Increasing the Biostability of Concrete by the Introduction Of Additives

. To increase the resistance to fungal corrosion and improve the performance of concrete cement stone, it is proposed to introduce calcium stearate and active metal nitrates in an amount of 0.5% by weight of cement into the cement mixture at the manufacturing stage. To determine the degree of corrosion damage of cement stone by fungal microorganisms Aspergillus niger, porosity, density, water absorption and strength were determined. It has been found that the introduction of a hydrophobic additive of calcium stearate into the cement mixture during the manufacture of concrete reduces water absorption, reduces porosity and increases strength. Additives of nitrates of active metals do not affect the characteristics of the cement stone of concrete. As a result of 6 months of fungal corrosion, the water absorption, porosity and density of cement stone containing calcium stearate did not change, and the strength decreased by 4%. Metal nitrates do not prevent damage to the cement stone by microorganisms, but slow down the flow of aggressive substances to the surface of the reinforcement in concrete. The combined introduction of calcium stearate and nitrates into the cement mixture will improve the characteristics of concrete, increase its corrosion resistance and ensure the safety of reinforcement in concrete.


Introduction
The destruction of concrete and reinforced concrete products and structures under the influence of microorganisms attracts more and more attention of scientists from the point of view of violation of structural integrity and reduction of the service life of bridge structures and various hydraulic structures [1][2][3][4][5][6][7][8].
Microorganisms affect the long-term strength and mechanical properties of concrete, creating acids that react with the components of the concrete mixture, reducing its stability and eventually leading to early destruction of the structure [3,[8][9][10][11]. Since microorganisms easily adapt to changing physical and chemical conditions of the environment and multiply intensively, their effect on materials should be considered dangerous and taken into account.
As a result of biocorrosion, the porosity of concrete increases [6,12,13] and diffusion processes accelerate, which contributes to the development of corrosion processes [14][15][16][17]. The reaction of biogenic acids and other products of the vital activity of microorganisms with the components of the cement stone of concrete leads to the destruction of the structure. The rate of destruction of cement stone largely depends on the solubility of the reaction products of organic acids released by microorganisms and the structural phases of cement stone. The more reaction products are dissolved and carried away by the aggressive solution, the faster the concrete is destroyed. Microorganisms dissolve the cement matrix with the leaching of structural elements and their accumulation in the biofilm [1,3].
From Rosstat data [18] presented in Fig. 1, it is obvious that the pace of construction and production of building materials in Russia is not decreasing. Construction is carried out under various weather conditions, which means that it is necessary to provide measures to prevent the development of corrosion processes of building materials and products.  Timely protection of concrete and reinforced concrete objects from biofouling will significantly reduce the economic damage from the effects of corrosion damage, increase the reliability of structures operated in conditions of high humidity, reduce the likelihood of accidents. It is necessary to focus research on expanding the complex of strength and anticorrosive properties of reinforced concrete in accordance with a variety of options for its application.
Despite the abundance of ways to protect against biofouling, there are still no radical methods to combat biocorrosion. The development of compositions of nanomodified additives based on calcium stearate hydrophobizer and reinforcement corrosion inhibitors introduced into concrete at the manufacturing stage is relevant to prevent the development of fungal microorganisms and to ensure the resistance of concrete products and structures to biological effects.

Materials and methods
Cement stone samples made of Portland cement CEM I 42.5 with W/C = 0.3 with additives of inhibitors and hydrophobizer in an amount of 0.5 % by weight of cement were infected with fungal microorganisms Aspergillus niger.
Calcium, sodium, potassium, magnesium and zinc nitrates were selected as inhibitory additives. Calcium stearate was introduced into the cement mixture as a hydrophobic additive.
After the surface of the cement stone was infected with fungi, the samples were placed in containers on a sintepon substrate, which was constantly wetted with water.
The strength was determined on samples of cement stone with a face length of 10 cm. During the compression test of the samples on the press, loading was carried out continuously with a constant rate of load increase until their destruction. At the same time, the loading time of the test sample before its destruction was at least 30 seconds. The maximum force achieved during the test was taken as a destructive load.
Water absorption is determined by testing a series of samples with dimensions 10х10х10 cm. The samples are placed in a container that is filled with water at a temperature of 20±2 °C above the upper level of the stacked samples by about 50 mm. Samples are weighed every 24 hours on a scale with an error of no more than 0.1%. When weighing, the samples are pre-wiped with a wrung-out damp cloth. The mass of water flowing out of the pores of the sample on the scale cup is included in the mass of the saturated sample. The test is carried out until the results of two consecutive weight measurements differ by no more than 0.1%.
The water absorption of concrete of a separate sample W m by weight as a percentage with an error of up to 0.1% is calculated by the formula: where: m c is mass of the dried sample, g; m w is mass of the water-saturated sample, g.
The water absorption of concrete of a separate sample W 0 by volume as a percentage with an error of up to 0.1% is calculated by the formula: where: ρ 0 is average density of dry concrete, g/cm 3 ; ρ w is the density of water assumed to be equal to 1, g/cm 3 .
The total pore volume of V p concrete is calculated as a percentage with an error of up to 0.1% according to the formula: where: ρ is the true density of concrete, kg/m 3 ; ρ d is density of dried concrete, kg/m 3 .
The volume of open capillary pores of concrete (sample) V o is assumed to be equal to the water absorption of concrete by volume W 0 .
The volume of conditionally closed capillary pores of concrete V c is calculated by the formula:

Results
The introduction of a hydrophobic calcium stearate additive into the cement mixture in the manufacture of concrete reduces water absorption, reduces porosity and increases strength ( Table 1). An increase in density due to a decrease in porosity helps to slow down the penetration of an aggressive medium deep into the cement stone concrete and inhibit the development of corrosion processes [19,20]. The characteristics of cement stone after 6 months of exposure to Aspergillus niger fungi are shown in Table 2. In concrete cement stone samples with inhibitor additives the loss of strength after 6 months of exposure to fungal microorganisms was 10%, and in cement stone samples without inhibitory additives it was 14%. In case of corrosion of hydrophobized samples, the strength as a result of exposure to micromycetes decreased by 4%. The water absorption of hydrophobized concrete cement stone after exposure to fungal microorganisms did not change, and in samples without additives it increased by 1.4 times, and with nitrate additives it increased by 1.3 times. The porosity of the samples changed in a similar way as a result of fungal corrosion. Table 3 shows that the combined addition of calcium stearate and metal nitrates has the same effect as volumetric hydrophobization without the addition of inhibitors.

Discussion
Additives of corrosion inhibitors do not significantly affect the mass transfer processes occurring in cement stone at the initial stage of concrete corrosion in an aggressive environment [21]. However, when nitrates of alkaline and alkaline earth metals are introduced into an aggressive environment, it is possible to slow down the anodic dissolution of steel reinforcement by 1.5 times [21]. Also, metal nitrate additives do not change the characteristics of concrete cement stone (Table 1).
From the results of the study presented in Table 2, it becomes obvious that as a result of the violation of the integrity of the surface structure of concrete cement stone and, accordingly, a decrease in its surface energy at the initial stage of biocorrosion, a significant deterioration in strength characteristics occurs. Microorganisms spread deep into the cement stone and change its pore structure. The process of bio-damage of cement stone is associated with a change in strength characteristics.
As a result of mass transfer and chemical reactions caused by the vital activity of microorganisms, bonds can form in cement concretes, but mainly the destruction of the formed structures occurs. The change in the number of bonds available in the structure of the material affects its strength characteristics [3,22,23]. In particular, therefore, strength indicators are often taken as the main characteristic of the durability of materials when assessing and predicting the resistance, including cement concretes, to the effects of various media, microorganisms and their waste products.
Nitrate ions in the pore fluid of cement stone interact with substances secreted by microorganisms and slow their flow deep into the concrete. As a consequence, the strength characteristics of cement stone concrete made with additives of inhibitors are reduced to a lesser extent. Hydrophobic additives make the pore surface immune to the waste products of microorganisms and other liquid media, preventing the dissolution and leaching of calcium-containing phases from the cement stone structure [24][25][26]. Also, calcium stearate can colmate the pores [19,27,28], preventing the entry of substances deep into the cement stone of concrete.
Since nitrates of alkaline and alkaline-earth metals are mostly used to inhibit corrosion of steel reinforcement during the operation of reinforced concrete in highly aggressive environments [21,29,30], in a complex additive they can have an auxiliary effect in those parts of cement stone where hydrophobization is absent or reduced due to insufficient mixing of concrete or as a result structure formation.

Conclusion
As a result of volumetric hydrophobization of cement stone, the performance characteristics of concrete are improved, its corrosion resistance increases. Inhibitory additives of nitrates of alkaline and alkaline-earth metals do not affect the characteristics of cement stone concrete. The addition of calcium stearate in an amount of 0.5 wt. % in the cement mixture at the stage of concrete production reduces the degree of corrosion damage of concrete to fungal corrosion.
Additives containing nanoscale components will be able to interact more actively with the components of the concrete mixture and have a positive effect on the structure formation of concrete during hydration and hardening, and, consequently, on its physical and mechanical properties and corrosion resistance.