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

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 with the complex additive. They revealed the formation of durable hydration products including thaumasite and calcium silicate hydrates of lower basicity that form a dense structure of cement matrix, increasing the physical and mechanical characteristics of cement-based composites.

1. Introduction

Cement mortar and concrete are among the most widely used, durable and reliable materials in modern construction; therefore, improving their structure and properties has always been one of the main priorities of construction materials science. Currently, there is a wide variety of technical possibilities for modifying the cement-based composites and 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], varying the hardening conditions [7], introducing the various types of fiber [8,9,10], etc. Moreover, many researchers around the world have claimed that the application of nano-sized additives can significantly improve the properties of the cement-based materials by affecting the processes of their structural formation [11,12,13].

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 nano-sized particles in the hardening mineral matrix stimulates the formation of the layer of hydration products on their surface. Here, the further recrystallization of hydration products into larger crystal blocks is limited due to the high surface energy of nanoparticles. This provides the conditions for creating the high-density and defect-free structure that unites the conglomerate and gives it high density and strength. Furthermore, it rearranges the pore structure of the cement gel and ettringite towards smaller sized pores and promotes 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 [17,18,19]. 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 [20,21,22]. 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. [23,24,25,26].

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 [27,28,29]. 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 [30,31,32].

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 amount of 10% of the total suspension volume with the C-3 superplasticizer in the amount of 2% by water mass using the cavitation disperser. The study of the microstructure of the chrysotile fibers (Figure 1a) shows that the diameter of individual fibers is around 30–50 nm, whereas the length of the fibers varies significantly. The pattern of the particle size distribution curve of the aqueous suspension of chrysotile fibers (Figure 1b) showed that the average particle diameter varied from 0.01 μm to 0.2 μm. The average value was equal to 0.046 μm.

Carbon black is a powdery product obtained in the process of controlled pyrolysis or thermal oxidative decomposition of liquid or gaseous hydrocarbons. The elementary composition of this synthetic material is represented by carbon (88.6–93.7%), hydrogen (0.7–0.8%) and oxygen (5.5–10.5). The particle size of carbon black (Figure 2) was found to be 30–120 nm.

In order to determine the properties of complex additive as well as the modified cement composite a comprehensive study was carried out using the following methods:

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particle size distribution analysis using the laser dispersion on a SALD-7500nano analyzer manufactured by Shimadzu with the 7 nm to 800 μm measurement range;

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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 second;

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differential thermal analysis of cement composites on a TGA/DSC1 Star system manufactured by Mettler Toledo in the temperature range from 60 °C to 1100 °C with the heating rate of 30 °C/min;

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IR-spectral analysis of cement composites on Shimadzu IRAffinity-1 spectrometer in the frequency range 400–4000 cm⁻¹;

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microstructure analysis and energy dispersive X-ray analysis (EDX) on scanning electron microscope Quattro ESEM Thermo Fisher Scientific with the resolution up to 0.8 nm.

The cement composite samples were tested at the age of 1 day after steam curing.

3. Results and Discussion

The optimal ratio of chrysotile nanofibers and carbon black in the composition of a complex additive was stated based on the particle dispersion level in the suspension as well as mechanical properties of the modified cement matrix.

Primarily, the ratio of chrysotile to carbon black by mass was taken equal to 10:1, 7.5:1, 5:1, 2.5:1 and 1:1. The chrysotile amount in the suspension was 10% of the full suspension mass. Higher concentrations lead to the fiber agglomeration that made it harder to provide the uniform particle distribution in cement composite that adversely affected the properties of the material [33]. The particle size distribution analysis of the complex additive was carried out on Shimadzu SALD-7500nano laser analyzer. The results are presented in Figure 3.

The highest dispersion level of the complex additive was obtained at the ratio of chrysotile fibers to carbon black equal to 5:1. In this case the average particle diameter was around 65 nm. It should be noted that carbon black particles do not significantly change the size of agglomerates in the complex additive apparently due to the stabilizing 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 of cement composites even considering the sufficient clarity of the results of the particle size analysis of the complex additive since the dispersion levels vary insignificantly. In this case, the most reliable option for stating the optimal amount of additives that provides the maximum increase in strength of cement composites is the preparation of standard samples and their mechanical testing.

The mechanical tests of samples modified with complex additives containing the different chrysotile to carbon black ratios were carried out on fine-grained concrete beams with the standard dimensions of 40 × 40 × 160 mm at the age of 1 day after steam curing. To determine the strength, a series of samples was prepared, containing three samples of each composition. The average compression and flexural strength values obtained via statistical analysis of the mechanical test results are indicated in

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

Chrysotile Amount, %	Compressive Strength, MPa						Flexural Strength, MPa
	Carbon Black Amount, %						Carbon Black 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

. In all cases, the deviation from the average value did not exceed 10%.

As can be seen from Table 1, 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 (Figure 4a), compositions modified with carbon black in the amount of 0.02% by the cement weight (Figure 4b) and chrysotile fibers in the amount of 0.05% by the cement weight (Figure 4c), as well as the composition modified with the complex additive based on chrysotile and carbon black (Figure 4d) 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 (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 additives are added into the composition together, which evidences their synergetic effect.

Thermal analysis of the reference sample and the sample modified with the complex additive based on carbon black and chrysotile fibers were carried out on a TGA/DSC1 Star system derivatograph. The results of the study are presented in Figure 5a,b.

The TGA curves show a change in the intensity of mass loss of the sample 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.

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 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 (Figure 8) confirmed that the formations belong to calcium silicate hydrates.

Moreover, the SEM of the cement matrix modified with the complex additive (Figure 9) 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).

Based on X-ray spectral analysis the hydration products of spherical shape can be attributed to thaumasite formations (CaSiO₃‧CaSO₄‧CaCO₃‧15H₂O) obtained in the course of the reaction of calcium silicate hydrates (C–S–H) with calcite and unbound sulfate ions or reaction of ettringite with C–S–H and carbonates/bicarbonates [36,37,38,39]. The noted hydration products of spherical shape are mainly formed on the surface of ettringite needle crystals and on the surface of calcium hydroxide plates.

The results of mechanical tests, microstructure studies, thermal analysis 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.

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.