Stabilized clay-gypsum binder

. Clay gypsum is a natural sedimentary rock of chemical origin, containing from 50 to 95% of fine-grained crystalline gypsum, evenly distributed between clay minerals. The purpose of the research was to develop the compositions of the stabilized clay-gypsum binder, taking into account the temperature regimes of its firing. The studies were based on mathematical planning of the experiment with subsequent statistical processing of the results and obtaining polynomials that adequately describe the phenomena under study and with analytical optimization of these polynomials. It has been established that the optimal firing temperature of the initial sedimentary rock is 294 –295 °C. The strength of the clay-gypsum binder is 8.8 – 8.6 MPa, which corresponds to the G-8 grade gypsum binder. The introduction of a superplasticizer additive makes it possible to increase the strength of the binder by 10 – 20%. As a result of the research, the technological parameters for obtaining a clay-gypsum binder, the modes of its heat treatment (roasting) were optimized, and the influence of technologies on the properties of the clay-gypsum binder, which can be considered as an analogue of a binder obtained from natural gypsum raw materials, was studied.


Introduction
One of the effective methods of cost reduction in construction work is the use of materials based on local raw materials, or materials containing secondary products from other industries.With regard to gypsum binders, positive experience with the use of phosphogypsum is known; the study of the possibilities of using clay gypsum is under development [1][2][3].
According to the volume of theoretical and experimental studies and scientific interest in the material, phosphogypsum can be distinguished.It is a by-product of the production of phosphate fertilizers.There are phosphogypsum dihydrate and hemihydrate (passivated), anhydrite that is determined by the technology peculiarities of phosphoric acid production.During phosphogypsum storage, the soluble compounds of phosphorus, fluorine, alkalis contained in it gradually penetrate into the soil and nearby water bodies, which significantly affects the sanitary state of the environment [4][5][6][7].
Gypsum-containing waste also includes products of desulfurization of flue gases of thermal units (power plants).Gases exhausting from fuel combustion containing inert sulfur oxides are treated with lime mud to produce calcium sulfide.Calcium sulfide, when oxidized in a reactor in the presence of oxygen, passes into calcium sulfate dehydrate.
Clay gypsum, heat-treated clay gypsum and stabilized clay gypsum binder in terms of their properties and material composition are representatives of this particular group of gypsum-containing materials.Since the basis of these materials is semi-aqueous gypsum, the theories of its hardening are the determining factor in the formation of the properties of all materials of this group [8][9][10][11].
Clay gypsum has a number of advantages compared to plastering and molding gypsum, namely: clay gypsum plaster is smooth, without pores, has a pleasant texture, is durable and is not afraid of water; setting is slower than with plaster.The mechanical strength is quite significant and has better results than molding plaster when mixed for sculptural castings [12][13][14][15].
The clay-gypsum binder is of particular interest for regions where there is no production of mineral binders, and the existing deficit is covered by imports.The Republic of Karakalpakstan, for example, uses imported gypsum binder; meanwhile, large reserves of gypsum explored on the territory are unused due to the low concentration of gypsum in the rock.
The solution to the issue of producing effective clay-gypsum is based on the methods for processing raw materials and producing a modified gypsum binder with stable properties for building mortars for various purposes, and, first of all, plaster compositions.

Materials and methods
Clay gypsum is a mechanical mixture consisting mainly of sulfuric acid substances.Usually these are loose, earthy formations mixed with fine-grained crystals.The clay-gypsum rock also contains organic impurities.Clay gypsum is gray, with a brownish-yellowish tint, the rock is skinny and dry to the touch.
The experiment is carried out on the basis of rotatable composition plans.As a result, regression equations are obtained; an accuracy of predicting the results up to 98%.Regression equations then used in the formation of optimization and interpolation solutions.Processing of the results, testing of statistical hypotheses and evaluation of the significance of the coefficients of the regression equation, is carried out by methods standard for statistical modeling [16,17].The following factors are variable: crushed clay gypsum strength (X1), heat treatment time (X2), heat treatment temperature (X3), superplasticizer consumption (X4).The experimental conditions are presented in Table 1.

Results and discussion
As a result of the experiment, quadratic nonlinear relationships were obtained between the response functions and variable factors.After evaluating the significance of the coefficients (Δb for the full factorial experiment are listed below), the following regression equations were obtained (only significant coefficients are taken into account, coefficients less than Δb are equated to 0).The strength of the modified clay-gypsum binder in compression (R c , Y 1 ), Δb1 = 0.4 MPa: The strength of the modified clay-gypsum binder is affected by the heat treatment parameters (coefficients 1.4 and 1.6 at Х 2 and Х 3 ), and by the superplasticizer consumption (coefficient 1.8 at Х 4 ).The effect of the joint influence of the degree of grinding of claygypsum and the heat treatment time is significant (the coefficient at Х 1 Х 2 is equal to +1.2).At the same time, the effect of temperature and superplasticizer consumption is not linear (coefficients "-0.9" at Х 3 2 and "-1.1" at Х 4 2 ).At heat treatment temperatures exceeding 250 °C, the increase in the strength of the clay-gypsum binder slows down, which can be explained by a deeper dehydration of the binder components.The average density of the modified clay-gypsum binder (ρ, Y 2 ) is determined by the regression equation (Δb 2 = 22 kg/m 3 ): To the greatest extent, the average density is influenced by the heat treatment temperature (coefficient is equal to 52 at Х 3 ), the heat treatment time has a lesser effect (coefficient is equal to 28 at Х 4 ); at the same time, a simultaneous increase in the temperature and duration of firing leads to an increase in density (the coefficient is 30 at Х 2 Х 3 ).An increase in the superplasticizer consumption in the intervals provided for by the experimental conditions (Table 1) leads to a decrease in the density of the hardened samples of the modified clay-gypsum binder.
The optimization of the obtained polynomials for the strength and density of the modified clay-gypsum binder is carried out in three stages.

FORM-2023
https://doi.org/10.1051/e3sconf/202341001014 E3S Web of Conferences 410, 01014 (2023) At the third stage, taking into account the data in Table 1, we calculate the natural values of the optimal firing temperature (t) and the optimal consumption of the superplasticizer (C s ): t = 250 + 50•0.89= 294-295 °С; C s = 0,9 + 0,3•0,82 = 1,1-1,2 %.Equations ( 3) and ( 4) can be solved analytically or presented in the form of a nomogram (Figure 1), which establishes the relationship between the characteristics of the modified clay-gypsum binder, bulk density and heat treatment time at optimal values of heat treatment temperature and superplasticizer consumption.The products of thermal decomposition of clay gypsum, which are substances of the same chemical nature, but with a different dissolution rate, are of interest for studying the role of these processes in structure formation.The optimum firing time is between 2 and 4 hours.The softening coefficient during firing up to 4 hours increases from 0.3 to 0.58.The change in compressive strength most significantly depends on the firing time at a temperature of 250°C, which provides the best flaw of binder material.The best result is obtained by firing for 2-3 hours.Then there is a (slight, up to 10%) decrease in compressive strength.
The conducted researches allow us to draw a conclusion that the properties of the fired clay-gypsum binder depend on the firing modes: firstly, non-linearly, and secondly, simultaneously on changing several technological parameters.This makes it necessary to conduct additional studies using the technique of mathematical planning of the experiment.
The clay-gypsum binder produced after thermomechanical processing of clay-gypsum raw materials has all the necessary properties, but it is necessary to correct them, taking into account the technological and regulatory requirements for plaster mixtures, as well as the peculiarities of the interaction of clay and gypsum modifications.
Stabilization of the properties of the clay-gypsum binder was carried out by application the additive C-3, which was introduced during the binder grinding.In the experiments, the influence of the content of the superplasticizer additive on the mobility of the clay dough, the setting time, and the strength characteristics of the hardened mortar was studied.The consumption of the plasticizing additive varied from 0.4 to 1.2% of the mass of claygypsum.

FORM-2023
https://doi.org/10.1051/e3sconf/202341001014 E3S Web of Conferences 410, 01014 (2023) The results of the experiment evidenced that the application of a superplasticizer (Table 2) leads to a decrease in the water demand of the binder, reduces the normal density from 0.56 to 0.41%.The plasticizer molecules, sorbed on the surfaces of clay particles, form water-repellent interlayers, which changes the equilibrium of states at the "gypsum binder-clay mineralswater" interface and reduces the amount of moisture penetrating into the clay particles.The hydrophobization of clay particles surface possibly leads to a decrease in the DEL potential of micelles of clay minerals and an increase in the concentration of hydration products in the solution part of the hardening clay-gypsum binder.
A decrease in the water demand of the binder and the formation of favorable conditions for the crystallization of neoplasms in the solution part (outside the field of action of the adsorption and electrostatic forces of the micelles) leads to a change in the strength characteristics of the hardened clay-gypsum stone (Figure 2, Table 3).It could be noted that coefficient of softening K s of clay-gypsum stone is also increased.

Conclusions
Clay gypsum is a sedimentary rock, widespread in many regions with developing economies.Producing a clay-gypsum binder seems possible as a result of a complex effect on clay-gypsum of various heat treatment parameters, as well as types of modifying additives.
It has been established that the optimal firing temperature of the initial sedimentary rock is 294-295°C.The strength of the clay-gypsum binder is 8.8-8.6 MPa, which corresponds to the G-8 grade gypsum binder.The application of a superplasticizer additive in the amount of 1.1-1.2%makes it possible to increase the strength of the binder by 10-20%.
The most appropriate areas of application of a binder based on clay-gypsum is the production of clay-gypsum binder and dry building mixtures based on it, as well as gypsum-containing products.This is especially important for regions operating on imported gypsum raw materials.This work was financially supported by the Ministry of Science and Higher Education of Russian Federation (grant # 075-15-2021-686).Tests were carried out using research equipment of The Head Regional Shared Research Facilities of the Moscow State University of Civil Engineering

Fig. 2 .
Fig. 2. Change in the strength characteristics of the clay-gypsum binder after application of C-3 superplasticizer.

Table 1 .
Experimental conditions.The compressive strength of the clay-gypsum binder (Rc, Y1) was an optimization parameter of the experiment.It was determined 2 hours after mixing with water.The average density (ρ, Y2) was the response functions.

Table 2 .
Influence of the content of superplasticizer C-3 on the B/T ratio of the clay-gypsum binder.

Table 3 .
Change in the strength characteristics of the clay-gypsum binder after application of C-3 superplasticizer