Influence of compound additive on concrete in hot and dry climate

. In the conditions of hot and dry climate, the concrete mixture quickly loses its mobility and workability. In this regard, it is necessary to plasticize it by introducing various surfactants; at the same time, the surfactant additive should not slow down the process of hydration and hardening. To ensure salt resistance, it is necessary to increase the density and strength of concrete. The proposed complex additive is obtained by joint grinding in a ball mill of soda sulfate melt and gossypol resin to a specific surface of 2800 cm 2 /g at the rate of 0.5-1.5% and 0.1-0.3% by weight of cement. From the obtained results, it can be seen that with an increase in the content of gossypol resin from 0.1 to 0.3%, the normal density decreases by 1.4-4 points, and the setting time due to the accelerating action of the soda sulfate melt is reduced at the beginning by 20-50 minutes and the end by 60 - 240 min. Reference cement for 28 days gained a strength of 56 MPa, with a complex additive of 60-68 MPa. The coefficient of salt resistance of the reference 0.55-0.65 and with a complex additive of 0.80-0.86 is observed due to the strength and density of the samples. Increasing the strength makes it possible to save 17-21% of cement.


Introduction
In connection with the development of new mineral deposits in the Republic of Uzbekistan, capital construction is being developed in desert areas characterized by a hot and dry climate and the population of soils and groundwater.The increase in saline territories is explained by the drying up of the Aral Sea and the blowing out of a large amount of salt.Chemical additives primarily affect cement paste and stone's structure formation and properties [1][2][3][4][5].
Studies were carried out to study the effect of a complex chemical additive based on gossypol resin, oil, and fat plant waste and soda-sulfate melt, caprolactam production waste, on the properties of cement paste, stone, and on salt resistance coefficient.several characteristics of concrete at once; the possibility of strengthening and deepening any effect, the maximum achieved with the introduction of any one-component additive; the reduction and elimination of undesirable side effects of a separate component of the complex additive; great versatility of their action, which means the independence of the effect obtained from the chemical and mineralogical composition of cement and, to a certain extent from the composition of the concrete mixture [6][7][8][9][10].The composition of the proposed complex additive from the point of view of the mechanism of its influence on the cement system under hot-dry climate conditions.
It is known that under the conditions of a hot and dry climate at an elevated temperature, the cement mixture quickly loses its mobility and workability.Therefore, it must be plasticized by introducing additives of surfactants.At the same time, adding surfactants should not slow down the process of hydration and hardening of the cement.At the same time, to ensure salt resistance, it is necessary to increase the density of concrete and reduce capillary permeability and the accumulation of salts in its cavity.

Results and Discussion
Individual surfactants cannot provide such an effect.For example, plasticizing additives slow down the processes of hydration and hardening of the cement.And under the conditions of the hot and dry climate, the addition of surfactants should simultaneously have both a plasticizing and accelerating effect on the processes of hydration and hardening of the cement.This cannot be ensured by introducing a one-component additive [11][12][13][14][15][16].To increase the salt resistance of concrete, it is necessary not only to improve its pore structure but also to make it hydrophobized, i.e., to give it water-repellent properties in volume.Therefore, it became necessary to use a complex additive, including a plasticizer and a hardening accelerator.Let us analyze the expediency of using the proposed complex additive from these positions.It is known that water-repellent additives, from the point of view of plasticizing action, are effective for lean concrete mixtures, i.e., with low cement content.Gossypol resin is a water-repellent admixture; therefore, it plasticizes fine concrete mixtures well.These include the composition of concrete mixtures for monolithic foundations.Since water-repellent substances, including gossypol resin, are waterinsoluble, water-dilutable emulsions are obtained from them using a special technology using emulsifiers.Water-repellent additives are characterized by a multifunctional action: they plasticize concrete mixtures and, being adsorbed in pores, give concrete waterrepellent properties, contributing to an increase in density, water permeability, frost, and salt resistance.However, they slow down the processes of gyration and hardening of the cement.
To compensate for this effect of the action of gossypol resin in the composition of the complex additive, it is recommended to use the soda sulfate melt of the hardening accelerator.The Na₂SO 4 and NаCL contained in the soda sulfate melt increase the solubility of the original cement clinker minerals, slowing down the formation of reignite and shielding shells around the cement particles, thereby accelerating the setting and hardening of the binder.
To obtain complex additives, in addition to liquid ones, a method was proposed for agglomeration of hydrophobic additives in the form of combined products (briquettes of granules, tablets, granular powder) [17][18][19][20].However, additives in the form of briquettes, granules, and tablets differ not only in the complexity of their compositions and preparation methods but also in their introduction's technology.
Taking this into account, the proposed complex additive was obtained by joint grinding of soda sulfate melt and gossypol resin at the rate of 0.5-1.5% and 0.1-0.3%,respectively, by weight of cement.Grinding of the complex additive was carried out in a ball mill to a specific surface area of 2800-3000 cm²/g.The resulting additive was introduced into the mixing water of the cement mixture.In the research process, we studied the additive's effect on the cement paste's normal density, setting time, plastic, and goblet strength of the cement stone.Cement from the Akhangaran cement plant was used as a binder.The plastic strength of cement stone of normal density was determined within the limits of its curing time.The research results are presented in fig. 1, fig. 2, and table 1.The data obtained shows that the best results in plasticizing and accelerating action on the cement system are provided with a complex additive at soda sulfate melt and gossypol resin content of 1.0 + 0.2%.This is evidenced by the data on the effect of the additive on the process of initial structure formation of the cement binder (Fig. 1).From those shown in fig. 1 shows that the complex additive accelerates the process of structure formation to a greater extent, the higher the soda sulfate melt content.In addition, there is an increase in the plastic strength of the hardening cement by 25-57% in comparison with the reference.At the same time, with a relatively longer duration of the coagulation period, structure formation with the addition of soda sulfate melt and gossypol resin 1.0 + 0.2% is not significantly lower by the end of the binder setting time and at a dosage of 1, + 0.3%.This also positively affects the increase in pot strength over time (Fig. 2).Quality cement under normal conditions hardens in 28 days.Gaining strength 56 MPa and with a complex additive 60-68 MPa or 15-24% more.One of the ways to increase salt resistance is associated with a decrease in its permeability; it is achieved by increasing its density and strength in the early stages of hardening, which is confirmed by the data obtained.The introduction of a complex additive into the composition of the cement mixture is observed to increase the strength of the cement stone by 2.5-3.5 times the initial hardening time.From table 1, it can be seen that with the increase in the content of gossypol resin from 0.1 to 0.3%, the normal density of the cement paste decreases by 1.5-4 points, and the setting time due to the accelerating effect of the soda sulfate melt action is reduced by 20-50 minutes and the end of 60-240 min.
The effect of the complex additive on the salt resistance of the samples was determined in two solutions: 55% Na 2 SO4 and 55% Na 2 SO4 + 5.5% Na 2 Cl.The tool life coefficient (C TL ) was determined by the following formula: Where: R b.c is the strength of the samples MPa, after testing the samples for capillary suction and evaporation of saline solution after 6 months; R n.h. is the strength of samples of normal hardening within 28 days.The data obtained indicate that the complex additive increased the salt resistance of the samples and amounted to 0.55-0.65 for the reference ones and 0.80-0.86 with the complex additive.The increase in salt resistance in samples with complex additives can be explained by an increase in the density of the samples.Consequently, technology was proposed for obtaining a complex additive by joint grinding of soda sulfate melt and gossypol resin in a ball mill to a specific surface area of 2800 cm 2 /g.The positive effect of the complex additive on the properties of the cement paste and stone was revealed, and the rational dosage of soda sulfate melt and gossypol resin in the composition of the complex additive (1.0 + 0.2%) by weight of cement was established.With an increase in the content of gossypol resin, the normal density of the cement paste decreases, and the setting time, due to the accelerating action of the soda sulfate melt, is reduced.An increase in the plastic strength of hardening cement is observed and amounted to 25-57% compared to the reference cement.Reference cement for 28 days gained strength of 56 MPa and, with a complex additive, 60-68 MPa.

Conclusion
The complex additive increased the coefficient of salt resistance of the samples and amounted to 0.55-0.65 for the reference ones, and with the complex additive 0.80-0.86.An increase in the salt resistance coefficient is observed due to increased strength and density of the samples.Due to the increase in strength, it is possible to save 17-21% of cement.

Table 1 .
Influence of complex additive on the normal density and setting time of cement