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