The effect of fillers’ composition on thermal swelling of silicate materials

. The article presents the results of studies of the process of thermal pore formation of materials based on sodium alkaline silicates. The aim of the study is to substantiate fillers composition for liquid glass mixtures. Substandard rocks and technogenic materials were used as fillers. There were determined the temperature intervals of intensive formation of the gas phase during heat treatment of fillers. Comparative analysis of swelling characteristics revealed the preference for a combination of several fillers containing substances with different resistance to temperature transformations. The results of studies of the structure and physical and mechanical properties of porous materials confirmed feasibility of using multicomponent fillers. The cellular material developed is characterized by predominantly closed, high porosity, heat-shielding properties and resistance to mechanical stress.


Introduction
Structure determines properties of each substance. Important elements of the structure of many materials are pores, which are characterized by different configurations and sizes. Technologies for building materials can be divided into two groups. The first group combines technologies aimed at the formation of dense structures with a minimum number of pores [1 -3]. In this case, pores are considered as structural defects that reduce materials strength and durability. The second group includes technologies that ensure the directed formation of materials' porous structure [4 -6]. For such kind of structures, pores guarantee the materials low density and thermal insulating capacity.
Purposeful pore formation is an important component of technological processes for manufacturing materials. Swelling is a common method of pore formation, which is implemented in the materials technologies [4,5,7,8]. Swelling includes saturation of viscous and plastic mass with a gas phase. Various technological methods for regulating the viscosity of plastic mass, the use of chemical, mechanical, and physical methods for saturating the mass with a gas phase ensure creation of cellular structures of various origins and purposes [6 -11].
Swelling is very effective in technology of materials based on the liquid glass. Liquid glass is an aqueous solution of alkali silicates. Liquid sodium glass (Na2O•n SiO2 + m·H2O) is widespread, the composition of which is controlled by changing the silicate modulus (n = SiО2 : Na2О). Liquid glass has chemical activity, binding properties, and adhesion to various materials. The unique properties of liquid glass provide multifunctionality in the composition of various technological mixtures [12 -16].
Liquid glass is very sensitive to heat. Heating up to a temperature of 100 -120⁰С is accompanied by evaporation of free and adsorbed water, partial formation of pores, and hardening of liquid glass [12 -14]. Crystallization water is removed from the liquid glass at a temperature of 130 -180⁰С, and the next stage of pore formation occurs. Hydrated water is removed from the liquid glass at a temperature of 240 -245⁰С. Dehydration process is completed at a temperature of 600 -650⁰С. Porosity of liquid glass is the result of lowtemperature swelling. The resulting structure consists of cells separated by thin partitions. Highly porous structure of expanded liquid glass is characterized by uneven distribution of cells and low mechanical strength. Therefore, to obtain porous materials, substances providing technological properties of moulding sands are added to the liquid glass [15 -22].
Mineral fine fillers are used as components of liquid glass materials regulating moulding sands viscosity. In this case, liquid glass binds finely dispersed components; activates participation of fillers in the processes of structure formation; serves as a source of a gas phase that inflates the pyroplastic mass. Fillers, performing a frame-forming function, affect swelling ability of the liquid glass mixture. According to the nature of the effect on thermal transformations and structure of the porous liquid glass material, fillers are divided into inert, gel-forming, and thermosetting. Therefore, the choice of fillers is very important for the formation of porous structure of materials based on liquid glass.
The objective of the work is to study the effect of fillers composition on thermal swelling of liquid-glass mixtures and properties of porous materials.

Materials and methods of research
The experiments were carried out using liquid sodium glass, which has a silicate modulus n=2.7 and a density of 1350 kg/m³. Based on liquid glass and fillers of various compositions (Table 1), the raw mixtures were prepared ( Table 2). The amount of liquid glass ensured the raw mixtures of the same consistency were obtained.
Preliminary studies revealed desirability of combination of silica fillers, which allows you to control technological properties of moulding sands [11]. When choosing fillers, their effect on technological properties of the mixtures and porosity of the bloated material was taken into account.
Glass cullet is a powdered material made from amorphous silica, obtained by fine grinding of sheet glass fragments and glass containers. Glass powder increases viscosity of the liquid-glass mixture, contributes to uniform distribution of cells in the mass, and serves as a source to form a small amount of pores.
Opoka sedimentary rock (opoka) is formed from skeletal particles of marine organisms. Opoka rock consists of amorphous opal, contains a small amount of tridymite, cristobalite and clay minerals. The experiments used opoka rock extracted as overburden during iron ore deposits development. Opoka rock helps to improve moulding properties of liquid glass mixture, contains minerals involved in pore formation.
Liquid-glass mixtures filled with glass powder and opoka acquire satisfactory technological properties for moulding products of a given configuration. However, such liquid glass mixtures are characterized by high swelling temperature; require stepwise firing and cooling to ensure structure's strength. This necessitated expansion of raw material base of fillers by introducing iron ore enrichment waste into the raw mixtures; clay containing carbonized wood; oil shale. Iron ore enrichment waste is a loose fine-grained mass of particles no larger than 0.5 mm in size. The waste contains minerals as follow: pyroxenes, garnets, epidote, scapolite, chlorites, feldspars, pyrite, and calcite. When polymineral wastes are heated, stepwise thermal transformations occur, accompanied by the release of gaseous compounds [11].
Clay containing carbonized wood (clay) is a rock of sedimentary origin, consisting of kaolinite, hydrargillite and boehmite. Mineral composition of clay is due to proximity to bauxite deposits. Presence of carbonized wood and hydrated minerals ensures gas phase formation during clay's heat treatment.
Oil shale is a clayish sedimentary rock containing kaolinite, hydromicas, montmorillonite, feldspars, calcite, quartz, and pyrite. Oil shale is a carrier of 15 -20 % bituminous substance, which, if heated, serves as a source of the gas phase.
Fillers used in the experiments were preliminarily ground to a specific surface area of 430 -470 m 2 /kg. Moulding sands were prepared by thorough mixing of finely dispersed fillers with the liquid glass. State of the moulding sand was assessed using a conical plastometer according to the value of plastic strength equal to 0.06 -0.08 MPa.
Samples -cylinders (diameter 20 mm, height 10 mm) were pressed from moulding sands using pressure of 0.5 MPa. Samples were fired in a laboratory muffle furnace. The mode of samples' heat treatment was assigned taking into account the objectives of the experiment. Pyrogenic transformations in the raw mixtures were studied by the method of derivatographic thermal analysis on a modernized Derivotograph Q -1500 unit manufactured by MOM brand. Phase composition of materials was determined using a modernized DRON -3M diffractometer. The microstructure of porous materials was studied using a JSM -649 OLV Energy scanning electron microscope.
Condition of the samples fired was assessed by the nature of porous structure. The swelling coefficient was calculated as the ratio of sample volumes before and after firing. Total porosity of fired materials was calculated taking into account the values of substance's true density and samples' average density. The true density of the substance was determined by the pycnometric method. Water absorption of the fired materials was determined taking into account original sample's weight and the weight of a sample saturated with water for 24 h. Thermal conductivity coefficient of porous materials was evaluated using an ITP -MG4 thermal conductivity meter. The strength of fired materials was determined by testing samples on a hydraulic press.

Results
Thermal analysis revealed transformations accompanied by gas phase formation in the mixtures of fillers. To exclude the effect of liquid glass, which is a source of gas formation, the mixtures of dry components were subjected to the thermal analysis. Analysis of the results of thermal transformations in the mixtures (Figures 1 and 2) indicates influence of material composition of fillers on the amount and intensity of gas phase evolution. Maximum weight loss was noted during the firing of mixtures containing fuelcontaining components (clay, oil shale). weight loss due to burning of carbonized wood and partial dehydration of hydrargillite Al(OH)3 to form boehmite AlO(OH). Intense and extended endothermic effect at a temperature of 120⁰С on the thermograms of SOC and SORC mixtures containing oil shale is due to bituminous substance's burnout.
With further heating of the mixtures, the mass loss sharply decreases. In the temperature range from 400 to 600⁰С, the amount of the substance removed is 5 -19 %; in the temperature range from 600 to 800⁰С it reaches 11 -23 % of the total mass loss. With comparable values of the mass removed during heating at a temperature of 400 -800⁰С, the gas phase proportion released from the mixtures containing iron ore enrichment waste is the largest.
Heating of the mixtures SOR (Figure 1a), SORG ( Figure 1d) and SORC (Figure 1e) containing ferruginous waste at a temperature of 400 -800⁰С is accompanied by decarbonization of calcite, dehydration of actinolite, chlorite, epidote, scapolite, as well as melting of the mixture (endothermic effect at a temperature of 685⁰С).
Firing the mixtures SOG and SORG at temperatures above 400⁰С is accompanied by dehydration of boehmite and kaolinite. This is marked by endothermic effects in the temperature range of 400 -500⁰С. The temperature range of 600 -800⁰С is characterized by a gradual softening of the mass. Appearance of a liquid phase in the SORG mixture containing ore dressing waste characterizes the extended endothermic effect at a temperature of 700⁰С. SOG mixture melting is completed at a temperature of 810⁰С. In the mixtures of SOC and SORC containing oil shale, the endothermic processes at temperatures from 500 to 700⁰С are also associated with dehydration of clay minerals and decarbonization of calcite. Low-temperature formation of a melt in the studied mixtures is due to the content of the glass component. The amount and viscosity of the liquid phase in the fired mixtures is determined by the mineral composition of fillers.

Discussion
Important criterion for the pore formation process is the swelling coefficient, which is defined as the ratio of samples' volumes before and after firing. To evaluate swelling, the samples of mixtures with liquid glass were fired at temperatures that ensured release of the main part of the gas phase and sintering of materials. For comparison, we used a mixture of SO prepared from glass powder, flask rock and liquid glass, which ensures production of foam glass material (Figure 3). The mixtures studied swell intensively in the temperature range of 725 -775⁰С, the fired samples acquire a density of 330 -370 kg/m³. At higher firing temperatures, compaction and melting of the samples are observed. Decrease in the expansion coefficient is due to decrease in the gas-holding capacity of the fired mixtures. At temperatures above 775⁰С, most of the fired mixtures acquire low viscosity, inter pore partitions in such mass do not prevent the intense pressure of the gas phase. As a result, a significant part of the gas phase breaks out into the environment, and shrinkage of the fired samples occurs. This process is expressive during firing of the SOR mixture, which is characterized by a narrow range of intense swelling. Mixtures of SORC and SORG are preferred for producing highly porous materials. The use of combined four-component fillers makes it possible to combine low-temperature expansion with the expansion of the temperature range of the viscous-plastic state of the foamed mass.
The values of the expansion coefficients of the mixtures SORC and SORG are comparable with those of the foam glass mixture SO. However, intense pore formation of the SORG and SORC mixtures occurs at low temperatures. High porosity of the studied mixtures with a limited proportion of the glass component was achieved due to gas formation processes with the participation of additionally introduced fillers.
The structure of materials fired at intense swelling temperatures reflects the nature of pore formation of mixtures of various compositions (Figure 4).
Introduction of additional fillers to the SO mixture is accompanied by a change in the nature of porosity and material properties ( Figure 4, Table 3). The SOR mixture containing a fusible technogenic component is characterized by the lowest swelling temperature and reduced material porosity. This is due to the relatively small formation of the gas phase during firing a mixture based on iron ore dressing waste.    When firing mixtures of SOG and SOC containing burnable organic substances, structures with predominance of large pores are formed. This contributes to increase in water absorption of porous materials SOG and SOC. Structures formed during firing of the mixtures SORG and SORC, consisting of four-component fillers, are characterized by more uniform distribution of pores and a smaller cell size. This provides porous materials SORG and SORC with high physical and mechanical properties.
Phase composition of porous materials synthesized on the basis of multicomponent mixtures using silica fillers is represented mainly by crystalline compounds ( Figure 5). The glass-crystal state of the developed porous materials SORG and SORC contributes to increase in their strength compared to foam glass SO ( Table 3).
Advantages of SORG and SORC multicomponent mixtures are due to combination of fillers that ensure participation of the gas phase of various origins in the swelling processes in an extended temperature range. Swelling of SORG and SORC multicomponent mixtures occurs due to gradual participation of the gas phase formed during heating of fillers of various origins. Expansion of the temperature range of swelling of multicomponent mixtures is provided by convergence of the temperatures of pyroplastic state of mixtures and active gas evolution during transformation of various fillers. Features of thermal transformations in the mixtures SORG and SORC provide the formation of polymodal porosity of the material. Along with cells with a diameter of 0.5 -3.0 mm, the cavities of 1 -5 µm in size are concentrated in the inter-pore partitions of the structure ( Figure 6).
Significant volume of porosity achieved due to cells of various sizes and strengths of inter-pore partitions, the closed nature of most of the pores provide high heat-shielding properties of the material, surpassing similar indicators of foam glass material.

Conclusions
Material composition of fillers significantly affects moulding properties and thermal swelling of materials based on sodium alkaline silicates.
Expediency of using multicomponent fillers, increasing technological properties of moulding sands and contribute to saturation of pyroplastic liquid glass masses with the gas phase, is substantiated.
Polymineral silica-containing mixtures based on fillers with different temperatures of dissociation and formation of the gas phase during the period of thermal softening of liquid glass mixtures were proposed.
Presence of silica-containing crystalline phases, isolation and poly modality of pores provide the developed heat-insulating cellular material with high technical properties.