The main disadvantage of gypsum and gypsum concrete products molded by casting, vibrating, rolling, and pressing methods is low water resistance, which manifests itself in a significant decrease of strength at humidification. The authors have shown the possibility of increasing the strength and water resistance of pressed gypsum products by modifying their structure with an additional crystallization framework of sparingly soluble calcium hydrogen phosphate dihydrate. The purpose of the research is to obtain an adjustment equation that makes it possible to determine the rational dosages of modifying additives depending on the values of the specified strength and water resistance of pressed gypsum composites, as well as to assign the optimal duration of mixing the semi-dry molding mixture until the sealing pressure is applied to it. Using mathematical experimental planning methods, there was investigated the effect of the mixing time of the molding mixture and the dosages of the modifying additives on the basic physical and mechanical properties of pressed gypsum composites compacted at a pressure of 40 MPa. Ammonium dihydrogen phosphate and carbonate-containing sludge from the chemical water treatment of thermal power plant were used as modifiers. It has been revealed that in the proposed technology, along with the dosages of modifying additives, the physical and mechanical properties of the material are significantly affected by the duration of mixing the molding mixture, during which chemical interaction occurs between its components, which increases the strength and water resistance of gypsum modified pressed composites. Experimental and statistical models of the most important technical characteristics of the proposed material have been developed, depending on the main prescription and technological factors, which make it possible to determine the conditions for obtaining pressed gypsum products with specified properties. An adjustment equation has been obtained that makes it possible to establish rational dosages of modifying additives and the optimal mixing time of the molding mixture at a given value of the softening coefficient.
1.Introduction
Gypsum construction products molded from semi-dry mixtures under high pressures are characterized by a simple production technology and have a number of valuable properties. First of all, they have a high compressive strength (up to 60–70 MPa), which can be attained even with low-grade gypsum binders, and they do not require artificial drying [1, 2]. However, they are united with other types of gypsum and gypsum concrete products formed by casting, vibrating, or rolling methods by low water resistance, which manifests itself in a significant decrease in strength when wet [3, 4]. This is explained by the dissolution of calcium sulfate dihydrate at the contact points of crystalline junctions in its structure, as well as the wedging effect of aqueous films adsorbing in microcracks and capillary pores during water saturation [5, 6].
The hardened gypsum binder’s structure can be made more resistant to the dissolving and wedging action of water, as many researchers claim, by adding an additional crystallization framework composed of sparingly soluble compounds [7–9].The results of our previous studies show that the water resistance of pressed gypsum composites in-creases significantly in the case of the formation of shielding protective films from sparingly soluble calcium hydrogen phosphate dihydrate on the elements of their fine-crystalline structure due to the modification of gypsum binder by the addition of ammonium dihydrogen phosphate NH4H2PO4 [10, 11]. In the technology of gypsum products made by casting, chemical additives that form sparingly soluble phase films, such as phosphate films, on the surface of grains of hydrated neoplasms are usually used in small dosages to slow down the setting of gypsum binders [12–14]. The pressing method allows for a significant increase in the dosage of the chemical additive because right after molding the material has a high structural strength. Excessive inhibition of the processes of structure formation in this case is not dangerous from the point of view of in-house transportation of freshly molded products. However, in the future, more complete shielding of the formed fine-crystalline structure of the material with films of sparingly soluble compounds will ensure an increase in its water resistance.
In gypsum mixtures with a reduced water content, various fine fillers can have a positive effect on the processes of binder formation. At the same time, in order to increase the strength and water resistance of gypsum products, it is appropriate to use fillers that can form water-resistant complexes with gypsum, as well as with chemical additives used [15–17]. In this case, the water adsorbed by the filler grains can serve as a reserve for the hydration of gypsum binder in a freshly molded product, and calcium sulfate dihydrate, which crystallizes with some increase in volume, will be deposited in the pores of the material, thereby healing defects in its structure [18–20]. At the same time, choosing the appropriate water-binding ratio is essential. It is necessary to ensure, on the one hand, proper compaction of the press powder and the absence of water separation, and on the other hand, complete hydration of the gypsum binder and, consequently, stability of properties and the absence of volumetric deformations of the artificial stone-like material over time. Given the aforementioned, in previous research, we used carbonate-containing sludge from the chemical water treatment of thermal power plants as a filler. The possibility of replacing a significant part of the gypsum binder (up to 60 %) with a carbonate-containing filler was established. Lower product costs and improved environmental conditions around thermal power plants are possible additional outcomes of using a secondary resource as a modifying filler [10, 11]. However, the analysis of the results of the pre-experiments revealed that in our proposed method of modifying the composition and structure of pressed gypsum composites, one of the most important factors is the appropriate selection of the molding mixture preparation time. It is during the mixing of gypsum binder with a modifying filler sealed with an aqueous solution of NH4H2PO4 that the binder starts to hydrate and the chemical additive interacts with both CaSO4·0.5H2O binder and CaCO3 sludge. The ongoing chemical processes lead to the formation of films of sparingly soluble calcium hydrogen phosphate dihydrate CaHPO4·2H2O (brushite) on the surface of semi-aqueous gypsum grains. This compound is isomorphic to dihydrate gypsum, but it is more than ten times less soluble. The formation of calcium hydrogen phosphate dihydrate also occurs on the surface of the grains of the modifying filler due to the interaction of NH4H2PO4 with CaCO3 of the chemical water treatment sludge. Further, these films protect the crystallization contacts of already hardened composites from dissolution at humidification, which largely increases their water resistance.
The chemical and physicochemical processes occurring when the molding mixture is mixed are described in more detail in [21, 22]. However, the nature and intensity of these processes, as well as the energy state of the system, as demonstrated by the research findings of the scientific school of Professor O.P. Mchedlov-Petrosyan [23, 24], must be coordinated with the moment of technological impact on the molding mixture obtained after mixing. In this instance, the use of modifying additives is likely to have the biggest impact. In the technology of molding gypsum products by pressing, such an effect is reached by, first of all, the application of sealing pressure to the prepared mixture. Pressing at the moment of changing the hydration stage of the binder activates and enhances the structural processes occurring in the system, ensures the formation of a more durable and water-resistant crystallization framework at further stages of structure formation.
The purpose of this work is to obtain an adjustment equation that makes it possible to determine the rational dosages of modifying additives depending on the values of the required strength and water resistance of pressed gypsum composites, as well as to assign the optimal mixing time of the semi-dry molding mixture until sealing pressure is applied to it.
To achieve the goal, it is necessary to consistently solve the following tasks:
to study the effect of the duration of mixing the molding mixture on the basic physical and mechanical properties of pressed gypsum composites at various consumption rates of modifying additives; to establish rational intervals for varying the mixing time of the molding mixture, the dosages of the chemical additive and the modifying filler, and to perform experimental and statistical modeling of the basic physical and mechanical properties of the final product, depending on these factors; to analyze the experimental statistical models and with these results to obtain an adjustment equation that enables the control of the structure formation processes of pressed gypsum and demonstrates that each of the consumption rates of carbonate filler and ammonium dihydrogen phosphate corresponds to its optimal mixing time.
2.Methods
In experimental studies, we used G-5 B II grade gypsum binder according to Russian State Standard GOST 125-2018 “Gypsum binders. Specifications.” Finely dispersed carbonate-containing sludge from the chemical water treatment of a thermal power plant in combination with a chemical additive, ammonium dihydrogen phosphate, was used as a modifying filler in the composition of molding mixtures.
The carbonate-containing filler used in these studies is waste generated in the technological process of softening the water of the Don River with slaked lime and soda at a thermal power plant. As a result of this preparation, the calcium and magnesium salts present in the river water are converted mainly into carbonates of these metals. The resulting sludge is a secondary resource, which is commonly referred to as sludge from chemical water treatment. After drying, it is a fine powder with a specific surface area of 780 m2/kg. Ammonium dihydrogen phosphate (NH4H2PO4) is a salt formed as a result of the neutralization of orthophosphoric acid by ammonia. Details regarding the additives, their mechanism of action, and effects are provided in [10, 11].
When molding gypsum products by casting or vibration methods, it is impossible to modify the carbonate filler with acid additives directly during the preparation of the molding mixture, since the carbon dioxide released during this process causes the gypsum dough to become porous. This feature is used in the manufacture of gypsum products by casting. In a semi-dry mixture of gypsum binder and finely dispersed calcium carbonate, suitable for the manufacture of products by pressing, carbon dioxide can be freely released into the environment [21, 22]. In this regard, selecting the appropriate molding mixture preparation method and duration based on the ongoing chemical processes is a crucial task [25, 26].
The components of the molding mixture (gypsum binder and sludge sealed with ammonium dihydrogen phosphate solution) were mixed in a laboratory slider mixer. The completeness of neutralization of the acidic solution of the chemical additive during mixing was controlled by changes of the pH of the aqueous extracts (1:5) using a potentiometric method on a universal EV-74 ionomer with platinum and glass electrodes.
Control samples, cylinders with a diameter and height of 50.5 mm, were formed from the press powder obtained after mixing at a pressure of 40 MPa in special molds. The prepared samples were stored for 24 hours in air-dry conditions and then dried in a drying cabinet at a temperature of (55±5) °C to a constant weight. Half of the samples from each batch were immersed in water for 24 hours. The dried and water-saturated samples were tested for compressive strength and, based on the results obtained, the coefficient of softening was calculated as the ratio of the strengths of the material in the dry and water-saturated state.
Experimental and statistical modeling of the technical characteristics of gypsum modified pressed composites was carried out using mathematical experimental planning methods. For these purposes, a complete factorial experiment on the B3 Box composition plan was carried out [27]. Mathematical models in the form of a second-order polynomial that adequately describe the technical characteristics of the proposed material were constructed with the help of our original software products. The adequacy of the obtained models was assessed by the value of the Fisher criterion calculated for them [27].
3.Results and Discussion
As already mentioned in the introduction of this paper, as a result of our earlier experimental findings [10, 11], there was revealed the possibility of increasing the strength and water resistance of pressed gypsum composites by modifying their composition and structure with additives of carbonate sludge from chemical water treatment of thermal power plants and ammonium dihydrogen phosphate. The proposed technology does not require the development and manufacture of special molds and devices. The amount of time the mixture is held under pressure does not exceed the time spent when pressing products based on other mineral binders. It was found that the time of mixing is an important factor in the proposed technology to produce gypsum products, providing for the compaction of a semi-dry molding mixture by pressing. At the same time, an important element of the mechanism of action of a chemical additive is its interaction not only with calcium sulfate hemihydrate of the gypsum binder but also with calcium carbonate of the filler. An aqueous solution of the chemical modifier has an acidic environment (pH ≈ 3.7). This is explained by the fact that when NH4H2PO4 is dissolved in water, along with the hydrolysis of this salt, dissociation of the dihydrogen phosphate ion occurs:
The hydrogen ions present in the solvent water interact with the CaCO3 of the filler, which leads to the formation of calcium hydrogen phosphate dihydrate on the surface of its grains.
Due to the high concentration of calcium carbonate in the mixture, these processes gradually lead to a change in the pH of its medium from acidic to neutral. Control of the neutralization of an aqueous solution of ammonium dihydrogen phosphate during its mixing with the dry components of the molding mixture was carried out according to the procedure described above. The mixture was sampled for pH measurement after every minute of stirring. The pH change of the mixture is shown in Table 1.
Table 1. Changes in the pH of components and mixtures depending on the duration of mixing.
Number
of composition
Content of components, %
Consumption of NH4H2PO4, %
of the mass of the dry components of the mixture
Duration of mixing, min
рН of
water
extracts
gypsum binder
sludge
from chemical water treatment of thermal power plants
1
100
0
0
1 and more
7.15
2
100
0
2
1
2
3 and more
6.80
7.10
7.30
3
0
100
0
1 and more
9.00
4
80
20
0
1
2 and more
8.10
8.30
5
80
20
2
1
2 and more
7.00
7.40
6
60
40
2
1
2 and more
7.20
9.00
As seen from Table 1, after 2 minutes of mixing, the pH of the mixture rises to approximately neutral (pH = 7.00). This indicates a high rate of reactions between ammonium dihydrogen phosphate, calcium sulfate, and calcium carbonate. As a result of chemical processes occurring during mixing of the molding mixture, there appear hydrate neoplasms on the resulting microcrystals and in the places of their contact with the surface of the filler grains – sparingly soluble protective films of calcium hydrogen phosphate dihydrate (CaHPO4·2H2O). These films play an important role in the organization of the fine-crystalline structure of the artificial stone-like material and later complicate the dissolution of crystallization contacts at humidification, thereby providing increased water resistance.
In the proposed method of increasing the strength and water resistance of pressed gypsum composites, it should be taken into account that at the stage of their structure formation, a significant effect is exerted by slowing down the hydration of gypsum binder due to the formation of shielding films of sparingly soluble calcium hydrogen phosphate dihydrate on the surface of its grains [10, 21, 22].
Fig. 1 shows the dependence of the compressive strength of dried samples, and Fig. 2 shows the dependence of water-saturated samples made by pressing from various compositions on the duration of mixing after which they were molded.
The duration of mixing varied from 2 to 20 minutes. All the compositions studied were prepared with a water-solids ratio of W/S = 0.19, which ensures complete hydration of semi-aqueous gypsum, and the samples were molded at a pressing pressure of P = 40 MPa.
As seen from Figs. 1a and 2a, the strength of the pure gypsum binder material is significantly affected by the quantitative ratio between the additives introduced and the duration of mixing the mixture. Longer mixing of the mixture leads to a significant decrease in the strength of dried and, to an even greater extent, water-saturated samples. This is due to the high rate of hydration of the gypsum binder, which is largely completed already during the mixing process. As the mixing time increases, more calcium sulfate dihydrate is formed and the amount of active hemihydrate decreases, the hydration of which in the molded product mainly determines the strength of the resulting material structure.
With the introduction of 1 % NH4H2PO4 additive (Figs. 1a and 2a, Curve 2), the dependence of the strength of the material on the mixing duration is of a different nature. The samples molded after 2 minutes of stirring have approximately the same strength as the samples without the addition of ammonium dihydrogen phosphate. An increase in the duration of mixing leads to an increase in the strength of the material. The samples molded from this composition have the maximum strength when the mixing time of the molding mixture is 12–14 minutes. With a longer mixing duration, the strength of the samples decreases. In compositions that do not contain a carbonate filler, an increase in the consumption of the NH4H2PO4 additive negatively affects the strength and water resistance of the samples made.
а)
b)
c)
Figure 1. The dependence of the compressive strength of dry samples on the duration of mixing of the molding mixture of compositions: а) without sludge; b) with 20 % of sludge; c) with 40 % of sludge; 1 – without ammonium dihydrogen phosphate; 2–4 – with 1, 2, and 3 % of ammonium dihydrogen phosphate respectively.
A further increase in the mixing duration from 14 to 20 minutes leads to a decrease in the strength of the samples with 1 % of the NH4H2PO4 additive but practically does not affect the strength of the samples with 2 % of this additive. This is explained by a significant slowdown in the hydration of the gypsum binder due to the formation of blocking films of sparingly soluble calcium hydrogen phosphate dihydrate on the surface of its grains, which is especially pronounced at high dosages of the chemical additive.
When a 3 % ammonium dihydrogen phosphate additive is added to the gypsum binder, a significant decrease in the strength of the pressed gypsum composites occurs. This is explained, on the one hand, by the formation of too thick phase films on the surface of binder particles, characterized by increased porosity and reduced elasticity, and, on the other hand, by the fact that when ammonium dihydrogen phosphate is consumed in excess of the optimal amount, an excessive amount of ammonium sulfate (NH4)2SO4 is formed in the molding mixture [11, 21]. This compound is a strong electrolyte that increases the solubility of gypsum and thereby accelerates its crystallization.
When replacing a part of the gypsum binder with a fine filler, mixing must be carried out for a sufficient time to obtain a homogeneous mixture. As our research has shown, with the combined use of sludge additives and ammonium dihydrogen phosphate, in order to produce the material with the highest strength, the mixing time must be carefully chosen.
а)
b)
c)
Figure 2. The dependence of the compressive strength of water-saturated samples on the duration of mixing of the molding mixture of compositions: a) without sludge; b) with 20 % of sludge; c) with 40 % of sludge; 1 – without ammonium dihydrogen phosphate; 2–4 – with 1, 2, and 3 % of ammonium dihydrogen phosphate, respectively.
Figs. 1b and 2b show that the composition containing 20 % additives of chemical water treatment sludge and 2 % additives of ammonium dihydrogen phosphate has the maximum strength (Curve 3) with an ideal mixing period of 8 to 12 minutes. With an additive content of 1 % in the mixture (Fig. 1b), the highest strength is achieved after 8–10 minutes of mixing, and at 3 % – after 12–14 minutes. Moreover, an increase in the mixing time of a mixture containing 3 % NH4H2PO4, has a negative effect on the strength of the samples to a much lesser extent, compared with a mixture with 1 % chemical additive. This confirms our assumptions about different hydration rates during the formation of the structure of pressed gypsum composites in the presence of different amounts of film-forming agent additives.
When replacing 40 % of the gypsum binder with sludge (Figs. 1c and 2c, Curve 1), samples without ammonium dihydrogen phosphate additives showed the greatest strength when the mixing duration was 10 minutes. The same mixing time can be called optimal for the composition of the molding mixture with 1 % ammonium dihydrogen phosphate. An increase in the consumption of ammonium dihydrogen phosphate while prolonging the mixing time of the mixture improves the physical and mechanical characteristics of the samples made from them. It is most advisable to mix mixtures with 2 % of the additive for 12–14 minutes, and with 3 % – for 14–16 minutes.
Therefore, the conducted studies have revealed that the duration of mixing gypsum binder with modifying additives is a crucial technological component of the proposed method of modifying the structure of pressed gypsum composites to improve their strength and water resistance. The highest strength (45–60 MPa) and softening coefficient (0.65–0.70) were demonstrated by samples of composite binders containing 20–40 % of sludge and 2 % of ammonium dihydrogen phosphate with an optimal mixing duration of 10–14 minutes.
It has been established that the determination of optimal dosages of additives and the duration of mixing of the molding mixture must be carried out considering the combined influence of these factors on the physical and mechanical properties of the material. Therefore, at the second stage of the research, we conducted experimental and statistical studies, the purpose of which was to obtain mathematical models of the physical and mechanical characteristics of the proposed material, allowing us to obtain products with specified properties. For this purpose, a complete factorial experiment was planned and implemented using the B3 Box composition plan [27], and the studied prescription and technological factors and rational intervals of their variation were adopted based on the above analysis of the performed pre-tests.
The conditions for planning and conducting the experiment are presented in Table 2.
Table 2. Conditions of the experiment.
Code
Code value
The factors under study and the intervals of their variation
X1 – chemical water treatment sludge
additive, %
X2 – ammonium
dihydrogen phosphate additive, %
X3 – duration of mixing, min
Main level хi 0
0
20
2
10
Variation range Δхi
∆
10
1
5
Upper level хi max
+1
30
3
15
Lower level хi min
−1
10
1
5
The effect of the water-solids ratio (W/S) and pressing pressure (P) on the studied physical and mechanical properties of gypsum composites was not considered in the paper. Based on the results of the previous studies, these factors were stabilized for all points of the experiment (V/S = 0.19; P = 40 MPa).
During the experiment, six control samples were produced for each point of the plan. The changes in the compressive strength of dry and water-saturated samples ( and respectively), the average density of the material its water absorption by mass , and softening coefficient were studied.
The average values of the experimental data for each point of the experiment, as well as the calculated theoretical values of the studied technical characteristics of gypsum modified composites are presented in Table 3.
Table 3. Test results of control samples and calculated values of physical and mechanical properties of gypsum modified composites.
Experiment number
Experiment plan
Property under study
X1
X2
X3
Y1 – strength in the dry state, MPa
Y2 – strength in the water-saturated state, MPa
Y3 – average density, kg/m3
Y4 – water absorption by mass, %
Y5 – softening coefficient
1
+1
+1
+1
47.3 / 48.1
30.5 / 30.6
1920 / 1918
7.0 / 7.1
0.63 / 0.64
2
+1
+1
–1
41.0 / 39.9
24.8 / 25.5
1921 / 1923
7.1 / 7.2
0.62 / 0.62
3
–1
+1
+1
40.5 / 39.4
22.4 / 23.6
1928 / 1926
8.2 / 8.2
0.58 / 0.59
4
–1
+1
–1
28.3 / 30.1
17.9 / 17.4
1933 / 1932
8.0 / 8.0
0.56 / 0.57
5
0
+1
0
54.7 / 54.4
38.3 / 37.1
1948 / 1950
6.0 / 5.9
0.69 / 0.68
6
0
0
+1
58.2 / 59.4
42.1 / 41.3
1941 / 1945
6.0 / 6.2
0.71 / 0.70
7
+1
0
0
54.3 / 55.5
39.1 / 37.9
1926 / 1923
6.1 / 6.0
0.68 / 0.69
8
0
0
0
63.6 / 62.6
45.1 / 44.4
1948 /1949
5.8 / 5.4
0.71 / 0.72
9
–1
0
0
48.9 / 48.2
31.5 / 32.0
1930 / 1932
6.6 / 6.8
0.66 / 0.65
10
0
0
–1
58.2 / 57.5
39.5 / 40.6
1950 /1946
6.2 /6.2
0.70 / 0.70
11
0
–1
0
54.2 / 55.0
34.1 / 35.7
1950 / 1947
6.2 / 6.6
0.64 / 0.65
12
+1
–1
+1
41.8 / 39.9
22.3 / 22.7
1920 / 1921
7.9 / 7.9
0.58 / 0.57
13
+1
–1
–1
44.3 / 45.4
28.1 / 27.1
1915 / 1917
8.2 / 8.2
0.60 / 0.59
14
–1
–1
+1
34.0 / 35.0
19.6 / 18.8
1930 / 1928
8.8 / 8.7
0.55 / 0.55
15
–1
–1
–1
40.5 / 39.6
22.6 / 22.4
1925 / 1926
8.7 / 8.6
0.57 / 0.57
16
0
0
0
63.4 / 62.6
46.3 / 44.4
1948 /1949
5.7 / 5.4
0.73 / 0.72
17
0
0
0
65.2 / 62.6
46.9 / 44.4
1948 /1949
5.8 / 5.4
0.72 / 0.72
Note: before the slash – an experimental result, after the slash – a theoretical result calculated based on the experimental statistical models obtained.
The obtained experimental and statistical models of the most important technical characteristics of gypsum modified pressed composites allow for a comprehensive analysis of their relationship with the factors studied in the accepted ranges of their variation. This makes it possible to set the conditions for obtaining pressed gypsum products with specified properties. The results of the experimental statistical modeling are in good agreement with the results of the above-described pre-tests and therefore are not reanalyzed further in this paper. Considering the goals of the study, the fifth three-factor quadratic model is of the greatest relevance since it describes how the modifying filler, the amount of NH4H2PO4 additive, and the duration of mixing the gypsum mixture impact the softening coefficient (water resistance) of pressed composites:
(1)
As a result of the transformation of the equation (1), three quasi-one-factor models can be obtained that make it possible to evaluate the individual effect of the three studied indicators on the water resistance of gypsum modified pressed composites:
(2)
(3)
(4)
Quasi-one-factor models of the effect of the carbonate filler consumption the dosage of the chemical additive , and the duration of mixing of the molding mixture are graphically presented in Fig. 3.
Figure 3. Quasi-one-factor models of the effect of additives of chemical water treatment sludge (Х1) and ammonium dihydrogen phosphate (Х2), as well as the duration of mixing (Х3) on the softening coefficient (water resistance) of the material: “+” – upper level; “–” – lower level.
As shown in Fig. 3, the modifying effect of the carbonate filler has a significant impact on increasing the softening coefficient of pressed gypsum composites. Moreover, the highest values of the softening coefficient can be achieved if the other two factors studied ( and ) are at the upper level. If the dosage of the ammonium dihydrogen phosphate additive and the duration of mixing are at the lower level, then the softening coefficient, on the contrary, decreases with an increase in the consumption of the sludge additive from chemical water treatment of thermal power plants.
The effect of the addition of ammonium dihydrogen phosphate on the water resistance of pressed gypsum composites is of a different nature. The material has the highest softening coefficient with the lowest consumption of ammonium dihydrogen phosphate additive, if the other two factors are at the lower level. If = +1 and = +1, then the softening coefficient increases with the increasing dosage of the NH4H2PO4 additive.
The duration of mixing the molding mixture, in comparison with the dosages of the carbonate filler and the chemical additive, has a less significant effect on the water resistance of the material. Fig. 3 clearly shows that the optimum is always located in the experiment area. In this regard, when looking for a combination of factors which will result in the material with the maximum softening coefficient, that is, when solving the optimization problem of the first type, using the method described in [25], the optimal control function for the duration of mixing the mixture can be calculated. This function will look like:
(5)
It follows from the equation (5) that each of the consumptions of carbonate filler and ammonium dihydrogen phosphate has its own optimal mixing time.
After substituting (5) into (1), the adjustment equation of the softening coefficient of the material is obtained for
(6)
The adjustment equation shows how the softening coefficient changes depending on factors and if the duration of mixing the mixture is optimal for any combination of dosages of chemical water treatment sludge and ammonium dihydrogen phosphate additives. This equation can be used to calculate the appropriate additive dosages for a given softening coefficient and to determine the rational mixing time for the molding mixture.
4.Conslusion
As a result of the research conducted, it is possible to draw the following conclusions:
It is shown that it is possible to increase the strength and water resistance of gypsum pressed composites by replacing a significant part of the gypsum binder with a finely dispersed carbonate filler and creating protective shielding films from sparingly soluble calcium hydrogen phosphate dihydrate on the surface of forming microcrystals of gypsum and filler due to the modifying effect of the ammonium dihydrogen phosphate additive. It has been found that in the proposed technology, along with the dosages of modifying additives, the physical and mechanical properties of the material are significantly affected by the duration of mixing of the molding mixture, during which chemical interaction occurs between its components, which increases the strength and water resistance of gypsum modified pressed composites. Making use of mathematical experiment planning techniques, experimental and statistical models of the most important technical characteristics of the proposed material have been developed, which make it possible to determine the conditions for obtaining pressed gypsum products with specified properties. An adjustment equation is proposed that makes it possible to establish rational dosages of modifying additives and the optimal duration of mixing of the molding mixture for a given softening coefficient.