Modification of Cement Matrix with Complex Additive Based On

applied sciences

Article Modiﬁcation of Cement Matrix with Complex Additive Based on Chrysotyl Nanoﬁbers and Carbon Black

Zarina Saidova 1, Grigory Yakovlev 1, Olga Smirnova 2,* , Anastasiya Gordina 1 and Natalia Kuzmina 1

1 Department of Construction Materials, Mechanization and Geotechnics, Kalashnikov Izhevsk State Technical University, Studencheskaya Str. 7, 426069 Izhevsk, Russia; [email protected] (Z.S.); [email protected] (G.Y.); [email protected] (A.G.); [email protected] (N.K.) 2 Department of Constructing Mining Enterprises and Underground Structures, Saint-Petersburg Mining University, 21-st Line V.O., 2, 199106 Saint-Petersburg, Russia * Correspondence: [email protected]

Abstract: This paper presents the results of studying the properties of cement-based composites modified with a complex additive based on chrysotile nanofibers and carbon black. The optimal composition of complex additive was stated due to the particle size analysis of suspensions with different chrysotile to carbon black ratios and the mechanical properties study of the fine-grained concrete modified with the complex additive. It was found that the addition of chrysotile in the amount of 0.05% of cement mass together with carbon black in the amount of 0.01% of cement mass leads to a 31.9% compression strength increase of cement composite and a 26.7% flexural strength increase. In order to explain the change in the mechanical properties of the material, physical and

chemical testing methods were used including IR-spectral analysis, differential thermal analysis, energy dispersive X-ray analysis as well as the study of the microstructure of the samples modiﬁed

Citation: Saidova, Z.; Yakovlev, G.; with the complex additive. They revealed the formation of durable hydration products including Smirnova, O.; Gordina, A.; Kuzmina, thaumasite and calcium silicate hydrates of lower basicity that form a dense structure of cement N. Modification of Cement Matrix matrix, increasing the physical and mechanical characteristics of cement-based composites. with Complex Additive Based on Chrysotyl Nanofibers and Carbon Keywords: cement matrix; chrysotile nanofibers; carbon black; thaumasite; hydration products Black. Appl. Sci. 2021, 11, 6943. https://doi.org/10.3390/app11156943

Academic Editor: Joan Formosa 1. Introduction Mitjans Cement mortar and concrete are among the most widely used, durable and reliable materials in modern construction; therefore, improving their structure and properties has Received: 13 June 2021 always been one of the main priorities of construction materials science. Currently, there is Accepted: 14 July 2021 a wide variety of technical possibilities for modifying the cement-based composites and Published: 28 July 2021 improving their physical and mechanical characteristics. These include decreasing the water-to-cement ratio [1,2], increasing the binder fineness [3,4], using plasticizers [5,6], Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in varying the hardening conditions [7], introducing the various types of fiber [8–10], etc. published maps and institutional affil- Moreover, many researchers around the world have claimed that the application of nano- iations. sized additives can significantly improve the properties of the cement-based materials by affecting the processes of their structural formation [11–13]. In practice, the creation of favorable conditions for the effective hydration of Portland cement is achieved by adding the additives that have high activity due to the large surface area and the ability to compact the cement matrix structure [14,15]. The presence of Copyright: © 2021 by the authors. nano-sized particles in the hardening mineral matrix stimulates the formation of the layer Licensee MDPI, Basel, Switzerland. This article is an open access article of hydration products on their surface. Here, the further recrystallization of hydration distributed under the terms and products into larger crystal blocks is limited due to the high surface energy of nanoparticles. conditions of the Creative Commons This provides the conditions for creating the high-density and defect-free structure that Attribution (CC BY) license (https:// unites the conglomerate and gives it high density and strength. Furthermore, it rearranges creativecommons.org/licenses/by/ the pore structure of the cement gel and ettringite towards smaller sized pores and promotes 4.0/).

Appl. Sci. 2021, 11, 6943. https://doi.org/10.3390/app11156943 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, 6943 2 of 12

the intensive formation of lower based calcium silicate hydrates [16] that increase the strength of cement matrix. Recently, carbon-based nanomaterials such as carbon black, isostatic graphite, graphene oxide, graphene nanoplates, carbon nanotubes and fibers have gained wide popularity in the field of construction composites modification [17–19]. In addition, many researchers believe that a promising direction in the modification of concretes and mortars is the use of nano-dispersed oxide systems SiO2, Al2O3, Fe2O3, CaO that are similar in composition and structure to the products of cement hydration [20–22]. An example of such materials can be micro- and nanosilica, metakaolin fly ash, granulated blast furnace slag as well as synthetic additives—fumed silica, nanosilica, colloidal silica, etc. [23–26]. At the same time, it is necessary to strive for the maximum reduction of the material cost while increasing the physical and mechanical characteristics. The cost of using carbon and synthetic silica-containing nano-sized materials, even considering their very low amount, is several times higher than the cost of natural silica-based additives as well as the wastes from various industries. The combined use of additives of different genesis can also be preferable as the mechanism of their influence on the processes of structural formation of the material has different natures [27–29]. It creates their synergistic effect on the processes of cement hydration and hardening, leading to the creation of composites with unique physical and mechanical characteristics. Namely, silicon-based additives introduced into the composition of the material are able to bind calcium hydroxide into low-basic calcium silicate hydrates C-S-H, which are characterized by increased strength. At the same time, carbon nanoparticles can change the morphology of cement hydration products, contributing to the compaction of the structure, which, in turn, leads to a strength increase [30–32]. Thereby, the purpose of this study is to develop a complex additive that combines both carbon nanoparticles and a silicon-based additive. The criteria for choosing the dispersed component were the ability to change the structure of the cement matrix, the possibility of stabilization in an aqueous medium with the use of surfactants and availability on the market. Dispersions of chrysotile nanofibers and carbon black were taken as the basis for the complex additive. Health safety that is currently limiting the use of chrysotile fibers in building materials is ensured by the chemically bound state of chrysotile fibers with cement in finished products, due to which the consumers are not exposed to direct contact with chrysotile fibers. In addition, the shape of chrysotile fibers allows their removal from the human lungs naturally in the process of breathing as their structure drastically differs from the structure of amphibole group asbestos, which is mainly known to cause cancer. Moreover, extremely small concentrations of chrysotile that are used for the modification of the composite materials prevent the negative influence on human health. Producers of such dispersions, however, should strictly follow state regulations on the working conditions with hazardous materials.

2. Materials and Methods Portland cement CEM I 32.5 N produced by Timlyui Cement Plant LLC was used as a binder. Natural river sand was used as a fine aggregate obtained from sand deposit of the Kama river (Novy village, Udmurtia, Russia) with the size modulus equal to 2.0. The fine aggregate-to-cement ratio was 3:1. The water-to-cement ratio was 0.45. Suspensions of chrysotile nanofibers and carbon black were added into the cement– sand mortars together with mixing water. In order to stabilize the suspensions of chrysotile fibers and prevent the re-agglomeration of ultrafine particles, the C-3 superplasticizer was used that is produced on the basis of naphthalene sulfonic acid and formaldehyde. Chrysotile is a natural mineral of the serpentine group which can be chemically described as a hydrous magnesium silicate with the theoretical composition corresponding to 3MgO·2SiO2·2H2O. In this research, chrysotile was added into cement–sand mortars in the form of an aqueous suspension that was prepared by mixing the chrysotile fibers in the Appl. Sci. 2021, 11, 6943 3 of 12

Appl. Sci. 2021, 11, x FOR PEER REVIEW 3 of 11 Appl. Sci. 2021, 11, x FOR PEER REVIEW 3 of 11 amount of 10% of the total suspension volume with the C-3 superplasticizer in the amount the amount of 10% of the total suspension volume with the C-3 superplasticizer in the theof amount 2% by water of 10% mass of the using total the suspension cavitation vo disperser.lume with The the study C-3 ofsuperplasticizer the microstructure in the of amount of 2% by water mass using the cavitation disperser. The study of the micro- amountthe chrysotile of 2% by fibers water (Figure mass1 a)using shows the that cavi thetation diameter disperser. of individual The study fibers of the is aroundmicro- structure of the chrysotile fibers (Figure 1a) shows that the diameter of individual fibers structure30–50 nm of, whereasthe chrysotile the length fibers of (Figure the fibers 1a) variesshows significantly.that the diameter The pattern of individual of the particlefibers is around 30–50 nm, whereas the length of the fibers varies significantly. The pattern of is sizearound distribution 30–50 nm, curve whereas of the the aqueous length suspension of the fibers of varies chrysotile significantly. fibers (Figure The1 patternb) showed of the particle size distribution curve of the aqueous suspension of chrysotile fibers (Figure thethat particle the average size distribution particle diameter curve of varied the aq fromueous 0.01 suspensionµm to 0.2 ofµm. chrysotile The average fibers value (Figure was 1b) showed that the average particle diameter varied from 0.01 μm to 0.2 μm. The aver- 1b)equal showed to 0.046 thatµ them. average particle diameter varied from 0.01 μm to 0.2 μm. The average value was equal to 0.046 μm. age value was equal to 0.046 μm.

(а) (b) (а) (b) Figure 1. Suspension of chrysotile fibers: (a) microstructure of chrysotile fibers at 20,000-fold magnification; (b) distribu- FigureFigure 1. 1.SuspensionSuspension of of chrysotile chrysotile fibers: fibers: ( (aa)) microstructure microstructure of of chrysotile fibersfibers atat 20,000-fold20,000-fold magnification; magnification; ( b(b)) distribution distribution of chrysotile particles after processing in the cavitation disperser. tionof chrysotileof chrysotile particles particles after after processing processing in thein the cavitation cavitation disperser. disperser. Carbon black is a powdery product obtained in the process of controlled pyrolysis or CarbonCarbon black black is isa powdery a powdery product product obtained obtained in the in theprocess process of controlled of controlled pyrolysis pyrolysis or thermal oxidative decomposition of liquid or gaseous hydrocarbons. The elementary thermalor thermal oxidative oxidative decomposition decomposition of liquid of liquid or orgaseous gaseous hydrocarbons. hydrocarbons. The The elementary elementary composition of this synthetic material is represented by carbon (88.6–93.7%), hydrogen compositioncomposition of of this this synthetic synthetic material material is isre representedpresented by by carbon carbon (88.6–93.7%), (88.6–93.7%), hydrogen hydrogen (0.7–0.8%) and oxygen (5.5–10.5). The particle size of carbon black (Figure 2) was found to (0.7–0.8%)(0.7–0.8%) and and oxygen oxygen (5.5–10.5). (5.5–10.5). The The particle particle size size of of carbon carbon black black (Figure (Figure 2)2 )was was found found to to be 30–120 nm. bebe 30–120 30–120 nm. nm.

(а) (b) (а) (b) Figure 2. Carbon black particles: (a) microstructure at 50,000-fold magnification, (b) particle size distribution. Figure 2. Carbon black particles: (a) microstructure at 50,000-fold magnification, (b) particle size distribution. Figure 2. Carbon black particles: (a) microstructure at 50,000-fold magniﬁcation, (b) particle size distribution. In order to determine the properties of complex additive as well as the modified In order to determine the properties of complex additive as well as the modified cement compositeIn order toa comprehensive determine the propertiesstudy was carried of complex out using additive the following as well as methods: the modiﬁed cementcement composite composite a acomprehensive comprehensive study study was was carried carried out out using using the the following following methods: methods: - particle size distribution analysis using the laser dispersion on a SALD-7500nano - particle size distribution analysis using the laser dispersion on a SALD-7500nano -analyzerparticle manufactured size distribution by Shimadzu analysis with using the the 7 nm laser to dispersion800 μm measurement on a SALD-7500nano range; analyzer manufactured by Shimadzu with the 7 nm to 800 μm measurement range; - mechanicalanalyzer tests manufactured (compressive by Shimadzu and flexur withal strength) the 7 nm toon 800 hydraulicµm measurement press model range; - mechanical tests (compressive and flexural strength) on hydraulic press model PGM-100 MG4-A with the maximum load of 100 kN and loading rate of 0.5 MPa per PGM-100 MG4-A with the maximum load of 100 kN and loading rate of 0.5 MPa per second; second; Appl. Sci. 2021, 11, 6943 4 of 12

Appl. Sci. 2021, 11, x FOR PEER REVIEW- mechanical tests (compressive and flexural strength) on hydraulic press model PGM-1004 of 11 MG4-A with the maximum load of 100 kN and loading rate of 0.5 MPa per second; -‐ differentialdifferential thermalthermal analysisanalysis ofof cementcement compositescomposites onon a a TGA/DSC1 TGA/DSC1 StarStar systemsystem manufacturedmanufactured by by Mettler Mettler ToledoToledo in in the the temperature temperature rangerange from from 60 ◦ C60 to °C 1100 to ◦1100C with °C thewith heating the heating rate of rate 30 ◦ofC/min; 30 °C/min; -‐ IR-spectralIR‐spectral analysisanalysis ofof cementcement compositescomposites onon ShimadzuShimadzu IRAfﬁnity-1IRAffinity‐1 spectrometerspectrometer in in thethe frequencyfrequency rangerange 400–4000400–4000 cmcm−−11;; -‐ microstructuremicrostructure analysisanalysis andand energyenergy dispersivedispersive X-rayX‐ray analysisanalysis (EDX)(EDX) onon scanningscanning electronelectron microscopemicroscope QuattroQuattro ESEM ESEM ThermoThermo Fisher Fisher ScientiﬁcScientific withwith the the resolution resolution up up to 0.8to 0.8 nm.nm. TheThe cementcement compositecomposite samples samples were were tested tested at at the the age age of of 1 1 day day after after steam steam curing. curing.

3.3. ResultsResults andand DiscussionDiscussion TheThe optimaloptimal ratioratio ofof chrysotilechrysotile nanofibersnanofibers andand carboncarbon blackblack inin the the composition composition ofof a a complexcomplex additiveadditive waswas statedstated basedbased onon thethe particleparticle dispersiondispersion levellevel inin the the suspension suspension as as wellwell as as mechanical mechanical properties properties of of the the modified modified cement cement matrix. matrix. Primarily,Primarily, thethe ratioratio of of chrysotile chrysotile to carbonto carbon black black by massby mass was was taken taken equalequal to 10:1, to 7.5:1, 10:1, 5:1,7.5:1, 2.5:1 5:1, and 2.5:1 1:1. and The 1:1. chrysotileThe chrysotile amountamount in the suspension in the suspension was 10%was of the10% full of the suspensionfull sus‐ mass.pension Higher mass. concentrations Higher concentrations lead to thelead fiber to agglomeration the fiber agglomeration that made itthat hardermade to it provide harder theto provide uniformthe particleuniform distributionparticle indistribution cement composite in cement thatcomposite adversely that affected adversely the properties affected ofthe the properties material of [33 the]. The material particle [33]. size The distribution particle size analysis distribution of the analysis complex of additive the complex was carriedadditive outwas on carried Shimadzuout SALD-7500nanoon Shimadzu SALD laser‐7500nano analyzer.laser The analyzer. results areThe presented results are in Figurepresented3. in Figure 3.

Chrysotile and carbon Particle size, μm black ratio 10:1 0.068 7.5:1 0.070 5:1 0.065 2.5:1 0.072 1:1 0.083

(а) (b)

FigureFigure 3. 3.Particle Particle size size distributiondistribution in in the the complex complex additive: additive: (a) at(a) different at different additivesadditives ratios;ratios; (b) at ( chrysotileb) at chrysotile to carbonto carbon black ratioblack equal ratio to equal 5:1. to 5:1.

TheThe highesthighest dispersiondispersion levellevel ofof thethe complexcomplex additiveadditive waswas obtainedobtained atat thethe ratioratio ofof chrysotilechrysotile fibersfibers to to carbon carbon black black equal equal to 5:1.to 5:1. In thisIn this casecase the averagethe average particleparticle diameter diameter was around 65 nm. It should be noted that carbon black particles do not significantly change was around 65 nm. It should be noted that carbon black particles do not significantly the size of agglomerates in the complex additive apparently due to the stabilizing effect of change the size of agglomerates in the complex additive apparently due to the stabilizing another component [19,34,35] in this case the chrysotile fibers. effect of another component [19,34,35] in this case the chrysotile fibers. Thus, it is hard to make specific conclusions on the effect of additives on the properties Thus, it is hard to make specific conclusions on the effect of additives on the prop‐ of cement composites even considering the sufficient clarity of the results of the particle erties of cement composites even considering the sufficient clarity of the results of the size analysis of the complex additive since the dispersion levels vary insignificantly. In this particle size analysis of the complex additive since the dispersion levels vary insignifi‐ case, the most reliable option for stating the optimal amount of additives that provides cantly. In this case, the most reliable option for stating the optimal amount of additives the maximum increase in strength of cement composites is the preparation of standard that provides the maximum increase in strength of cement composites is the preparation samples and their mechanical testing. of standard samples and their mechanical testing. The mechanical tests of samples modified with complex additives containing the differentThe chrysotile mechanical to carbontests of black samples ratios modified were carried with out complex on fine-grainedadditives concretecontaining beams the withdifferent the standardchrysotile dimensions to carbon of black 40 × 40ratios× 160 were mm carried at the age out of on 1 day fine after‐grained steam concrete curing. Tobeams determinewith the the standard strength, dimensions a series of of samples 40 × 40 was × 160 prepared, mm at the containing age of 1 day three after samplessteam curing. To determine the strength, a series of samples was prepared, containing three samples of each composition. The average compression and flexural strength values ob‐ tained via statistical analysis of the mechanical test results are indicated in Table 1. In all cases, the deviation from the average value did not exceed 10%. Appl. Sci. 2021, 11, 6943 5 of 12

of each composition. The average compression and ﬂexural strength values obtained via statistical analysis of the mechanical test results are indicated in Table1. In all cases, the deviation from the average value did not exceed 10%.

Table 1. Compressive and flexural strength of samples at various ratios of components in complex additive.

Compressive Strength, MPa Flexural Strength, MPa Chrysotile Carbon Black Amount, % Carbon Black Amount, % Amount, % 0 0.005 0.01 0.02 0.05 0.1 0 0.005 0.01 0.02 0.05 0.1 0 27.6 30.7 29.9 30.3 27.8 27.4 4.39 4.45 4.76 4.82 4.42 4.51 0.01 28.2 31.0 30.7 30.8 31.5 30.1 4.84 4.73 4.72 4.90 4.87 4.79 0.025 29.7 31.3 31.4 32.8 31.5 30.2 4.79 4.98 4.87 5.22 4.98 4.80 0.05 32.3 34.2 36.4 35.4 33.2 30.0 4.58 5.12 5.56 5.48 5.29 4.77 0.075 30.9 34.6 33.9 33.1 32.3 28.7 4.57 4.91 5.17 5.11 4.84 4.56 0.1 29.7 30.9 30.2 29.9 28.3 28.2 4.51 4.60 4.42 4.58 4.42 4.41

As can be seen from Table1, the maximum increase of concrete strength is achieved with the combined addition of chrysotile fibers in the amount of 0.05 % by cement mass and carbon black in the amount of 0.01% by cement mass. In this case, the compressive and flexural strengths of composite increased by up to 31.9% and 26.7%, respectively. A further increase of the amount of chrysotile fibers leads to a strength decrease, supposedly due to the tendency of fibers to re-agglomerate due to the action of van der Waals forces. These local fiber agglomerates are held together only by the forces of intermolecular interaction and can promote the destruction of the material. In order to interpret the results obtained during physical and mechanical tests, the chemical analysis of concrete samples was carried out as well as the microstructure study. IR spectral analysis of cement matrices of the reference composition (Figure4a), compositions modified with carbon black in the amount of 0.02% by the cement weight (Figure4b) and chrysotile fibers in the amount of 0.05% by the cement weight (Figure4c), as well as the composition modified with the complex additive based on chrysotile and carbon black (Figure4d) was carried out using Shimadzu IRAffinity-1 spectrometer in the frequency range 400–4000 cm−1. Significant change of the IR spectrum of the sample, modified with the complex additive can be observed compared to spectrum of the reference sample and the spectra of the samples, modified with carbon black in the amount of 0.01% by the cement weight and chrysotile fibers in the amount of 0.05% by the cement weight. In the spectra of the samples, the intensity of the absorption line corresponding to calcium silicate hydrates decreases in the frequency range of 1008.7 cm−1 and a shift of the absorption lines is noted from 1089.78 cm−1 to 1080.14 cm−1 as well as from 1008.77 cm−1 to 993.34 cm−1. A change in the character of a doublet with a shift of the maximum peak to a lower frequency region can be caused by a change in the basicity of calcium silicate hydrates predetermining the increase of cement matrix strength. In addition, the IR analysis of the reference sample (Figure4a) showed the presence of OH- hydroxyl group (3421.72 cm−1) and chemically bound water (1662.24 cm−1) that in the case of composite modification with the complex additive were taken over by multiple peaks in the frequency range 3400–3900cm−1. Here, the decrease of the amount of free water (3800–3400 cm−1 and 1653.00 cm−1) can be explained by a different degree of OH- hydroxyl group bonding in calcium silicate hydrates as well as a decrease in the total amount of free calcium hydroxide due to its replacement by calcium silicate hydrates. It can be noted that the maximum change in the IR spectra curve is observed when the additives are added into the composition together, which evidences their synergetic effect. Appl. Sci. 2021, 11, 6943 6 of 12

Appl. Sci. 2021, 11, x FOR PEER REVIEW 6 of 11

(а) (b)

(c) (d) Figure 4. IR spectra of: (a) reference sample, (b) sample modified with the carbon black in the amount of 0.01% by cement Figure 4. IR spectra of: (a) reference sample, (b) sample modified with the carbon black in the amount of 0.01% by cement mass, (c) sample modified with chrysotile dispersion in the amount of 0.05% by cement mass, (d) sample modified with mass,the complex (c) sample additive. modified with chrysotile dispersion in the amount of 0.05% by cement mass, (d) sample modified with the complex additive. SignificantThermal analysis change of of the the reference IR spectrum sample of and the the sample, sample modified modified with with the the complex additive basedcan be on observed carbon blackcompared and chrysotile to spectrum fibers of werethe reference carried out sample on a TGA/DSC1and the spectra Star ofsystem the samples, derivatograph. modified The with results carbon of theblack study in the are amount presented of 0.01% in Figure by 5thea,b. cement weight Appl. Sci. 2021, 11, x FOR PEER REVIEWand chrysotile fibers in the amount of 0.05% by the cement weight. In the spectra 7of of the 11 samples, the intensity of the absorption line corresponding to calcium silicate hydrates decreases in the frequency range of 1008.7 cm−1 and a shift of the absorption lines is noted from 1089.78 cm−1 to 1080.14 cm−1 as well as from 1008.77 cm−1 to 993.34 cm−1. A change in the character of a doublet with a shift of the maximum peak to a lower frequency region can be caused by a change in the basicity of calcium silicate hydrates predetermining the increase of cement matrix strength. In addition, the IR analysis of the reference sample (Figure 4a) showed the presence of OH- hydroxyl group (3421.72 cm−1) and chemically bound water (1662.24 cm−1) that in the case of composite modification with the complex additive were taken over by multiple peaks in the frequency range 3400–3900cm−1. Here, the decrease of the amount of free water (3800–3400 cm−1 and 1653.00 cm−1) can be explained by a different degree of OH- hydroxyl group bonding in calcium silicate hydrates as well as a decrease in the total amount of free calcium hydroxide due to its replacement by calcium silicate hydrates. It can be noted that the maximum change in the IR spectra curve is observed when the ad- (а) (b) ditives are added into the composition together, which evidences their synergetic effect. Figure 5. DSC andThermal TGA spectrum analysis of of samples: the reference (a) reference sample sample, and (theb) modified sample modifiedsample. with the complex Figure 5. DSC and TGA spectrum of samples: (a) reference sample, (b) modified sample. additive based on carbon black and chrysotile fibers were carried out on a TGA/DSC1 Star systemThe TGA derivatograph. curves show Thea change results in ofthe the intens studyity are of masspresented loss ofin theFigure sample 5a,b. modified with the complex additive. When the samples were heated from 90 to 200 °C, the mass loss was equal to 4.64% for the control sample and 3.26% for the sample modified with the complex additive. With further heating up to 465 °C, the control sample lost 1.74% of its original weight, whereas the modified sample lost only 1.15%. Later on, in the temperature range of 650–750 °C, the mass loss of the samples was calculated to be 2.58% to 1.61% for the reference and modified samples, respectively. This additionally confirms the higher degree of OH- hydroxyl group binding in calcium silicate hydrates as well as a decrease of the total amount of free calcium hydroxide due to its replacement by C-S-H. To assess the influence of the complex additive on the morphology of hydration products in the cement composites, the microstructural analysis of samples was carried out using the Thermo Fisher Scientific Quattro S microscope. Figure 6 shows a significant change of the micromorphology of cement matrix modified with the complex additive based on carbon black in the amount of 0.01% by the cement mass and chrysotile fibers in the amount of 0.05% by cement mass, resulting in the replacement of the porous structure of the cement hydration products by the gel phase of calcium silicate hydrates C-S-H. In this case, the addition of the complex additive contributes to the formation of a dense microstructure, creating cement composites with increased strength.

(а) (b) Figure 6. Microstructure of cement composite at 1000-fold magnification: (a) reference sample, (b) modified sample.

The microstructure of the sample in Figure 7 clearly shows that chrysotile nanotubes are evenly distributed among the phases of calcium silicate hydrates and their surface is fully covered with cement hydration products. This confirms the tendency of new formations to grow along the modifier fibers and evidences for the high adhesion between Appl. Sci. 2021, 11, x FOR PEER REVIEW 7 of 11

(а) (b) Appl. Sci. 2021, 11, 6943 7 of 12 Figure 5. DSC and TGA spectrum of samples: (a) reference sample, (b) modified sample.

The TGA curves show a change in the intensity of mass loss of the sample modified with theThe complex TGA curves additive. show When a change the in samples the intensity were of heated mass lossfrom of 90 the to sample 200 °C, modified the mass with the complex additive. When the samples were heated from 90 to 200 ◦C, the mass loss was equal to 4.64% for the control sample and 3.26% for the sample modified with loss was equal to 4.64% for the control sample and 3.26% for the sample modified with the the complex additive. With further heating up to 465 °C, the control sample lost 1.74% of complex additive. With further heating up to 465 ◦C, the control sample lost 1.74% of its itsoriginal original weight, weight, whereas whereas the modifiedthe modified sample sample lost only lost 1.15%. only Later1.15%. on, Later in the on, temperature in the tem- peraturerange of range 650–750 of 650–750◦C, the mass °C, the loss mass of the loss samples of the was samples calculated was tocalculated be 2.58% to 1.61%be 2.58% for to 1.61%the reference for the reference and modified and samples,modified respectively. samples, respectively. This additionally This additionally confirms the higherconfirms thedegree higher of degree OH− hydroxyl of OH- hydroxyl group binding group in binding calcium in silicate calcium hydrates silicate as hydrates well as a as decrease well as a decreaseof the total of the amount total ofamount free calcium of free hydroxide calcium hydroxide due to its replacementdue to its replacement by C-S-H. by C-S-H. ToTo assess assess the the influence influence of thethe complexcomplex additiveadditive on on the the morphology morphology of of hydration hydration productsproducts in in the the cement cement composites, composites, thethe microstructuralmicrostructural analysis analysis of of samples samples was was carried carried outout using using the the Thermo Thermo Fisher Fisher Scientific Scientific QuattroQuattro S S microscope. microscope. Figure Figure6 shows 6 shows a significant a significant changechange of of the the micromorphology micromorphology of cementcement matrixmatrix modified modified with with the the complex complex additive additive basedbased on on carbon carbon black black in in the the amount amount of 0.01%0.01% byby thethe cement cement mass mass and and chrysotile chrysotile fibers fibers in in thethe amount amount of of 0.05% 0.05% by by cement cement mass, mass, resultingresulting inin thethe replacement replacement of of the the porous porous structure structure ofof the the cement cement hydration hydration products products by thethe gelgel phasephase of of calcium calcium silicate silicate hydrates hydrates C-S-H. C-S-H. In In this case, the addition of the complex additive contributes to the formation of a dense this case, the addition of the complex additive contributes to the formation of a dense microstructure, creating cement composites with increased strength. microstructure, creating cement composites with increased strength.

(а) (b) Figure 6. Microstructure of cement composite at 1000-fold magnification: (a) reference sample, (b) modified sample. Figure 6. Microstructure of cement composite at 1000-fold magniﬁcation: (a) reference sample, (b) modiﬁed sample.

TheThe microstructure microstructure of of the the sample inin FigureFigure7 7clearly clearly shows shows that that chrysotile chrysotile nanotubes nanotubes areare evenly evenly distributed distributed among among th thee phases phases of of calcium calcium silicate silicate hydrates and their surfacesurface is fullyis fully covered covered with with cement cement hydration hydration products products.. This This confirms confirms the the tendency tendency of ofnew new for- mationsformations to grow to grow along along the the modi modifierfier fibers fibers and and evidences evidences forfor thethe high high adhesion adhesion between between chrysotile fibers and cement hydration products. The location of fibers in the pore space as well as the formation of hydration products on their surface contribute to a change in the pore structure of cement matrix increasing its density and leading to an improvement of mechanical characteristics of material. X-ray microanalysis of crystalline hydrates formed on the surface of chrysotile fibers (Figure8) confirmed that the formations belong to calcium silicate hydrates. Moreover, the SEM of the cement matrix modified with the complex additive (Figure9) shows that the main hydration products are presented by calcium hydroxide, C-S-H and spherical formations about 100 nm in diameter. The EDX analysis was carried out on samples modified with the complex additive in order to study the elemental composition of the observed cement hydration products (Figure 10). Appl. Sci. 2021, 11, x FOR PEER REVIEW 8 of 11

chrysotile fibers and cement hydration products. The location of fibers in the pore space as well as the formation of hydration products on their surface contribute to a change in the pore structure of cement matrix increasing its density and leading to an improvement of mechanical characteristics of material.

Appl. Sci. 2021, 11, x FOR PEER REVIEW 8 of 11

chrysotile fibers and cement hydration products. The location of fibers in the pore space Appl. Sci. 2021, 11, 6943 as well as the formation of hydration products on their surface contribute to a change8 of 12 in the pore structure of cement matrix increasing its density and leading to an improvement of mechanical characteristics of material.

(а) (b) Figure 7. Microstructure of cement matrix modified with the complex additive: (a) at 14000-fold magnification, (b) at 70,000-fold magnification. (а) (b) Figure 7. MicrostructureX-ray microanalysis of cement matrix of crystalline modified with hydrates the complex formed additive: on (a) theat 14000-fold surface magnification, of chrysotile (b) fibersat Figure 7. Microstructure of cement matrix modified with the complex additive: (a) at 14000-fold magnification, (b) at 70,000-fold magnification. 70,000-fold(Figure magnification. 8) confirmed that the formations belong to calcium silicate hydrates. X-ray microanalysis of crystalline hydrates formed on the surface of chrysotile fibers (Figure 8) confirmed that the formations belong to calcium silicate hydrates.

Figure 8. X-ray microanalysis of crystalline hydrates on the surface of chrysotile fibers.

Moreover, the SEM of the cement matrix modified with the complex additive (Fig- Figure 8. X-ray microanalysis of crystalline hydrates on the surface of chrysotile ﬁbers. ure 9) shows that the main hydration products are presented by calcium hydroxide, Figure 8. X-ray microanalysis of crystalline hydrates on the surface of chrysotile fibers. C-S-H and spherical formations about 100 nm in diameter. Moreover, the SEM of the cement matrix modified with the complex additive (Fig- ure 9) shows that the main hydration products are presented by calcium hydroxide, C-S-H and spherical formations about 100 nm in diameter. Appl. Sci. 2021, 11, x FOR PEER REVIEW 9 of 11

Appl. Sci. 2021, 11, 6943 9 of 12

Appl. Sci. 2021, 11, x FOR PEER REVIEW 9 of 11

(а) (b) Figure 9. Microstructure of cement matrix modified with the complex additive: (a) at 14000-fold magnification; (b) a fragment at 70,000-fold magnification.

The(а) EDX analysis was carried out on samples modified(b) with the complex additive in Figure 9. Microstructure of cement matrix modified with the complex additive: (a) at 14000-fold magnification; (b) a Figure 9. Microstructureorder ofto cement study matrix the modified elemental with the composition complex additive: of ( athe) at 14000-fold observed magnification; cement (bhydration) a products fragment at 70,000-fold magnification. fragment at 70,000-fold(Figure magnification. 10). The EDX analysis was carried out on samples modified with the complex additive in order to study the elemental composition of the observed cement hydration products (Figure 10).

Figure 10. X-ray microanalysis of spherical hydration products.

Based on X-ray spectral analysis the hydration products of spherical shape can be Figure 10.attributedFigure X-ray 10. microanalysistoX-ray thaumasite microanalysis formations of of spherical spherical (CaSiO hydration hydration3‧CaSO products.4‧CaCO products.3‧15H2O) obtained in the course of the reaction of calcium silicate hydrates (C–S–H) with calcite and unbound sulfate ions Basedor reaction Basedon X-ray of on ettringite X-ray spectral spectral with C–S–Hanalysis analysis and the thecarbonates/bicarbonates hydration hydration products products of spherical[36–39]. of The shapespherical noted can hy- be shape can be drationattributed products to thaumasite of spherical formations shape (CaSiOare mainly3·CaSO formed4·CaCO on3 ·the15H surface2O) obtained of ettringite in the needle course attributedcrystalsof the to reaction thaumasiteand on the of calciumsurface formations of silicate calcium hydrates (CaSiOhydroxide (C–S–H)3‧ CaSOplates. with 4‧CaCO calcite3‧ and15H unbound2O) obtained sulfate in the course of the reactionionsThe or reaction resultsof calcium of of ettringite mechanical silicate with tests, C–S–Hhydrates microstr and carbonates/bicarbonates(C–S–H)ucture studies, with thermalcalcite [36 analysisand–39]. Theunbound and noted IR sulfate ions or reactionspectroscopyhydration of ettringite products prove of that with spherical the C–S–Haddition shape are andof mainlythe carbonates/bicarbonates complex formed additive on the surface based ofon ettringite chrysotile [36–39]. needle and The noted hy- carboncrystals black and onincreases the surface the intensity of calcium of hydroxidethe cement plates. hydration and promotes the formation drationof products a denseThe results structure of ofspherical mechanical of cement shape matrix tests, microstructure contaiare mainlyning thaumasite studies,formed thermalglobular on the analysis hydration surface and products IRof spec- ettringite needle crystalsandtroscopy and calcium on prove the silicate thatsurface thehydrates addition of calciumof a of lower the complex hydroxidebasicity additivethat increases plates. based the on mechanical chrysotile and character- carbon Theisticsblack results of increases the material. of themechanical intensity of thetests, cement microstr hydrationucture and promotes studies, the thermal formation ofanalysis a and IR spectroscopy prove that the addition of the complex additive based on chrysotile and carbon black increases the intensity of the cement hydration and promotes the formation of a dense structure of cement matrix containing thaumasite globular hydration products and calcium silicate hydrates of a lower basicity that increases the mechanical characteristics of the material. Appl. Sci. 2021, 11, 6943 10 of 12

dense structure of cement matrix containing thaumasite globular hydration products and calcium silicate hydrates of a lower basicity that increases the mechanical characteristics of the material.

4. Conclusions The results have confirmed the possibility of the combined use of chrysotile nanofibers and carbon black in order to improve the structure and properties of cement composites. The optimal ratio of the components in the complex additive was stated that ensured the highest degree of dispersion of the system and the maximum increase of mechanical characteristics of material. Based on the experimental data, it was proven that the modification of cement composite with the dispersion of chrysotile fibers in the amount of 0.05% by cement mass and carbon black in the amount of 0.01% by cement mass leads to the increase of compressive strength of the studied cement composites up to 31.9% and a bending strength up to 26.7%. A significant change in hydration of Portland cement was also noted. The study of the microstructure of the modified sample indicated that modification of the cement matrix with complex additive based on chrysotile and carbon black suspensions results in the formation of a dense matrix structure containing globular hydration products (thaumasite) and calcium silicate hydrates of lower basicity contributing to the increase in mechanical characteristics of the material. The perspectives for the development of this topic is the further implementation of the presented ultra-dispersed additives in combination with various superplasticizers in order to study their effect on the rheological characteristics of the material as well as the development of compositions based on other carbon-containing materials, for example, isostatic graphite, which can be promising for the use in modified composites in the range of higher temperatures to create heat- and heat-resistant composite products.

Author Contributions: Conceptualization, G.Y. and Z.S.; methodology, A.G.; software, N.K.; valida- tion, G.Y. and O.S.; formal analysis, O.S.; investigation, G.Y. and Z.S.; resources, Z.S.; data curation, G.Y.; writing—original draft preparation, Z.S. and G.Y.; writing—review and editing, Z.S., G.Y., A.G., N.K. and O.S.; visualization, Z.S., A.G. and N.K.; supervision, G.Y.; project administration, G.Y. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Protocol of the Faculty of Construction No. 9 dated 2 July 2021, Saint- Petersburg Mining University. Informed Consent Statement: Informed consent was obtained from all subjects involved in the study. Data Availability Statement: Data is contained within the article. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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