The use of superplasticizers in multicomponent concrete mixtures

. In addition to the positive effect of the introduction of a superplasticizer, there are also negative ones: an increase in the period of structure formation, a significant air entrainment and, as a result, a decrease in strength. One of the ways to reduce the negative impact on the structure and properties of concrete composites is the method of preparing a suspension, which consists in first dissolving 10% of the amount of cement in all the mixing water in a concrete mixer for 1-2 minutes. Then load the rest of the cement and aggregate. Preliminary dissolution in water mixing with superplasticizer leads to the formation of a colloidal solution containing an increased amount of micro-and nanoparticles. When a superplasticizer is introduced into a cement-water suspension, particles with a high degree of dispersion are formed, as well as calcium hydrosilicate cristolites with a size of 1-3 nm. This sharply increases the activity of the superplasticizer and, consequently, improves the mobility of the concrete mixture. For example, from P2 to P3, from P3 to P5. This allows to reduce the consumption of the additive by 1.5 - 2 times. It has been established that the superplasticizer can be adsorbed only on hydrate neoplasms. For pre-treatment, it is necessary to use special equipment with high energy consumption such as RPA, fine grinding mills, disintegrators and various activators. Obtaining a cement-water suspension can be carried out in mortar, concrete mixers, in mixers of any type. The concrete mixture consists of cement, crushed stone, sand, water and superplasticizer C-3, determination of the draft of the cone using the Abrams cone, determination of the cubic strength using a hydraulic press in accordance with GOST 10180-2012. It becomes possible to increase the grade in terms of workability as a result of preliminary hydroactivation of the plasticizing additive, which leads to a decrease in its consumption. This method of hydroactivation of superplasticizers for the manufacture of cement concretes can be used to save the consumption of a plasticizing additive.


Introduction
Currently, concrete mixes with various additives of both mineral and organic origin have found wide application in construction [1][2][3][4][5][6]. The increased amount of the mortar component of concrete mixtures obtained during the preparation process affects their quality and is determined by the type and value of forces arising between the particles of the solid phase and the liquid.
The direction of industrial waste disposal is actively developing. Waste or industrial surpluses are added to the composition of the concrete mixture or binders, mainly in order to reduce the cost of the product [7].
Superplasticizers based on sulfated melamine formaldehyde resins and based on naphthalene sulfonic acid condensation products are used as organic additives. When using these additives [13,14], it is possible to obtain concrete mixtures with different mobility grades. There are both positive and negative effects, such as: there is an increase in the period of formation of the structure, causing significant air entrainment, resulting in a decrease in strength. When using these diluents, it becomes possible to obtain concrete mixtures with different grades of workability. To reduce the negative impact on the structure and properties of concrete materials [15][16][17], there are several methods, including pretreatment of cement binder in full or in part with a superplasticizer at water-cement ratio from 0.5 to 1 and subsequent mixing with other components of the concrete mixture.

Materials and methods
At the initial stage of hardening of the cement component, a colloidal solution is formed within 40-45 minutes. Thus, in order to increase the intensity of the process of initial formation of a colloidal solution, it was proposed that a part of the cement, about 10% of the calculated amount, be activated by stirring in an aqueous medium at V / C = 0.5-1 for 40-45 minutes, and the plasticizer be injected in an amount of 1% of this part of the binder. Cementwater suspension can be obtained in concrete mixers or mixers of any type ( Fig. 1,  2). As a result of this procedure, the consumption of plasticizer is reduced to 0.1% of the total amount of cement and the strength of cement concrete is increased by 12-14 MPa. The negative point in this case is the long activation time. When preparing the suspension in a conventional concrete mixer by dissolving 5-10% of the calculated amount of cement in the entire mixing water before mixing with the remaining amount of cement and aggregate, it was possible to reduce the preparation time of the cement-water suspension to 1-2 minutes.
The activity of the superplasticizer and the mobility of the concrete mixture increases due to the preliminary dissolution of part of the cement in the mixing water containing the superplasticizer additive, which leads to the formation of a colloidal solution with an increased content of micro-and nano-particles.  This method was tested on the example of a concrete mixture consisting of cement in the amount of 376 kg/m3, crushed stone in the amount of 1105kg/m3, sand in the amount of 647 kg/m3, water in the amount of 210 kg/m3 and superplasticizer C-3 in the amount of 0.6% of the binder weight. Examining the control composition, the brand was determined by mobility -P4 (17-19 cm of cone sediment). The cubic strength of concrete, at the age of 28 days when hardening under normal conditions was determined using a press, is 29.42 MPa (Fig. 3). When using a cement-water suspension prepared with cement in an amount of 37.6 kg, all the mixing water and 0.6% of the additive C-3 of the total binder weight according to the above method, the concrete mixture had a cone draft of 23-26 cm, and the cubic strength was 33.47 MPa (Fig. 4). Obtaining a modified concrete mixture with the same mobility as the control sample was possible with a reduction in the amount of superplasticizer C-3 by 1.5-2 times. This happens when cement interacts with water and the dissolution of clinker minerals on the surface of cement grains begins

Results
Depending on the values of the diffusion of the flow and the ratio of the dissolution rates of minerals, it follows that the highest dissolution rate has Dissolution products of all clinker minerals pass into the liquid phase of the cementwater suspension. Depending on the values of the diffusion of the flow and the ratio of the dissolution rates of minerals, it follows that C 3 A has the highest dissolution rate. However, in the first minutes, i.e. before the formation of dense screening films on the cement grains, the cement-water suspension has time to be saturated with the products of C 3 S dissolution. At the same time, Ca 2 + , OH -, SO 4 4ions are present. There is a rapid increase in the pH of the liquid phase of the cement-water suspension as a result of the formation of an excess of OHions. At the same time, highly basic calcium hydrosilicates are formed and the concentration of Ca 2 + in the liquid phase of the cement-water suspension exceeds, as a rule, 1.12 g/l.
The resulting particles of neoplasms are characterized by a high degree of dispersion with Ssp = 200-400 m2/g, i.e. crystallites of calcium hydrosilicates reach a thickness of 1-3 nm. With such a value, from half to 2/3 of the atoms and ions that compose them are on the surface and, therefore, they are characterized by the presence of free energy, which determines the connection of particles of neoplasms and the hardening of the entire system.
With the introduction of effective diluents such as superplasticizer C-3 into a cementwater suspension, it was found that the superplasticizer can be adsorbed only on hydrate neoplasms. If the superplasticizer is introduced a minute after mixing the cement with water, the system acquires a predominantly electronegative ξ-potential and the effect of the superplasticizer is more effective. Modification of a cement-water suspension with a superplasticizer shows that highly dispersed hydration products are active adsorbents of superplasticizers. When the superplasticizer interacts with various materials, for example, with quartz sand, only physical adsorption is observed, its value is not great. Adsorption on limestone and especially on cement is due to chemisorption, in which particles of the adsorbed substance and particles of the absorber interact chemically, in contrast to adsorption, when the absorber adsorbs only on the surface. Adsorption of the superplasticizer by new formations is largely determined by the content of these products in the medium of the hydrating cement. The introduction of a superplasticizer into a cement-water suspension increases the rate of adsorption action, which can be estimated by the first exo-effect. This phenomenon is due to the fact that in the first minutes the surface of the cement particle is wetted and its hydrophilicity increases due to the adsorption of polyphilic macromolecules of the superplasticizer.
The increase in the rate of the adsorption effect of the superplasticizer was also estimated by the strength of the current passing through the cement-water suspension with different water-binding activation ratios (Table 1.).

Conclusion
The data in Table 1 indicate that the intensive growth of the current strength in a cementwater suspension subjected to constant mixing (and in solutions there is ionic conductivity) continues most intensively for 2-2.5 minutes. During this period, the ions Ca 2 + , OH -, SO 4 4are formed. And after 2 minutes, ettringitis is already formed in the form of dense screening films. Preliminary hydroactivation of the plasticizing additive leads to an intensification of its action in concrete mixtures: the mobility of concrete mixes increases, for example, the grade and higher from P2 to P3 from P3 to P5, this allows you to reduce the consumption of plasticizing additives by 1.5-2 times.