Modified concrete for irrigation and drainage construction

. Within the framework of these studies, issues related to the modification of the composition of heavy concrete based on a polydisperse binder with a complex modifier and its influence on the formation of the structure and properties of cement stone, as well as hardened concrete, are considered. The purpose of the study is to substantiate the possibility of obtaining effective heavy concrete for irrigation and drainage construction with improved indicators of operational properties in terms of strength and hydrophysical characteristics. Object of study: heavy concrete based on a polydisperse binder with a complex modifier reinforced with basalt fiber. Subject of study: the structure and properties of heavy concrete based on a polydisperse cement binder with a high packing density together with a complex modifier and reinforced basalt fiber. The influence of the three-level dispersed composition of the clinker component of cement particles on the reduction of interparticle voidness and the increase in the strength of the cement stone was studied. The resulting modified concrete is characterized by an increase in strength to 77.3 MPa, a stretching of 8.62 MPa and a decrease in water absorption to 1.9%; % in comparison with the control composition.


Introduction
Concretes used in irrigation and drainage construction are subject to high requirements for durability, which is evaluated primarily by their strength and water resistance [1].The main direction of solving problems of this type is the creation of a high-density concrete structure that rationally combines the necessary technological and operational characteristics by using high-quality binders and complex modifiers [2][3].In particular, the use of polydisperse mineral components that provide a high packing density of the initial particles in each microvolume of the material, as well as plasticizers and polymer modifiers that modify the structure and properties of the material [4][5].
A promising direction is the development and implementation of effective measures to strengthen and protect reinforced concrete with materials with a high degree of resistance to aggressive environmental factors, for example, polymer compositions [6][7][8].Polymer concretes have not yet found wide distribution due to their scarcity and high cost, and thus, concrete and reinforced concrete remain the main building material in the construction of reclamation structures.Thus, solving the problems of obtaining concrete, in particular for irrigation and drainage construction, with high performance properties by modifying its structure and properties with complex additives [9][10][11][12] becomes more and more relevant every year.

Methods
The properties and structure of modified concrete were studied using the following materials.Portland cement of class CEM I 42.5N, manufacturer Holsim (Rus) Building Materials (GOST 31108-2016) was used as a binder; fine aggregate -natural sand with a fineness modulus of 2.5 (supplier of Khromtsovsky Quarry; GOST 8736-2014); coarse aggregate -crushed granite.To regulate the properties of the concrete mixture and the concrete obtained on its basis, a complex chemical additive was introduced: Melflux 5581 F superplasticizer based on polycarboxylate ether (NPO SINTEZ; GOST 24211-2008) plus a water-soluble polymer additive "POLYDON-A" ( Orgpolimersintez; TU 9365-002-46270704-2001) and a dispersed reinforcing additive -chopped basalt fiber (InReS LLC; TU 5952-002-13307094-08).The selection of the preliminary composition of the heavy concrete mixture was carried out in order to determine the optimal consumption of starting materials per 1 m 3 of the concrete mixture, under which the following conditions are provided: the specified physical-mechanical, hydrophysical characteristics and operational properties of the designed concrete are maintained.
The work was carried out using a system-structural approach based on the methodology of building materials science in the system: composition -structure -properties.A high degree of reliability is ensured by conducting experimental work using research and testing equipment with sufficient reproducibility of research results; a well-founded complex of modern research methods: laser granulometry, X-ray phase, electron microscopic and chemical analyses; using standard techniques that provide sufficient accuracy of the results obtained with a probability of 0.95; application of mathematical and statistical methods of data processing.

Results
In the work, a study was carried out on streamlining the structure of a heterogeneous binder by choosing the optimal dispersion and creating finely dispersed fractions of the clinker component.It was envisaged to determine the various fractions of the clinker component, providing the maximum packing density of the cement particles used.The study was carried out with three fractions: (coarse -average particle diameter d av -12 µm with a specific surface S sp = 150m 2 /kg; medium -dav = 6.6 µm, S sp = 300m 2 /kg; fine -d av = 4.9 µm, S sp = 450m 2 /kg).
In order to obtain the optimal composition of the polydisperse binder, the topological (geometry) characteristics of the resulting cement compositions were studied using a software-computing complex based on the DropAndRoll algorithm.
The analysis of scientific literary sources showed that in order to reduce the influence of the "wall effect", the ratio of the particle diameter to the packing size should be> 20.Due to the fact that the calculation of a packing of 20 diameters of the largest sphere will take a very long time, it was decided to study a single cell, with the ratio of the largest diameter of the sphere to the side 1 to 14.In our case, the largest diameter is 12, so the size of the cell will be 168x168x168.The initial data for calculating the topological characteristics and a graphical representation of the change in the packing density of the binder depending on the composition of the composition of the fractions of cement particles of various dispersions are presented in Table 1 and Fig. 1.Data analysis (Fig. 1) showed a decrease in packing density in the proposed composition by 0.5% (Fig. 1 a, point A) in comparison with the maximum calculated value (Fig. 1 b point B).Taking into account the fact of the increase in energy costs in the production of cement particles with a fraction of 450 m 2 /kg, it was decided to take composition 12 (Table 1) with the ratio of cement particles in %: 15 -with an average diameter dav = 12 μm and a specific surface S sp =150m 2 /kg; 75 -d av =6.6 µm, S sp =300m 2 /kg; 10 -d av = 4.9 µm, S sp =450m 2 /kg (see graph in Fig. 1a, point A).
At the same time, there was a positive dynamics in the dependence of strength on the packing density of cement particles: the optimal composition 12 showed an increase in strength at the age of 7 and 28 days by 57 and 29% compared to the control composition 1, which is 44.6 and 56.4 MPa, respectively.
The phase composition of the cement stone samples, calculated on the basis of the obtained XPA data, is shown in Fig. 2 and table 2. The results of X-ray phase analysis showed that in composition 2 (PV) the content of C3S and C2S increased in comparison with composition 1, and the degree of hydration decreased from 70 to 52%, respectively.The decrease in the degree of hydration is explained by the presence in the composition of 2 large non-hydrated binder grains (fraction 150 m2/kg), which form the reserve of the clinker fund.When Melflux and Polydon-A (compositions 3 and 4) were added to the HP separately, a slight decrease in C 3 S and C 2 S by 1-2% was observed in comparison with the control (composition 2).Their joint introduction into HP (composition 5) showed a slight decrease in the main clinker minerals, while the degree of hydration increased to 57% in comparison with compositions 3 and 4. To improve the resistance of cement stone against leaching corrosion, 15% MKU-95 was added to composition 6.At the same time, the content of residual clinker minerals decreased slightly, and the amount of Ca(OH)2 decreased by more than 26% compared to the control.This fact is evidence of the occurrence of the pozzolanic reaction, which is accompanied by the binding of Ca  To confirm the hypothesis about the preservation of the clinker fund and its gradual involvement in the process of structure formation over long periods of hardening, composition 6 was additionally studied after 6 months.At the same time, a decrease in C 3 S and C 2 S by 10 and 11%, portlandite -up to 10.4 and an increase in the degree of hydration from 62 to 82%, respectively, were found in comparison with composition 6 (28 days of hardening).The results obtained indicate the positive role of the clinker fund in the cement stone (the stock of coarse particles is a fraction of 150 m 2 /kg), which should further increase the durability of concrete made on its basis.
Studies of the microstructure of cement stone showed (Fig. 3) that the presence of a complex modifier in the composition (Fig. 3c) made it possible to obtain a dense cement stone of an ordered structure with evenly distributed pores throughout the volume with the accumulation of ettringite needle crystals in the zone of micropore formation.It was found that in composition 5 (Table 2) the crystal sizes decrease from 70 to 90 nm (Fig. 3b), and in composition 6 (Table 2) from 60 to 75 nm (Fig. 3c), which is much lower than in control 2 (Table 2) -from 100 to 120 nm (Fig. 3a).Modified cement stone composition 6 has a denser and more uniform finely porous structure with a micropore size of 0.1 to 0.6 µm (Fig. 3c); in composition 5, the main range is from 0.5 to 2 µm (Fig. 3b); in the control sample, composition 2 is from 1 to 5 µm (Fig. 3a), with the presence of pores up to 50 µm.
The results of studies of modified cement stone formed the basis for the development of a composition of heavy concrete with a given set of performance properties.In table 3 shows the compositions of heavy concrete on which the research was carried out.Analysis of the results of studies (Table 4) of the properties of concrete mixtures showed that the proposed compound 6 significantly reduced the indicators of water separation and solution separation by 2.5 and 2.2 times in comparison with the control compound 1.This fact has a positive role in reducing the negative consequences: delamination concrete mixture, settling of coarse aggregate and reduction of concrete heterogeneity.An analysis of the results of the properties of modified concretes showed an increase in indicators: for compression in the composition of HP + (0.3% Melflux + 0.2% Polydon-A + 15% MK) by 47.9% relative to the control (HP + 0.3% Melflux ) and by 19.5% relative to the compound (HP+ 0.3% Melflux), which is 24 and 12.1 MPa in absolute terms, respectively.The maximum increase in tensile strength in bending (Rtb) was shown by concrete with micro-reinforcing basalt fiber (compound 6).The R tb index increased by 49.1% in comparison with the control and by 11.5% in comparison with compound 5 (without fiber).The indicators of the maximum load increased by 40.92% and the conditional stress intensity factor by 40.60%, the bending stress by 35.1% compared to the control.Based on the results of hydrophysical tests of modified concrete (compound 6, Table 4), it was established that the water absorption index decreased by -57.8%; the water resistance mark increased by 4 loading levels in comparison with the control compound 1.

Discussion
A technological solution for obtaining heavy modified concrete based on a cement binder of optimal composition of various fineness (15% -average particle diameter dav-12 µm with a specific surface S sp = 150 m 2 /kg; 75% -d av = 6.6 µm) has been substantiated and experimentally proven., S sp = 300m 2 /kg; 10% -d av = 4.9 µm, S sp = 450m 2 /kg) with a high packing density together with a complex modifier (0.3% Melflux + 0.2% Polydon-A + 15% MK) and basalt fiber (0.7% BF).As a result, a dense hardened cement stone of an ordered fine-grained structure is formed with crystals from 60 to 75 nm and micropores uniformly distributed throughout the volume from 0.1 to 0.6 μm.The results obtained indicate that the presence of a fine fraction (fr.450 m 2 /kg) allows for high rates of strength growth in the early stages of hardening, and a large fraction (fr.150 m 2 /kg) increases the stock of the clinker fund in the cement stone: the amount of low-basic calcium hydrosilicates increases by 17% compared with the control composition, and the degree of hydration reaches 82% (at the age of 6 months).The resulting modified heavy concrete has increased strength, hydrophysical properties and high corrosion resistance, which positively affects its durability.The results obtained are consistent with scientific works [10][11][12][13][14][15][16][17][18][19][20].

Conclusions
The creation of the structure of high-density heavy concrete, rationally combining the necessary technological and operational characteristics, is substantiated and experimentally proven by optimizing the composition of a cement binder of various dispersion with a complex modifier (hyperplasticizer + polymer + microsilica) + basalt fiber.The resulting modified concrete has the following indicators: compressive strength -77.3 MPa; ultimate tensile strength in bending -8.62 MPa; conditional stress intensity factor -0.074169 MPa × m0.5, water absorption -1.9%; waterproof grade -W14; frost resistance F1=600, increased resistance to aggressive environments.

Fig. 1 .
Fig. 1.Change in the packing density of the binder depending on the composition of the composition of cement particle fractions of different dispersion: a) the maximum calculated packing value at point A; b) the actual packing value at point B.

Table 1 .
Initial data for calculating topological characteristics.
№ Dispersed compositions of cement particles

Table 2 .
Phase composition of cement stone at the age of 28 days of normal hardening.

Table 3 .
Investigated compositions of modified heavy concrete.

Table 4 .
Results of studies of concrete mixes and concretes.