Service properties of porous liquid glass concrete

. The article presents studies’ results of cementless lightweight concretes based on porous granular aggregate. Lightweight concrete components are specially synthesized from mixtures containing liquid sodium glass and thermal energy waste with various fineness. Thermal hardening of a matrix based on liquid glass and technogenic fillers at a temperature of 350ºС provided heat-insulating concrete with 480 kg/m 3 density and compressive strength of 4.7 MPa. The aim of the work is to study operational stability of lightweight concrete from genetically related components. Durability of lightweight concrete was evaluated in terms of hydro physical properties, resistance to frost and salt aggression, and cyclic heating. Methods of physical and mechanical testing of concrete have been used in the work. X-ray phase analysis and electron microscopy were used to study materials’ composition and structure. The results of complex tests showed stability of the structure of liquid glass concrete based on porous aggregate to the impact of operational factors. The lightweight concretes developed are characterized by a softening coefficient of 0.81; they withstood 50 cycles of alternating freezing and thawing, 20 cycles of cyclic exposure at a temperature of 1050ºС and 20 thermal cy cles at a temperature of 250ºС ; staying in aggressive sulfate and chloride magnesium solutions.


Introduction
Energy efficiency of construction is largely determined by durability of materials that have heat-shielding properties and maintain stability of the structure.Long-term operation of building objects erected using expanded clay concrete confirms reliability of lightweight concretes based on porous aggregates [1,2].Increasing requirements for thermal resistance of enclosing structures requires expanding the range of lightweight concretes.
Thermal engineering and physicomechanical properties of lightweight concretes significantly depend on structural characteristics of porous aggregate.Studies aimed at reducing aggregates' density and energy intensity of production are relevant for the development of lightweight concrete technology [3,4].The resource saving technology of porous aggregates is facilitated by involvement of technogenic materials of mineral and organic origin.Significant experience has been accumulated in the use of ashes formed at thermal power plants to produce lightweight concretes.Ashes from hard coal combustion are introduced into concrete mixtures, into raw materials to obtain unburned and thermally expanded granular materials [5 -7].An aluminosilicate microsphere is a type of thermal energy waste and is also used in the technology of porous materials [8,9].
Highly porous granular materials are obtained by thermal expansion of mixtures based on the liquid glass [10,11].Physical and chemical properties of liquid glass provide its versatility in the composition of raw mixes: binding of finely dispersed components of the moulding mass; formation of porosity of materials [4].
New varieties of porous aggregate are often characterized by the presence of noncrystalline or weakly crystalline silica, siliceous glass.This causes the probability of alkaline corrosion development as a result of reactions between aggregate and alkali hydroxides of the cement stone [12,13].To increase the resistance of lightweight concretes based on artificial porous aggregates, composite materials from cementless binders are expedient.
The need to use clinker-free binders is also due to the high energy intensity of Portland cement production, which is characterized by a significant carbon footprint.The technologies of alkali-silicate binders based on alkaline activation of silica-containing materials are promising [9,10].The impact of alkalis, for example, liquid glass, promotes dissolution and subsequent polycondensation of silica-containing filler with the formation of a durable stone.
Porous structure can contribute to materials destruction under environmental influences.Lightweight concretes are often subjected to alternating wetting and drying, freezing and thawing, and exposure to aggressive environments under operating conditions [14 -16].
Creation and use of new components in the composition of moulding sands increases the relevance of the research of operational reliability of lightweight concretes.
The objective of the work is to study the performance properties of lightweight concretes, the components of which are obtained from raw mixes based on liquid glass.

Materials and methods of research
Specially synthesized porous aggregate and binder were used to obtain lightweight concrete.The chemical composition of raw materials for the production of lightweight concrete components is presented in Table 1.As part of the raw masses, the liquid glass performed several functions as follow: binding of dispersed components and regulation of rheological properties of moulding sands; activation of fillers participation in the processes of structure formation; thermal swelling with the formation of thin-walled cells.The experiments used liquid soda glass, characterized by a silicate modulus n = SiO2 : Na2O = 2.7 and density of 1350 kg/m3.Liquid glass served as a component of the raw mixture for obtaining aggregate and contributed to thermal porosity of the granules.Liquid glass has been used as the binder's basis in the concrete mixture.
Waste from thermal power engineering has been used as filler for liquid glass mixtures.Fly ash is a finely dispersed residue formed during pulverized coal combustion.Fly ash contains aluminosilicate opaque glass, quartz, mullite and particles of unburned coal.Fly ash is characterized by a specific surface area of 300 m2/kg and a bulk density of 720 kg/m3.
Aluminosilicate microsphere is a loose material consisting of glass-ceramic hollow particles with a diameter of 50 -200 microns.The aluminosilicate microsphere is formed during high-temperature flaring of coal and has a bulk density of 400 kg/m3.
Raw mixture for obtaining a porous granular aggregate was characterized by the ratio of «liquid glass : waste of thermal energy», equal to 0.7.Granules with a diameter of 7 -10 mm were obtained by pelletizing the mixture on a laboratory drum machine.Raw granules were fired at a temperature of 200°C [17].The main characteristics of the porous aggregate are shown in Table 2. Binder based on liquid sodium glass and thermal energy waste at a ratio of components equal to 1.2 has been the matrix of lightweight concrete.
The method for preparing moulding sands for lightweight concrete included two stages.The first stage is preparation of a binder by mixing liquid glass with fly ash and aluminosilicate microsphere for 2 minutes.The second stage is introduction of porous aggregate into the liquid glass suspension.Moulding sands compositions were characterized by the ratio of «binder volume : aggregate volume» equal to 0.4 -0.5.Stirring the mass for 3 min ensured uniform distribution of the binder between the granules.Mobility of the sands, determined using the Abrams cone, was characterized by a cone draft of 2 -4 cm.
To obtain samples of lightweight concrete, the moulding sand was placed in a metal mold and vibrated for 30 s.Samples sized 70x70x70 mm after preliminary one hour exposure were subjected to heat treatment in a drying chamber at a temperature of 350ºС.
To assess service properties, the samples of lightweight concrete were subjected to resistance tests under conditions of variable values of humidity and temperature, and exposure to an aggressive environment.
Impact of water on concrete was characterized by the magnitude of water absorption, water resistance, changes during alternating water saturation and drying.Water resistance of concrete samples was evaluated by the softening coefficient, calculated as the ratio of strength of the sample, which was in water for 10 days, and original sample's strength.Resistance to alternating water saturation and drying was determined by visual evaluation and testing of samples for strength after every 10 cycles.Test cycle as follows: 4 hours is saturation with water, 4 hours is drying at a temperature of 25 -30ºС.
Resistance of concrete to frost exposure was determined by comparing the strength of samples subjected to alternate freezing (at a temperature of «minus» 20ºС) and thawing in water (at a temperature of 20ºС) after 25 and 50 test cycles.Samples were saturated with water before freezing.Test cycle as follows: freezing -3 hours and thawing in water -3 hours.At the end of each cycle, the samples were examined for signs of destruction.
To study concrete's behavior in aggressive environments, the samples were placed in solutions of magnesium sulfate (3% concentration), sodium sulfate (5% concentration) and magnesium chloride (7% concentration).To enhance impact of aggressive environments on the material, samples with cut ends were used, where access to the inside part of a granular aggregate is open.
The nature of high temperatures' impact on concrete was evaluated by testing samples for thermal resistance and heat resistance.Thermal resistance is the ability of a material to withstand thermal stresses during heating and cooling.The samples were placed in a drying chamber heated to a temperature of 250ºС, and kept for 2 hours.Then the samples heated were immersed into the water (20ºС) for 2 hours.
Heat resistance is the ability of a material to work when exposed to high temperatures.The heat resistance of concrete was determined by the change in appearance and «residual» strength.The samples were kept in a muffle furnace for 4 hours at a temperature of 1050ºC.After turning off the furnace, the samples were cooled to a temperature of 500ºC for 2 h, then again subjected to high-temperature exposure.For heat-resistant materials, the «residual» strength must be at least 80%.

Results
Lightweight concrete, obtained from granular aggregate and liquid glass binder, is characterized by a porous structure, heat-shielding properties (Table 3).The pores are concentrated in the aggregate and in the liquid glass matrix.Phase composition of the hardened binder is represented by β-quartz, α-cristobalite, wollastonite, and X-ray amorphous compounds (Figure 1).Studies of the contact zone in concrete indicate reliable adhesion of porous granules to the matrix substance (Figure 2).The results of determining concrete's hydro physical properties are shown in Table 4.During the testing period, no pronounced defects were found on the -concrete samples.However, white accumulations were formed on the cut of the samples in the thin capillaries of the lower layer.Visual assessment of the state of concrete samples subjected to alternate freezing and thawing did not reveal defects in the structure of the material.After 50 test cycles, the strength of concrete samples amounted to 75% of standard samples' indicators (Table 5).The nature of samples' destruction during strength tests indicates the preservation of contacts of the aggregate with the matrix substance.The results of concrete testing for resistance to salt solutions are shown in Table 6.The concrete samples were being examined visually within 8 months; a resistance coefficient was determined after completion of the tests (the ratio of samples' strength in aggressive environment to the strength of standard samples).The samples were accepted to pass the test if the value of the resistance coefficient is at least 0.85.Concrete samples in a solution of magnesium sulfate showed resistance and retained the integrity of structure; however, white insoluble accumulations of magnesium hydroxide were formed on the surface.White formations were revealed after three weeks of concrete's being in magnesium sulfate solution.Subsequently, the number of clusters did not change, but the size of formations increased by an average of 10 -15%.Holding concrete in magnesium chloride solution practically did not worsen samples condition.Signs of violation of samples integrity were observed in sodium sulfate solution four weeks later; small fragments of the material were separated from the surface of the concrete.Four months later, the activity of destructive processes decreased.

Before testing
After testing After determining the strength strength 6.4 MPa strength, MPa: 25 cycles -5.9 after 50 cycles 50 cycles -4.8All the concrete samples retained resistance to swelling deformations during the testing period.
Materials based on liquid glass are generally characterized by resistance to elevated temperatures.Tests of the concrete developed revealed that heat treatment contributes to the increase in density of samples by 6 -19% (Table 7).
The density of samples tested for heat resistance increases, apparently due to formation of additional hydrates.At the same time, the strength of concrete is reduced by 30% as a result of destructive processes.
Samples subjected to a heat resistance test are characterized by shrinkage of 2-3% and hardening.The nature of destruction of the heat-resistant composite during strength tests indicates the stability of adhesion of structure's components (Table 7).

Discussion
The nature of impact of various operational factors on the concrete developed largely depends on the composition and structure of the material under study.
The value of water absorption is much less than the total porosity of the material (Tables 3 and 4), which is predetermined by predominance of closed pores in the concrete structure.The value of the softening coefficient characterizes the concrete under study as a waterproof material.Ensuring resistance of liquid-glass concrete to the impact of aqueous medium is provided by the mode of heat treatment of the material, which contributed to the formation of solid solutions with the participation of technogenic aggregates, partial crystallization of silicate neoplasms [18 -20].The presence of crystalline phases (β-quartz, α-cristobalite and wollastonite) is mainly due to participation of fly ash in the processes of binder stone's structure formation.Heat treatment of concrete at a temperature of 350ºС contributed to increase in the crystalline component in the binder, strengthening and increasing water stone's resistance.The test results testify to satisfactory resistance of liquid-glass concrete to the effects of water and possibility of using products under high humidity.
After 25 cycles of testing for frost resistance, the strength of concrete samples was 92%, after 50 cycles it was 75% of standard samples' indicators.Appearance of defects in the concrete structure was not observed.The nature of destruction of the samples during the strength test testified to reliable adhesion of the aggregate to the matrix.The test results showed that frost resistance of liquid glass concrete meets the requirements for wall materials.Resistance of the structure of the studied material to frost damage is due to the strength of the structure and low values of water absorption of porous concrete.
Liquid-glass concrete showed satisfactory stability in solutions of magnesium salts.The aggressive effect of sodium sulfate solution on concrete is apparently associated with decrease in the content of silica gel, which is the basis of the adhesive.
The resistance of the studied water-glass concrete to the effects of elevated temperatures is predetermined by the material composition of the moulding sand.Wastes from thermal energy, generated during high-temperature flaring of fuel at coal-fired power plants were used to obtain concrete.The thermal stability of liquid glass is due to the nature of phase transformations during heating and is used in the technology of heat-resistant concrete.

Conclusions
Research results of heat-insulating cementless concrete containing granular aggregate testify to the resistance of porous structure to the cyclic effects of water and elevated temperatures, to frost and salt aggression.
Combinations of liquid glass and thermal energy wastes of various states, the choice of conditions for raw mixtures hardening ensured the formation of insoluble crystalline phases and a structure with predominantly closed porosity and resistance to operational factors.
Genetic relationship of the matrix substance and porous aggregate contributed to their reliable adhesion and exclusion of destructive reactions between the concrete components.
Operational stability of the lightweight concrete developed enhances the resourcesaving aspect of the cementless composite material obtained using large-tonnage waste from thermal power engineering

Table 2 .
Characteristics of porous granular aggregate.

Table 3 .
Properties of concrete.

Table 4 .
Indicators of concrete's hydro physical properties.

Table 5 .
Characteristics of frost resistance of concrete.

Table 6 .
Indicators of lightweight concrete's resistance in aggressive environments.

Table 7 .
Indicators of concrete resistance to thermal effects.