Composition and Properties of Fine-grained Concrete for Self-cleaning Coatings

. The paper considers the dependences of the properties of self-cleaning fine-grained concrete on the composition of the concrete mixture: the ratio of sand to white Portland cement, the content of plasticizer, the content of photocatalytic composite material. The photocatalytic composite material (PCM) is a diatomite-based anatase-silica material obtained by sol-gel deposition of nanoscale TiO 2 of anatase modification on a silica carrier. The investigated properties of fine-grained concrete were: average density, compressive strength and self-cleaning ability, which was determined by evaluating the photocatalytic decomposition of an organic dye – Rhodamine B. It was shown that an increase in the content of PCM to increase the ability of the coating to self-cleaning leads to a loosening of the structure of fine-grained concrete, but this can be successfully offset by an increase in the content of plasticizer, which additionally improves the distribution of the photocatalytic agent in the cement-sand matrix. An increase in the proportion of sand and the content of plasticizer also contribute to an increase in the ability to self-cleaning. The pozzolan activity of the PCM provides an increase in the strength of concrete. The results of comparing the self-cleaning ability of fine-grained concrete of various compositions indicate a high photocatalytic activity of samples with PCM, which allows its application for creating self-cleaning coatings.


Intoduction
Construction materials obtained using photocatalysts are becoming increasingly widespread due to the ability to self-cleaning, which ensures their whiteness and/or cleanliness, a more aesthetic appearance of buildings and structures for a long time [1][2][3][4].
The example of construction products where the use of photocatalysts is relevant and expedient is facial wall stones, which are used as protective or decorative design of the walls of buildings.Their decorative surface is subjected to contamination, is accessible to solar radiation and atmospheric precipitations, which allows full use of the photocatalysis process.Such products are usually obtained from fine-grained concrete based on white portland cement [5][6][7].
When obtaining any construction products with photocatalysts, from an economic point of view, it is advisable to introduce a photocatalyst only into the upper, facing layer to ensure its contact with pollutant molecules, access to electromagnetic radiation and water molecules, or to apply a photocatalyst in the form of a thin coating [8][9][10].
For the production of self-cleaning products, such technologies are used where twolayer molding of the concrete mixture and a high degree of compaction are possible to ensure high durability of products intended for outdoor use.One of these technologies is vibrocompression, where the use of a high-rigidity concrete mixture is provided.As the first (base) layer, ordinary concrete acts both on the basis of general construction cements and on the basis of white cements.The upper facial layer is formed mainly of white cement to preserve the aesthetics of the unpainted product, as well as to form a uniformly pigmented product of bright color.A photocatalyst, for example, nanoscale titanium dioxide of anatase modification, is introduced into the concrete mixture of the upper front layer.Composite photocatalytic additives are also known, in particular, TiO 2 -SiO 2 systems.Their photocatalytic activity is not inferior to the activity of industrial photocatalysts based on anatase, and at the same time they have a positive effect on the processes of phase and structure formation of Portland cement [4,[11][12][13][14][15][16].
The aim of the work was to study the effect of the composition of a fine-grained concrete mixture with a photocatalytic composite additive of TiO 2 -SiO 2 composition on the properties of fine-grained concrete.

Materials and Methods
The concrete mixture was prepared using the following materials: white portland cement CEM I 52,5 R produced by Adana Cimento (Turkey), certified in composition and properties for compliance with the norms and requirements of the European standard EN 197-1:2012.It contains at least 95 % by weight of high-quality white clinker and up to 5 % of auxiliary components; fine aggregate -natural white sand (M f =2.0) (Khokholsky sand quarry, Russia), bulk density in dry condition -1500 kg/m 3 , content of pulverized and clay particles -0.8 %, wetness -2.7 %; the Melflux 5581F hyperplasticizer is manufactured by BASF Construction Additives (Trostberg, Germany).A photocatalytic composite material (PCM) synthesized by the sol-gel method based on tetrabutoxytitanium and finely dispersed diatomite powder was used as a photocatalyst.The chemical composition of PCM is represented by about 55 % SiO 2 and 38 % TiO 2 .The phase composition includes quartz and anatase crystals, as well as the opal-cristobalite-tridymite phase [15,16].AEROXIDE TiO 2 P25 produced by Evonik Industries AG (Germany) was used as a control photocatalyst.
The production and testing of concrete was carried out in accordance with the EN 12390 series of standards "Concrete testing methods".
The ability of fine-grained concrete to self-cleaning was determined by evaluating the photocatalytic decomposition of an organic dye -Rhodamine B (C 2 8H 31 ClN 2 O 3 ) by changing the color of samples after ultraviolet irradiation (according to the Italian standard UNI 11259 "Determination of the Photocatalytic Activity of Hydraulic Binders Radammina Test Method" (2008)).An aqueous dye solution in the concentration of 4•10 -4 mol/l was applied in an amount of 0.3 ml to samples with an area of 12.5 cm 2 .The samples were kept for 4 and 26 hours under ultraviolet radiation (UV-A, 1.1 ± 0.1 W/m 2 ).The color change of the samples was evaluated from photographs, for which the change in the value of the a* coordinate of the CIELAB color space was recorded and calculated [17].

Results
The effect of the composition of a fine-grained concrete mixture with a photocatalytic composite additive of TiO 2 -SiO 2 composition on the properties of fine-grained concrete was studied using a complete factor experiment [6,18].The initial composition of concrete was determined by calculation and experimental method on the basis of production compositions of fine-grained concrete, providing a strength of at least 10 MPa, according to State Standard 27006-2019 "Concretes.Rules for mix proposing" (2020).After the theoretical calculation, an experimental batch was made based on which the actual initial composition of the concrete mixture was adjusted: white portland cement -290 kg/m 3 , fine aggregate -sand -1860 kg/m 3 .The initial dosage of the photocatalytic composite material was selected taking into account its photocatalytic activity and influence on the processes of phase and structure formation of Portland cement [15,16].
For a complete factor experiment, the following variation parameters were selected: the ratio of sand to white portland cement (S/PC) -5-7, the content of Melflux hyperplasticizer from the mass of cement (HP) -1-2, the content of photocatalytic composite material from the mass of dry mixture -1.5-2.5.The output controlled parameters of fine-grained concrete are: average density, compressive strength and self-cleaning ability.According to the protocol of the EN 12390 series of standards "Concrete testing methods" and with the specified variation of components, samples of fine-grained concrete were obtained, their physical and mechanical properties were studied, regression equations were compiled, according to which dependencies were constructed (Fig. 1-3).To confirm the effectiveness of the use of PCM in the composition of concrete, a comparison of the self-cleaning ability of fine-grained concretes of various compositions was carried out: without photocatalyst, with industrial photocatalyst AEROXIDE TiO 2 P 25 and with PCM (Table 1).The content of the AEROXIDE TiO 2 P 25 photocatalyst in the concrete mixture was equal to the content of PCM in terms of TiO 2 .
Table 1.The ability to self-cleaning of fine-grained concrete of various compositions by the method of assessing the photocatalytic decomposition of the organic dye Rhodamine B.
Fine-grained concrete no additives with AEROXIDE TiO 2 P 25 with PCM

Before UV irradiation
After 26 hours of UV irradiation

Discussion
According to the data obtained, the average density of the produced samples linearly depends on the ratio of cement to aggregate and the proportion of plasticizer in the system (Fig. 1).Thus, an increase in the proportion of sand from 5 to 7 mass parts in the system leads to a certain decrease in density: by 10-12 %.This is due to the decompression of the structure of the material due to the lack of binder that monolithizes aggregate particles.At the same time, compositions with a higher concentration of plasticizing additives are characterized by a lower density.It should be noted that an increase in the concentration of the photocatalytic component responsible for self-cleaning from 1.5 to 2.5 % causes a lower density of samples (Fig. 1, b), which is probably due to the sorption of water by microporous diatomite when mixing the components.At the same time, in samples with a higher concentration of plasticizer (2 %) with an equal ratio of binder to filler and photocatalytic component, the density is slightly lower, which is due to the water-reducing function of the plasticizer.
Data on the strength of samples (Fig. 2) correlate well with the results of their density estimation.However, unlike density, strength depends more on the ratio of the main components forming the framework of the material structure.Thus, with an increase in the proportion of cement in the system, there is a significant increase in the strength of samples: by 40-60 %, depending on the concentration of PCM.At the same time, an increase in the proportion of PCM from 1.5 to 2.5 % provides a 1.2-fold increase in strength, regardless of the ratio of binder and filler, which confirms the pozzolan activity of the photocatalytic component.
The ability to self-cleaning of concrete will depend on a number of physical factors, including porosity and permeability, uniformity of the photocatalyst distribution, etc.A decrease in the density of the studied compounds (Fig. 1) is associated with the formation of a larger number of pores and capillaries, which cause a greater porosity of the system, and, as a consequence, its greater ability to absorb.Whereas the increase in the strength of the samples confirms the qualitative distribution of PCM in the volume of the concrete mixture.This means that such systems will have a higher self-cleaning ability, which is associated with physical and chemical factors of exposure.
The assumptions made are confirmed by analyzing the data on the self-cleaning ability of the developed compositions (Fig. 3), which was determined by the method with Rhodamine B. An increase in the proportion of the photocatalytic component slightly affects the degree of self-cleaning of samples: the increase in photocatalytic activity is within 2 % when comparing samples of comparable density with an equal ratio of components.To a greater extent, the degree of self-cleaning of samples is affected by a change in the ratio of the main components of concrete: with an equal proportion of PCM, the difference in photocatalytic activity with an increase in the proportion of sand from 5 to 7 is about 7 %.This is due to the greater porosity of materials with an increased content of fine aggregate, which is confirmed by the analysis of their density (Fig. 1).At the same time, an increase in the concentration of the plasticizing additive leads to a slight increase in the ability to self-cleaning, which indicates a better distribution of the photocatalyst in the system.
The results of comparing the self-cleaning ability of fine-grained concrete of various compositions (Table 1) indicate a high photocatalytic activity of samples with PCM.So fine-grained concrete without additives did not show the ability to self-cleaning: the removal of Rhodamine B was about 15 %.Concretes with the industrial photocatalyst AEROXIDE TiO 2 P 25 and with PCM with an equal content of TiO 2 showed a similar ability to self-cleaning the surface from the organic dye Rhodamine B -about 70 and 64 %, respectively.

Conclusions
Using the complete factor experiment, the dependences of the density, strength and selfcleaning ability of fine-grained concrete on the ratio of sand to cement, the amount of plasticizer and photocatalytic composite material were investigated.ranges of compositions are focused on the technology of obtaining concrete products by vibrocompression.
It is shown that an increase in the content of the photocatalytic composite material to increase the ability of the coating to self-cleaning leads to a loosening of the structure of fine-grained concrete, but this can be successfully offset by an increase in the content of the plasticizer, which further improves the distribution of the photocatalytic agent in the cement-sand matrix.An increase in the proportion of sand and the content of plasticizer also contribute to an increase in the ability to self-cleaning.The pozzolan activity of the photocatalytic composite material provides an increase in the strength of concrete.
The obtained nomograms can be used to solve practical problems of selecting compositions of self-cleaning products made of fine-grained concrete with specified physical and mechanical characteristics with a reduced proportion of cement due to the use of a pozzolan component, but maintaining photocatalytic activity.
The ability to self-cleaning of fine-grained concrete with a photocatalytic composite material and an industrial (control) photocatalyst is compared.The photocatalytic activity of fine-grained concrete samples with a sol-gel synthesized photocatalytic composite material of the TiO 2 -SiO 2 system reaches 67 % in the studied composition range.The proposed fine-grained concrete with PCM can be used to obtain self-cleaning coatings.
The work was realized within the framework of the implementation of the state task of the Ministry of Science and Higher Education of the Russian Federation No. FZWN-2023-0006 using equipment of High Technology Center at BSTU named after V.G.Shukhov