HYDRATION AND STRUCTURAL FORMATION KINETICS OF GYPSUM BINDERS DURING SOLIDIFICATION

ABSTRACT

In this study, the kinetics of hydration and structural formation of gypsum binders during solidification were comprehensively studied under the influence of plasticizers. The results of the study made it possible to determine the changes in water potential, hydration level and structural parameters over time. It was found that the type and amount of plasticizers have a significant effect on the crystallization process, capillary-pore structure and interparticle interaction. It is scientifically proven that the optimal addition amount serves to increase the density, strength and operational efficiency of gypsum materials.

АННОТАЦИЯ

В данном исследовании всесторонне изучена кинетика гидратации и структурообразования гипсовых вяжущих материалов в процессе затвердевания под влиянием пластификаторов. Результаты исследования позволили определить изменения водного потенциала, уровня гидратации и структурных параметров во времени. Установлено, что тип и количество пластификаторов оказывают существенное влияние на процесс кристаллизации, капиллярно-пористую структуру и межчастичное взаимодействие. Научно доказано, что оптимальное количество добавки способствует повышению плотности, прочности и эксплуатационной эффективности гипсовых материалов.

Keywords: gypsum binder, hydration, plasticizer, structural formation, water potential, capillary porosity, rheological properties, strength, dispersed system, crystallization kinetics.

Ключевые слова: гипсовое вяжущее, гидратация, пластификатор, структурообразование, водный потенциал, капиллярная пористость, реологические свойства, прочность, дисперсная система, кинетика кристаллизации.

Introduction. The use of highly efficient building materials in the construction of modern buildings and structures is of great scientific and practical importance. Currently, gypsum-based materials are gaining importance among building materials. Due to the sufficient strength, long service life, resistance to external environmental influences, and environmental and technological convenience of gypsum materials, they are widely used in almost all areas of construction. This material has been improved for centuries, and based on modern scientific approaches, its composition and properties are constantly being improved with various mineral and chemical additives [1,2,3].

One of the effective methods for improving the technical properties of gypsum-based materials and products is the use of plasticizing additives. Such additives improve the rheological properties of the material, reduce water demand, and optimize the formation process. As a result, the density, strength, and operational performance of gypsum mixtures significantly increase.

The physical and mechanical properties of gypsum materials are closely related to their internal structure and crystallization process. Therefore, a thorough study of the effect of plasticizing additives on the hardening process, structural formation and final operational properties of gypsum is one of the current scientific directions in building materials science. Research in this area serves as an important scientific basis for creating a new generation of effective and high-quality building materials [4,5,6].

During the hydration process, as a result of the interaction of semi-aqueous gypsum (CaSO₄·0.5H₂O) with water, dihydrate gypsum (CaSO₄·2H₂O) crystals are formed. Plasticizers significantly affect the speed of this process, the shape and size of the growth of crystals, and their mutual arrangement. As a result, the density, porosity and strength indicators of the resulting structure change [7,8].

In conducting research, gypsum binder produced by “Konchi” LLC, located in the Kogon district of the Bukhara region, was selected as the main raw material. Based on the research results, scientifically based recommendations were developed for the enterprise on optimal technological regimes and effective use. This allows to improve product quality, reduce raw material consumption, and improve production efficiency.

The main physical, mechanical, and technological properties of the gypsum binder used were determined based on experimental results, systematically analyzed, and presented in Table 1. These indicators serve as the main criteria for evaluating and comparing the material at subsequent stages of research.

As is known, the main technological task in the production of gypsum products is to obtain a material with high operational performance at minimal energy consumption. These indicators are determined, first of all, by the initial water-gypsum ratio. This ratio directly affects the molding properties of the gypsum mixture, its structural formation, and the efficiency of the drying process.

Table 1

Physical and mechanical properties of gypsum binders

As a result of adding excess water to the mixture, the capillary-porous structure develops excessively, forming a structure with low strength. In addition, the need to subsequently evaporate excess water sharply increases the consumption of additional thermal energy when drying products.

The introduction of plasticizers into gypsum mixtures allows you to improve the rheological properties of the system and reduce water consumption while maintaining its mobility. As a result, the mixture reaches a normal density with less water consumption, which ensures the formation of a denser and stronger structure and an energy-efficient technological process.

As plasticizers, naphthalene-formaldehyde-based - S-3, melamine-formaldehyde-based - Melment F15G and silicon organic - phenol formaldehyde-based - KOFF-3 were selected [9,10].

Research methodology. The hardening process of gypsum binders is a complex physicochemical process directly related to changes in the moisture state in the "binder - water" system. During this process, the solid phase in the phase state of the system, initially in the form of separate particles, gradually passes into a state of a unified structure, while the liquid phase, on the contrary, is transformed into a state separated into capillaries and pores.

As a result of these phase changes, the forces and energy of molecular interaction between particles in the "gypsum - water" system are redistributed. This, in turn, directly affects the structural formation, densification and development of strength properties of the material. The energy state of moisture in the system is expressed by the water potential. Water potential characterizes the energy level of free and bound water in a gypsum system and is the main thermodynamic indicator that determines the direction, intensity, and equilibrium state of moisture movement and is calculated based on the following formula (1):

θ = RTlnφ;                                                 (1)

Results and discussion. Figure 1 shows the curves of the change in water potential over time during the hardening process of gypsum dispersions based on the experimental results. The studies were conducted with the introduction of 0.5% plasticizers into the mixture at a constant water-gypsum ratio, and the results show that, regardless of the type of plasticizer used, the general character of all curves is similar.

Figure 1. Changes in water potential with plasticizers during the hardening of gypsum binders

According to the experimental results, an increase in water potential is observed in the initial stage after mixing, that is, in the period of about 5 minutes. This is explained by the discrete distribution of adsorption complexes formed in the system during the preparation of gypsum paste. At this stage, the liquid phase of the binder system exists as a continuous medium.

After this period, a tendency to decrease in water potential begins. This is due to the gradual formation of a continuous structure of the solid phase, that is, the formation of a coagulation skeleton. At the same time, the liquid phase loses its continuity and separates into separate volumes, and pores and capillary systems appear in the system. These structural changes characterize the main stages of the formation of a strong and stable internal structure during the hardening of the gypsum binder.

As the process progresses, the formation of a capillary-porous colloidal system becomes more and more intense. In this case, it is difficult to establish a clear boundary between different states of the solid and liquid phases, since structural changes occur continuously and merge seamlessly into each other. According to the experimental results, by t = 90 minutes, the water potential values ​​stabilize. This indicates the completion of the structural formation stage during the hardening of gypsum binders in a closed system.

From the analysis of Figure 1, it becomes clear that the water potential in the binder system in which the KOFF-3 type plasticizer is used has the lowest value. This indicates the maximum hydration binding of moisture to the material. Since the initial moisture content in all systems is the same, this situation is explained by the peculiarity of the capillary-porous structure that has arisen in this system. When a plasticizer is used, due to a decrease in water demand, the number of large-sized pores decreases, and the probability of the formation of small-sized pores and capillaries increases. As a result, high-energy interaction of moisture with the material is ensured, which leads to a significant increase in the physical and mechanical properties of the material.

As a result of these changes, interparticle contact-crystallization type bonds are formed in the binder system, resulting in the formation of a capillary-porous structure. The mechanism of formation of this structure is explained, first of all, by the gradual and almost complete release of water by the particles of the solid phase. This process ensures the formation of a strong spatial skeleton in the system.

Analysis of Figures 2 and 3 shows that when various natural plasticizers are used, the structural formation and degree of hydration of the gypsum binder during its hardening process significantly change. In particular, the type and amount of plasticizers directly affect the rate of the hydration process, the kinetics of crystal growth, and the parameters of the resulting capillary-pore system.

Figure 2. The effect of plasticizers on the degree of structure formation of gypsum binder

Analysis of the presented experimental data shows that the degree of structure formation in the binder system increases monotonically over time. In the kinetic curves, regardless of the nature of the surfactants, the phase of η decrease, which is characteristic of “cement-water” systems, is not observed. The decrease in the degree of structure formation is usually associated with the development of destructive processes in the binder system. However, the experimental results presented in Figure 2 show that such processes are practically absent in this system. Destructive processes are usually associated with recrystallization, which occur only in the presence of free water in the system. According to the results of the studies, after mixing gypsum with binder water, boundary liquid layers are formed in the system. These layers orient the polarization regions in the liquid phase. As a result, water molecules are bound to the solid phase and fixed. Therefore, the amount of free water in the “water-gypsum” system is very small. This sharply reduces the likelihood of recrystallization processes. According to the data of Figure 2, the degree of structure formation reaches the highest values in the gypsum system in which KOFF-3 is used. When using plasticizers based on melamine-formaldehyde and naphthalene-formaldehyde, relatively high efficiency is observed in the first type of plasticizer dispersion.

The degree of hydration also increases monotonically over time (Figure 3), which confirms the gradual strengthening of chemical bonds in the system and the strengthening of the final structure.

Figure 3. The effect of plasticizers on the degree of hydration of gypsum binder

Analysis of Figure 3 shows that in gypsum dispersions with the addition of plasticizers, the final degree of the formation of hydrate phases increases. This is explained by the effect of plasticizer additives on the water structure and the activation of active surface centers of the gypsum binder. As a result, the hydration process proceeds more completely and the crystalline structure formed in the system improves.

According to the results of scientific research, such a change in the water structure is directly related to the ionic composition of the sulfo groups contained in plasticizers, in particular S-3 and Melment F15G. That is, the Na⁺ cations and SO₄^2- anions in these additives interact with water molecules, reorganizing their spatial structure. As a result of such ionic-molecular interactions, the structural state of water changes, which in turn increases the reactivity of active centers on the surface of the gypsum binder. As a result, the hydration process accelerates and its final degree reaches high values. The increase in the degree of completion of the hydrate formation process in the system with the presence of KOFF-3 is mainly explained by its effect on the active centers on the surface of the solid phase. This, in turn, is associated with the high dispersing and surfactant properties of the organosilicon compounds contained in the plasticizer. As a result, the ability to react on the surface of the binder particles increases, and the hydration process occurs more completely. In addition, the increase in the final degree of the hydrate formation process can also be explained by the increase in particle mobility in dispersions containing plasticizing additives. In such systems, the viscosity of the dispersed medium is significantly reduced (Figure 4), which facilitates the free movement and interaction of particles. As a result, the contact surfaces between particles increase, diffusion processes accelerate, and favorable conditions for hydration reactions are created [11,12].

Figure 4. Change in the viscosity of gypsum suspension with plasticizers

It should be noted that the degree of completion of structure formation remains η > 0 throughout the hardening process. This means that the initial structure, i.e., a periodic colloidal structure of type I, is formed immediately after the binder system is mixed. This initial structure is the main factor determining the molding (rheological and technological) properties of gypsum paste [13].

When the dosage of plasticizers is reduced, no significant change in the water potential values is observed in mixtures with the same mobility and the same water-gypsum ratio (Figure 5). This indicates that the total moisture content in the system and its energy state remain stable.

However, a decrease in the amount of plasticizers, in particular S-3 and Melment F15G, leads to a decrease in the degree of completion of the structure formation process (Figure 6). This is explained by the fact that these additives form structured adsorption layers on the surface of solid phase particles. These layers regulate the interparticle interaction in the dispersed system and play an important role in the formation of a strong and stable capillary-pore structure of gypsum stone. At the same time, reducing the dosage of additives leads to a decrease in the degree of completion of the hydrate formation process (Fig. 7). This confirms that plasticizers directly affect not only the structural formation, but also the completeness of the hydration process.

Figure 5. Change in the hydration potential of a hardening binder with the presence of plasticizer additives

Figure 6. Effect of plasticizers on the degree of structure formation in a hardening gypsum binder

Figure 7. Effect of plasticizers on the degree of hydrate formation in a hardening gypsum binder

Conclusions. Thus, based on the conclusions drawn from the analyses, the effect of the noted plasticizer additives on the water structure and its physicochemical properties is effectively manifested only in a certain optimal concentration range. The amount of plasticizer introduced in the optimal amount affects the water-gypsum ratio, thereby reducing the water demand of gypsum and increasing the mechanical properties of the resulting material due to the reduction of large pores and an increase in the number of capillaries. Excessive or insufficient use of plasticizers can cause negative changes in material properties due to varying degrees of influence on the hydration period and structure formation.

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