with a diameter in the range of 50–100 nm [35]. Magnetite crystals in the nano-Fe3O4/SiO2 structures have a dense morphology in contrast to silica shell, which is transparent. The transmission electron microscopy (TEM) images also revealed that the mean thickness of the silica coating was ~20 nm. After carbonization, a porous structure with additional cracks and cavities (from the removed zinc oxide) was observed. According to the N2 adsorption isotherm, the surface area of the nano-Fe3O4/SiO2 was 60 m2/g. The composition of the nano-Fe3O4/SiO2 structures was confirmed with energy-dispersive X-ray spectroscopy (EDS) analysis. EDS analysis of the nano-Fe3O4/SiO2, show signal attribute to the silicon, oxygen and iron, from the detection of a silica and magnetite (Fig. 1c). Signal from the copper and carbon comes from the TEM grid. Detailed crystallographic phase analysis has been presented in a previous report [35]. 2.2. Preparation of the test specimens The specimens made from the pastes containing nano-Fe3O4/ SiO2 have been marked P0, P1, P2, P3, P4 and P5, according to the nanoadmixture content. The number corresponds to the percentage of the nanomaterial in respect to the cement mass in the paste. The nanoparticles were sonicated in water with simultaneous mechanical stirring for 30 min. Afterwards, the suspensions were added to the cement and mixed in a cement mortar mixer for 120 s at high speed (120 rpm). The cubic specimens of 20 mm size were then prepared for testing Young’s modulus and nano-hardness. Schematic presentation of the nano-Fe3O4/SiO2– incorporated cement paste preparation process is presented in Fig. 2. The samples were demoulded after 24 h, and then cured for 27 days in a water bath, at a temperature of 20 ± 2 °C. After 28 days of curing, the specimens underwent further examination. For AFM and nanoindentation testing, the surfaces of the specimens after 28 days of curing were specially treated. In order to achieve the surface as flat as possible, the specimens were trimmed to an appropriate size using a diamond saw. In the first step, cement paste specimens were wet ground with abrasive papers (of 200, 400, 800, 1000, 1200, 1500, 1800 fineness) for 5 min for each paper fineness. In the second step, the specimens were polished with diamond polishing cloths of fineness of 9, 6, and 3 lm. In the final step, the specimens were polished with a diamond polishing film of fineness 1 and 0.5 lm (in ethanol, for 10–15 min). Before indentation, each specimen was cleaned in an ultrasonic bath filled with ethanol for 5 min. 2.3. Methods In order to analyse the nanomaterials’ structure and chemical composition, a transmission (HR-TEM) electron microscope – Fei Tecnai G2 F20 STwin equipped with energy dispersive X-ray 3 E. Horszczaruk et al. / Construction and Building Materials 251 (2020) 118945 Table 1 Chemical composition of Portland cement [wt.%]. Material CaO SiO2 Al2O3 Fe2O3 FeO MgO Na2O K2O SO3 BaSO4 TiO2 CEM I 42.5 R 63.3 19.5 4.9 2.9 0.01 1.3 0.1 0.9 2.8 0.1 0.01 Fig. 1. TEM micrographs of iron oxide/silica (Fe3O4/SiO2) (A-B) core–shell structures and EDS (C) graphs. Fig. 2. Scheme of the preparation of the cement composite containing nano-Fe3O4/SiO2 particles. spectroscopy (EDS) (TEM; Tecnai F30, Thermo Fisher Scientific, Waltham, MA, USA) was used. The specific surface area was calculated by the Brunauer-Emmett-Teller (BET) method. The Young’s modulus was measured by means of atomic force microscopy (AFM – Nanoscope V MultiMode 8, Bruker). The Young’s modulus of the hydration products was determined by nanoindentation method, using the Nanoindenter XP (Agilent) with the use of three-sided pyramidal Berkovich indenter. 3. Results and discussions 3.1. Young modulus of the cement paste by AFM The specimens of the cement pastes, after preparation of the surfaces as it is described in Section 2.2, were tested using AFM method. Nanomechanical mapping was performed using a Dimension of Icon instrument in PeakForce threading mode with quantitative nanomechanical analysis (QNM). The peak force value was set to 3.0 lN and the sample scan was 350  350 pixels (13.8  1 3.8 lm). Fig. 3 present Young’s modulus mapping of the reference sample (P0) and the cement paste modified with various amount of nano-Fe3O4/SiO2 (from 1% to 5% of the cement mass, samples P1–P5) estimated with AFM. Additionally, the comparison of minimum, maximum and dominating fraction of Young’s modulus (Y.M.) values is shown in Table 2. It was observed that the reference material presented the elastic modulus distribution in the range of 0.054–27.152 GPa with maximum peak at 0.054 GPa. Interestingly, the addition of 1% of nano-Fe3O4/SiO2 to the cement paste resulted in deterioration of its mechanical properties. However, increase of the admixture in amount from 1 to 3 wt% caused an enhancement of the Young modulus from 0.015 to 0.707 GPa. Further increase to 4% and 5% of nano-Fe3O4/SiO2 resulted in decrease to 0.056 and 0.013 GPa, respectively. The results showed that modification of cement paste with magnetite nanoparticles covered with silica shell had an effect on its Young’s modulus values and an appropriate amount of the admixture significantly improved the elastic modulus. 3.2. Properties of hydration products On the basis of the results obtained using AFM method, the specimens of the paste containing 1, 3 and 5% of the admixture of nano-Fe3O4/SiO2 (P1, P3 and P5) as well as the reference paste (P0) were selected for the further testing of the properties of the cement matrix. The Young’s modulus of the selected specimens of the cement pastes has been determined by nanoindentation 4 E. Horszczaruk et al. / Construction and Building Materials 251 (2020) 118945 technique, using three-sided pyramidal Berkovich indenter. The surfaces of the specimens were prepared before testing the same way as in the case of AFM method. The measuring grid 20  20 points has been adopted. There were 400 measurements of nanoindentation performed for each specimen of the cement paste. The distance between the measuring points was 5 mm. Three samples from each series were tested. The methodology proposed in [19,23,36,37] was used for analysing the results. The histograms of the obtained values of Young’s modulus were created for each specimen. Then, the obtained curves were decomposed onto the peaks defining the probability of distribution of the particular phases of the paste, based on the values of Young’s modulus attributed to the phases, using Origin software (the so-called peak analyzing protocol). On the basis of the literature data [15,17,18], the following ranges of Young’s modulus for the particular phases of the tested pastes were accepted in the statistical analysis:     Fig. 3. Young’s modulus maps of the paste specimens estimated by AFM: a) reference sample (P0), b) sample P1, c) sample P2, d) sample P3, e) sample P4, e) sample P5. Right panel presents histograms of Young’s modulus distribution in the specimens. porous LD-C-S-H phase of low stiffness: 20 ± 5 GPa, HD- C-S-H phase of high stiffness: 30 ± 5 GPa, CH/C-S-H phase: 40 ± 5 GPa. low stiffness/hardness phase of macroporosity (MP) – modulus below 10 GPa. The experimental and fitted theoretical frequency distributions of indentation modulus from the nanoindentation tests on paste specimens are shown in the Fig. 4. These diagrams translate the indentation modulus functions into the probability modulus functions, and provide useful and easily understood information about mechanical properties of distinct phases. There is some overlapping between bell curves in the plots, while distinguishable peaks verify existence of three C-S-H phases: LD C-S-H, HD C-S-H and CH/ C-S-H [4]. The mean and standard deviation values of indentation modulus of hydration products of the specimens, obtained from maximum probability statistical analysis, is reported in Table 3. The values are in good agreement with those found on the basis of the probability density function approach. The measured indentation parameters for different C-S-H phases in the specimens are consistent with each other within the ranges of variability. The measured volume fraction of LD-C-S-H, HD-C-S-H, CH/C-S-H and phase of low stiffness/hardness of macroporosity (MP) in the tested samples are presented in Fig. 5. Analysis of the Figs. 3 and 4 as well as the data presented in the Table 3 shows that the paste P1 contained LD-C-S-H phase with the mean value of stiffness equal to 17.2 MPa; this value was by 9.6% lower than in the case of the reference paste (P0). For the HD/CS-H phase, the increase of the mean value of the modulus in regard to the reference paste by 14.4% has been noted, while in the case of CH/C-S-H an increase by 11% has been observed. Very big growth of the amount of the porous phase with the low stiffness was observed for the paste P1 (37.7%); this is more than ten times larger volume of the porous phase as compared to the reference paste (P0). Such large volume of the phase with the low stiffness can be explained by high water demand of nanosilica covering the particles of nano-Fe3O4. An increase of the mean values of the indentation modulus of HD/C-S-H and CH/C-S-H, in turn, can be a consequence of the impact of the admixture of nano-Fe3O4/ SiO2. The mean value of indentation modulus for CH/C-S-H phase in the paste P3 was by 14.7% higher than in the paste P0. The volume fraction of this phase in the paste P3 was more than twice (2.33) larger than in the paste P0. The volume fraction of HD/C-S-H phase in the paste P3 was also more than 1.5 times larger as compared to the paste P0, although the mean modulus of this phase was by 10% lower than in the case of the paste P0. The volume fraction of the phases with the low stiffness in the paste P3 was 15.3%, i.e. more than twice less than in the paste P1, but almost 5 times more than 5 E. Horszczaruk et al. / Construction and Building Materials 251 (2020) 118945 Table 2 Young’s modulus (Y.M.) values of the paste samples estimated by AFM. Sample Nano-Fe3O4/SiO2 [wt.%] Y.M.min [GPa] Y.M.max [GPa] Y.M. domination fraction [GPa] P0 P1 P2 P3 P4 P5 – 1 2 3 4 5 0.054 0.015 0.034 0.041 0.056 0.013 27.152 7.506 17.296 27.152 28.598 6.456 0.054 0.015 0.170 0.707 0.056 0.013 Fig. 4. Experimental and theoretical probability distribution function of indentation modulus: a) reference sample (0% nano-Fe3O4/SiO2), b) content of 1% nano-Fe3O4/SiO2 (P1), c) content of 3% nano-Fe3O4/SiO2 (P3), d) content of 5% nano-Fe3O4/SiO2 (P5). Table 3 Mean values and standard deviation of Young’s modulus (Y.M.) for different C-S-H phases determined by nanoindentation. Sample designation P0 P1 P3 P5 LD-C-S-H HD-C-S-H CH/C-S-H Y.M. [GPa] SD Y.M. [GPa] SD Y.M. [GPa] SD 19.0 17.2 15.5 17.2 3.4 3.5 1.9 3.4 24.3 27.8 21.9 24.3 2.8 2.2 3.8 2.6 31.9 35.4 36.6 37.7 2.6 2.6 4.5 5.1 in the paste P0. For the paste P5, the volume fraction of the porous phase with the low stiffness was equal to 31.7%, which is similar to this value for the paste P1. The volume fractions of the other C-S-H phases were similar to each other, although the larger fraction was observed in the case CH/C-S-H (25.5%) that is 4.5-fold growth as compared to the paste P0. Significant increase of porosity as compared to the reference paste (P0) was observed for the paste P3, however, an increase of the fractions HD/C-S-H and CH/C-S-H with the high stiffness has also been observed (59.9%), which fraction in the reference paste P0 was only 33.3%. The admixture of nano-Fe3O4/SiO2 caused a significant increase of the volume of MP phase of low stiffness/hardness in the tested pastes (Fig. 5). To this phase the regions of the matrix dominated by the capillary pores are most often attributed [15,38]. The quantitative growth of this phase suggests the increase of the porosity of matrix under the influence of nano-Fe3O4/SiO2. This was 6 E. Horszczaruk et al. / Construction and Building Materials 251 (2020) 118945 Fig. 5. Volume fraction of hydration products in the tested pastes. confirmed by the measurements of the pastes porosity performed using the mercury porosimetry, presented in Fig. 6. The total porosity of the pastes decreases in the following order: P1 > P5 > P3 > P0, which corresponds to the volumes of MP phase observed during the nanoindentation testing (Fig. 5). The measurements inside the area of the pores are not recommended in the nanoindentation method [15]. Such results are often rejected during the statistical analysis. In order to avoid the measurements inside the pores, the impregnation of the sample with the epoxy resin was often used. As it was showed in the further research [38], however, the impregnation significantly affected the results of nanoindentation. Due to this fact, many researchers use the method described by Diamond [39] for the cement composites. The analysis of the pastes porosity in the tests conducted using MIP and nanoindentation can only be general and comparative. The reason of the increase of cement composites porosity under the influence of the nanomaterials is also seen in the uneven distribution of the nanomaterials in the matrix. The authors see the cause of the increasing porosity of the cement composites under the influence of nano-Fe3O4/SiO2 in an uneven distribution of the nanomaterial in the matrix. This refers particularly to the specimens of P3 paste. The agglomerates of the nanoparticles cause a local increase in porosity [36,40]. Due to a very large specific surface area of the nanoparticles, including nano-Fe3O4/SiO2, the growth of the nanoadmixture’s content is accompanied by the weakening of the matrix strengthening as a result of increasing porosity. The lowest porosity has been found for P3 specimens, which was slightly higher, however, than in the case of P0 (without nanoadmixture) specimens. A significant role in the growing porosity of the gel in all of the modified pastes can play the use of a shell made of nano-SiO2. The last researches on the cement composites containing nanosilica admixture show that even when used in small amounts, it can significantly improve the mechanical performance and durability of the composite, mainly by a filler effect and high reactivity of dissolved nanosilica [41,42]. Nanosilica can fill up the empty spaces inside the calcium silicate hydrate (C-S-H) gel. The result of the pozzolanic reaction of nano-SiO2 with calcium hydroxide (CH) is increase of the amount of C-S-H, which leads to the changes in the pores structure in the matrix. The results of testing of porosity of cement composites with nano-SiO2 admixture are not unequivocal tough. The MIP investigations of these composites [43–45] have proved that the total porosity distinctly diminishes, when cement is substituted with nanosilica. The other tests [46– 49] showed, however, that the total porosity of the composite not necessarily is changing when nanosilica is added, but the larger (capillary) pores, occurring originally in the cement matrix, are divided into the smaller (gel) pores. The research presented in [45,47,50] show that nanosilica increases porosity of the gel. In the MIP investigations of the cement pastes containing nano-SiO2 [51], however, a decrease of the gel porosity from 9.37% to 7.13% was observed. This downfall is explained by the authors [51] Fig. 6. Pore volume in tested pastes (MIP). E. Horszczaruk et al. / Construction and Building Materials 251 (2020) 118945 by a relatively large content of nanosilica in the paste (5% of the cement mass), which can lead to a bigger filler effect and decrease of the total porosity as compared to the pozzolanic effect, where additional C-S-H and gel pores are produced. A few studies on the influence of nano-Fe3O4 on the structure and porosity of the cement pastes show a favourable effect of this nanoadmixture [33,36,53]. A positive impact of nano-Fe3O4 on the total porosity of the composites and decreasing of the average pore diameter has been reported in [33]. In the carried out researches, the MIP analysis provided some evidences of the changes in the pores structure (i.e. pores diameters, distribution of the pores size, density), caused by adding nano-Fe3O4/SiO2. An increase of porosity of the tested pastes with increasing content of nanoparticles has been found, which did not lead, however, to the worsening of mechanical performance of the modified composites, as an increase in Young modulus was observed. The obtained results are not sufficient, however, for characterization of the actual changes. The MIP method does not provide an information on the actual shape and size of the pores in the tested composite. As it has been showed in the investigation reported in [51], the method can cause an overestimation of the small pores frequency. The AFM method has confirmed the changes in the porosity of the tested composites, observed when using MIP method, and provided more detailed information on the cement matrix structure in the tested pastes. More information about the changes in shape, volume and distribution of the pores in the cement matrix under the influence of nanoparticles with different morphology can be brought by the methods based on nano-CT technique [54] and the use of scanning electron microscopy (SEM) coupled with focused ion beam (FIB), so-called dual beam SEM/FIB [54]. The latter allows to describe the pores with equivalent diameters above 10 nm. The mean values of Young’s modulus obtained by the nanoindentation method were significantly higher than those obtained by AFM method. However, in both methods the best results for the pastes modified with nano-Fe3O4/SiO2 were obtained for the paste P3. Addition of the nanoadmixture has affected significantly the growth of porosity of the tested pastes. Together with increasing content of nano-Fe3O4/SiO2 in the composite, the volume of CH/C-S-H phase and its stiffness also increases. The average values of the Young”s modulus, obtained using AFM method, point to the firm predominance of MP phase in all tested samples. However, in both methods the best results for the pastes modified with nano-Fe3O4/SiO2 were obtained for the paste P3. Addition of the nanoadmixture has caused the significant growth of porosity of the tested pastes. Together with increasing content of nano-Fe3O4/SiO2 in the composite, the volume of CH/C-S-H phase and its stiffness also increases. The discrepancies in the results of testing of the Young’s modulus when using the AFM and nanoindentation are caused, among others, by packing density of C–S–H particles [55]. The values of the Young’s modulus obtained using AFM method are possibly affected also by the method of sample preparation. In the presented research no impregnation was applied for the samples surface treatment. When using the AFM technique, the epoxy resins are often used for the samples impregnation [56,57]. The epoxy can penetrate into the pores in the cement paste, affecting the topographic properties measured by AFM [58]. Considering the use of the AFM method for testing the mechanical nanoproperties of the cement composites in the future, it should be favourable to increase the tested surface of the single sample, e.g. to 80  80 mm. However, the consequence would be the extending of the single test time to more than 24 h. Peak force tapping in AFM with quantitative nanomechanical analysis ensures the tool for evaluation of the nanomechanical properties of the multiphase cement composites. 7 4. Conclusions The influence of the admixture of nano-Fe3O4/SiO2 on the structure and mechanical properties of the cement pastes was investigated in this research. The main findings are as follows: 1. The results of tests with the use of various techniques has shown the similar tendency in the changes of the cement matrix microstructure under the influence of the various amounts of the admixture of nano-Fe3O4/SiO2. This was confirmed by the mean values of Young’s modulus obtained in AFM testing and by nanoindentation, although the absolute values of the measured moduli are not the same. The reason for the increase of the porosity of the tested pastes under the influence of the nanoadmixture can also be the local creation of the nanoparticles agglomerates, resulted from their uneven distribution inside the cement matrix. 2. The MIP analysis provided some evidences of the changes in the pores structure, caused by adding nano-Fe3O4/SiO2. An increase in the porosity of the tested pastes modified with nanoadmixture has been found out, however, it was not proportional to the admixture content. P3 specimens had the lowest porosity as compared to the other pastes with nanoadmixture. The added nanoparticles, in spite of increasing of the total porosity of the composite, became the nuclei for the C-S-H phase formation, which improved the mechanical performance of the modified pastes. An increase in Young modulus was observed for the specimens modified with nano-Fe3O4/SiO2 as compared to the pastes without nanoadmixture (P0). The investigation of the pastes structure by AFM method showed that together with increasing content of nano-Fe3O4/SiO2 in the composite, the volume of CH/C-S-H phase and its mean Young modulus also increases. In the case of 5% content of nano-Fe3O4/SiO2 in the composite, 4.5-fold higher fraction of CH/C-S-H phase in the matrix was observed as compared to the unmodified paste. 3. A complex structure of the nanoparticles Fe3O4/SiO2, used for the modification of the cement pastes, has affected significantly the porosity and mechanical properties of the tested composites. The carried out investigation revealed the influence of the shell made of nano-SiO2 on the pores structure in the composite. The use of AFM method allowed to extend the characteristics of the changes in the cement gel structure, caused by the used nanoparticles. 4. The use of various test methods is necessary in the future investigations for the detailed description of the phenomena taking place in the cement composites under the influence of nanoFe3O4/SiO2 and the changes in the pores structure. These methods should allow to complete the data in the aforementioned range, particularly regarding to the nanopores. CRediT authorship contribution statement Elzbieta Horszczaruk: Conceptualization, Methodology, Writing - original draft, Writing - review & editing, Project administration. Małgorzata Aleksandrzak: Methodology, Investigation, Formal analysis. Krzysztof Cendrowski: Methodology, Investigation, Formal analysis. Roman Je˛drzejewski: Methodology, Investigation, Formal analysis. Jolanta Baranowska: Supervision. Ewa Mijowska: Supervision. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. 8 E. Horszczaruk et al. / Construction and Building Materials 251 (2020) 118945 Acknowledgment This research was supported by the National Science Centre of Poland within the project No. 2014/13/B/ST8/03875 (OPUS 7). References [1] N.A. Nematpour Keshteli, M. Sheikholeslami, Nanoparticle enhanced PCM applications for intensification of thermal performance in building: a review, J. Mol. Liq. 274 (2019) 516–533. [2] Z. Li, S. Ding, X. Yu, B. Han, J. Ou, Multifunctional cementitious composites modified with nano titanium dioxide: a review, Compos. A 111 (2018) 115– 137. [3] J. Chen, Y. Qiu, D. Han, Lau: Piezoelectric materials for sustainable building structures: fundamentals and applications, Renew. Sustain. 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